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Plant Physiology 133:1000-1010 (2003)
© 2003 American Society of Plant Biologists

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Agrobacterium-Mediated Root Transformation Is Inhibited by Mutation of an Arabidopsis Cellulose Synthase-Like Gene1

Yanmin Zhu, Jaesung Nam2, Nicholas C. Carpita, Ann G. Matthysse and Stanton B. Gelvin*

Departments of Biological Sciences (Y.Z., J.N., S.B.G.) and Botany and Plant Pathology (N.C.C.), Purdue University, West Lafayette, Indiana 47907; and Department of Biology, University of North Carolina, Chapel Hill, North Carolina 27599 (A.G.M.)


   ABSTRACT
TOP
 ABSTRACT
RESULTS
 DISCUSSION
 MATERIALS AND METHODS
 LITERATURE CITED
 
Agrobacterium-mediated plant genetic transformation involves a complex interaction between the bacterium and the host plant. Relatively little is known about the role plant genes and proteins play in this process. We previously identified an Arabidopsis mutant, rat4, that is resistant to Agrobacterium transformation. We show here that rat4 contains a T-DNA insertion into the 3'-untranslated region of the cellulose synthase-like gene CSLA9. CSLA9 transcripts are greatly reduced in the rat4 mutant. Genetic complementation of rat4 with wild-type genomic copies of the CSLA9 gene restores both transformation competence and the wild-type level of CSLA9 transcripts. The CSLA9 promoter shows a distinct pattern of expression in Arabidopsis plants. In particular, the promoter is active in the elongation zone of roots, the root tissue that we previously showed is most susceptible to Agrobacterium-mediated transformation. Disruption of the CSLA9 gene in the rat4 mutant results in reduced numbers and rate of growth of lateral roots and reduced ability of the roots to bind A. tumefaciens cells under certain conditions. No major differences in the linkage structure of the non-cellulosic polysaccharides could be traced to the defective CSLA9 gene.

The investigation of the molecular basis of Agrobacterium-mediated plant transformation has been a focus of research for more than 3 decades, driven most recently by its use in the generation of transgenic plants for the plant biotechnology community. This complex process can be separated into several steps: (a) induction of bacterial vir (virulence) genes by phenolic and sugar molecules released from wounded plants; (b) processing of the transferred DNA (T-DNA) region from the tumor-inducing plasmid by the VirD1, D2 endonuclease; (c) attachment of the bacteria to the plant surface; (d) transfer of T-DNA and virulence proteins from the bacterium to the plant cell via a Type IV secretion system composed of VirB and VirD4 proteins; (e) cytoplasmic trafficking and nuclear targeting of T-DNA and virulence proteins, most likely involving the plant actin cytoskeleton and importins; (f) integration of T-DNA into the plant genome; and (g) expression of T-DNA-encoded genes (for review, see Christie and Vogel, 2000Go; Gelvin, 2000Go, 2003aGo; Zupan et al., 2000Go; Tzfira and Citovsky, 2001Go). The ultimate outcome of this complex process is the horizontal transfer of genetic information from A. tumefaciens to the plant. Investigations on the contribution of the bacterium to this interaction generally have involved genetic analyses of bacterial genes involved in attachment, vir gene induction, T-DNA processing, and T-DNA transfer (e.g. Stachel and Nester, 1986Go; Stachel and Zambryski, 1986Go; Stachel et al., 1987Go; Veluthambi et al., 1988Go; Matthysse et al., 1996bGo), followed by molecular and biochemical analyses. Compared with the large volume of information regarding these bacterial processes, little is known about the role of plant genes and their products in the transformation process (Sheng and Citovsky, 1996Go; Tinland, 1996Go; Gelvin, 2000Go, 2003aGo). Further knowledge of the plant contribution to transformation almost certainly will be useful in extending the host range of A. tumefaciens and the "quality" of the transformation event (Gelvin, 2003aGo, 2003bGo).

To identify plant genes and proteins involved in Agrobacterium-mediated plant transformation, we devised a series of root-based transformation assays to identify Arabidopsis T-DNA insertion mutants, rat mutants (resistant to Agrobacterium-mediated transformation). Screening of the Feldmann Arabidopsis T-DNA insertion library (Feldmann and Marks, 1987Go; Winkler et al., 1998Go) and other similar libraries indicated that approximately 0.7% of the mutant lines are deficient in the process of Agrobacterium-mediated plant transformation (Nam et al., 1999Go; Zhu et al., 2003Go). Among the mutants that we identified is rat4. The initial characterization of rat4 indicated that the phenotype appears dominant and cosegregates with kanamycin resistance encoded by the mutagenizing T-DNA (Nam et al., 1999Go). We show here that the rat phenotype of rat4 results from the insertion of a T-DNA into the 3'-untranslated region (UTR) of the cellulose synthase-like gene CSLA9.

The Arabidopsis genome contains six families of CSL genes consisting of 30 members. These genes are structurally related to the CESA gene and contain conserved motifs considered necessary for nucleotide-sugar binding and catalytic activity of processive glycosyltransferases responsible for synthesis of (1->4)-{beta}-linked polysaccharide backbones in which each sugar residue is turned 180° with respect to its neighbor (Delmer, 1999Go; Richmond and Somerville, 2001Go). The divergent nature of CESA-like (CSL) sequences suggests that they are involved in the synthesis of several distinct cell wall polysaccharides during plant growth and development. To date, no biochemical function has been ascribed to any CSL protein, although disruption of the Arabidopsis AtCSLD3 gene results in the rupture of initiating root hairs (Favery et al., 2001Go; Wang et al., 2001Go).

We report here that the expression of the CSLA9 gene in rat4 is severely decreased relative to that found in wild-type plants. Genetic complementation of rat4 with wild-type genomic copies of the CSLA9 gene restores both the elevated transcript levels and transformation proficiency. The CSLA9 gene is expressed in regions of the plant root that we previously showed are most susceptible to Agrobacterium-mediated transformation (Yi et al., 2002Go). In addition, the rat4 mutant has decreased numbers of lateral roots, shows a decreased ability to bind A. tumefaciens, but displays no major alterations in cell wall sugar composition or linkage structure. These experiments indicate that CSLA9 is involved in an early step in the Agrobacterium-mediated plant transformation process.


   RESULTS
TOP
 ABSTRACT
 RESULTS
DISCUSSION
 MATERIALS AND METHODS
 LITERATURE CITED
 

The rat4 Mutant Contains a T-DNA Insertion in the 3'-UTR of the CSLA9 Gene

We previously described the identification of the rat4 mutant from the Feldmann T-DNA insertion library (Nam et al., 1999Go). Using a root-based tumorigenesis assay, we confirmed the previous finding that the transformation of homozygous rat4 root segments is severely decreased compared with that of wild-type parental Wassilewskija (Ws) plants. Figure 1A shows that the few tumors generated on rat4 roots were small and yellow, compared with the numerous large green teratomas incited by A. tumefaciens A208 on roots of wild-type plants. We obtained similar results using another transformation assay that measures the generation of phosphinothricin-resistant calli after infection of root segments with A. tumefaciens At849 that contains a plant-active bar gene within the T-DNA of a binary vector (Nam et al., 1999Go; Zhu et al., 2003Go; data not shown).



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  		Figure 1. Characterization of the rat4 mutant. A, Stable transformation crown gall tumorigenesis assays on root segments of wild-type Arabidopsis ecotype Ws and rat4 mutant plants. Sterile root segments were infected with A. tumefaciens A208. Two days after infection, the roots were transferred to Murashige and Skoog medium lacking phytohormones and containing timentin. Tumors were scored 4 weeks later. B, Sequence of the RAT4/T-DNA junction region. The T-DNA was inserted into the 3'-UTR of the RAT4 gene. The insertion was precise on one side of the T-DNA. However, on the other side, the insertion caused a deletion of 5 bp (orange letters in parentheses) with an insertion of 3 bp of "filler" DNA (black letters in parentheses). Underlined green nucleotides indicate the putative polyadenylation signal sequence for the CSLA9 gene. Nucleotides in brown indicate an unknown gene adjacent to RAT4.

 

We used a plasmid rescue technique to isolate T-DNA/plant DNA junction fragments from rat4. DNA sequence analysis of the junction regions identified a T-DNA insertion into the 3'-UTR of the CESA-like gene CSLA9 (At5g03760) 260 bp after the stop codon and downstream of a putative polyadenylation signal AATATA. DNA-blot and sequence analysis indicated that two copies of T-DNA were inserted in an inverted repeat conformation; T-DNA left border sequences were found at both junctions (Fig. 1B). Comparison of the T-DNA junction sequence with that of the wild-type CSLA9 sequence indicated that, on one side of the T-DNA, there is no deletion at the insertion site. However, on the other side, five nucleotides of the CSLA9 sequence are deleted, and three nucleotides appear as "filler" DNA. We established a rat4 T-DNA insertion line that is homozygous for the T-DNA insertion by PCR genotyping because kanamycin resistance encoded by the nos-nptII gene within the T-DNA was frequently silenced in homozygous rat4 plants (data not shown).


The rat Phenotype of rat4 Results from the T-DNA Insertion in the CSLA9 Gene

The rat phenotype of rat4 cosegregates with kanamycin resistance encoded by the mutagenizing T-DNA (Nam et al., 1999Go). To determine whether mutation of CSLA9 caused the rat phenotype, we performed genetic complementation experiments using genomic clones covering various regions that encompass the CSLA9 gene (Fig. 2). Although the mutants are resistant to transformation via roots, we were able to introduce these genomic clones into homozygous rat4 mutant plants using a flower dip protocol (Mysore et al., 2000aGo). We selected T1 plants containing the complementing T-DNA using hygromycin. To various extents, several complementation lines for each construction restored transformation proficiency to rat4 (Table I). In particular, a 7.1-kb BamHI-AvrII genomic fragment released from the P1 clone MED24 and containing only the CSLA9 gene (pRAT4CCC) was able to restore transformation competence to the rat4 mutant. Line 4 of RAT4CCC and line 3 of RAT4C showed tumorigenesis efficiencies similar to, or even higher than, those of wild-type Ws plants. This result indicates that the T-DNA insertion into the 3' end of CSL9 is responsible for the rat phenotype of rat4. Our ability to complement a homozygous rat4 line with various constructions containing the wild-type CSLA9 gene indicates that, although the rat4 mutation behaves as a dominant characteristic in genetic crosses (Nam et al., 1999Go), the mutant is actually haplo-insufficient. Another rat mutant, rat5, displays similar haplo-insufficient properties (Mysore et al., 2000bGo).



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  		Figure 2. Map of the CSLA9 genomic DNA region showing the various restriction endonuclease fragments used for complementation (above the map). The nine exons of the CSLA9 gene are shown in dark boxes. Below the map is a schematic diagram of pCB302, the T-DNA binary vector used to deliver the various RAT4 constructions into the rat4 mutant. RB, T-DNA right border; LB, T-DNA left border; RAT4, coding sequence and flanking sequences of the RAT4 CSLA9 gene; Pnos, nopaline synthase promoter; Tnos, nopaline synthase polyadenylation signal sequence; bar, phosphinothricin resistance gene. The SalI site in pRAT4C derives from the inserted T-DNA in the rat4 mutant and is not found in the genomic DNA sequence.

 

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  		Table I. Tumorigenesis assays on complemented rat4 lines and rat4 antisense lines

Root segments from wild-type and rat4 mutant plants containing various constructions of the wild-type CSLA9 gene or wild-type plants containing a CSLA9 antisense construction were infected with A. tumefaciens A208. The formation of crown gall tumors on the root segments was determined after 1 month.

 

In addition to the complementation experiments, we attempted to reconstitute the rat phenotype by expressing an antisense rat4 cDNA, under the control of a cauliflower mosaic virus 35S promoter, in wild-type Ws plants. Tumorigenesis assays on the roots of antisense lines L to N showed partial rat phenotypes, with a 30% to 70% reduction in tumorigenesis efficiency compared with that of wild-type root segments (Table I).


Correlation of the Level of CSLA9 Transcripts with the Observed rat Phenotypes

In the rat4 mutant, the T-DNA inserted downstream of the putative polyadenylation signal of the CSLA9 gene. The distance of the insertion site from the CSLA9 ORF raised the question of whether the insertion of T-DNA affected the CSLA9 transcript level. Therefore, we performed reverse transcriptase (RT)-PCR analyses using total RNA from wild-type Ws plants, a homozygous rat4 mutant line, and rat4 lines showing various degrees of complementation after introduction of the wild-type CSLA9 gene. Expression of the CSLA9 gene was barely detectable in rat4 mutant plants, whereas CSLA9 transcript levels increased to almost wild-type levels in a successfully complemented line (Fig. 3). However, CSLA9 transcript levels remained either undetectable or low in rat4 lines that were not successfully complemented by the wild-type CSLA9 cDNA.



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  		Figure 3. CSLA9 transcript levels in wild-type, rat4, and rat4 complemented lines. Plants were grown under identical conditions, and total RNA was isolated at the same time. Total RNA isolated from 2-week-old plants was subjected to RT-PCR. Samples were normalized to the ACT2 transcript. Lane 1, Wild-type Ws; lane 2, rat4; lane 3, RNA from a successfully complemented rat4 line; lanes 4 and 5, RNA from rat4 lines that did not show successful phenotypic complementation.

 


Expression Pattern of the CSLA9 Promoter

To determine the expression pattern of the CSLA9 promoter, we created a translational fusion consisting of 1.9 kb of DNA upstream of the CSLA9 start codon plus DNA encoding the first 24 amino acids of CSLA9 with a gusA gene and introduced this construction into wild-type Arabidopsis plants. We chose this promoter region because of convenient restriction endonuclease sites, and because this region was used in the RAT4CC and RAT4CCC constructions described above. We assayed the resulting transgenic plants for {beta}-glucuronidase (GUS) activity at different developmental stages to examine the expression pattern of the CSLA9 promoter. GUS expression in young plants (2–7 d old) was observed primarily in the cotyledons, at the base of the hypocotyls, in newly emerging lateral root primordia, and in the root elongation zone (Fig. 4, A–C). As the plant matured, GUS expression in the aerial portions of the plant was primarily in the vascular system of young leaves (Fig. 4E). In mature plants, GUS expression was detected primarily in the abscission zone of the pedicle and in the flower receptacle (Fig. 4, D and F). We routinely assay roots of Arabidopsis plants for transformation competence approximately 3 weeks after germination. At this stage, CSLA9 promoter activity in the root is restricted to the elongation zone and lateral root primordia (Fig. 4, B and C), the regions of the root that we previously demonstrated were most susceptible to Agrobacterium-mediated transformation (Yi et al., 2002Go).



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  		Figure 4. Expression pattern of the CSLA9 promoter. A CSLA9 promoter-gusA translational fusion was introduced into wild-type Ws plants, and the plants were examined for GUS expression various times after germination. A, Expression in a young (5-d-old) seedling. Note that GUS expression is primarily in the cotyledons, the base of the hypocotyls, the crown of the plant, emerging lateral root primordia, and the root elongation zone; B, Enlargement of a root of a young plant showing GUS activity limited to the elongation zone. C, Enlargement of a root of a mature plant showing GUS activity in lateral root primordia. D, Enlargement of the region at the crown of the rosette leaves. E, Young leaf showing GUS staining predominantly in the vascular system. F, Mature inflorescence tissues showing GUS expression in the abscission zone of the peduncle and in the flower receptacle.

 


rat4 Plants Have Fewer Lateral Roots Than Do Wild-type Plants

Because the expression pattern of the CSLA9 promoter was restricted to particular tissues in developing plants, we carefully examined rat4 plants for developmental abnormalities. During the 1st week of growth, we could not distinguish homozygous rat4 plants from wild-type plants. However, after 2 weeks of growth, rat4 plants showed differences in the number and rate of growth of lateral roots. When grown in vitro on agar, rat4 plants developed approximately one-half the number of lateral roots as did wild-type plants at 2 weeks after germination (Fig. 5A). There was also a visible difference in lateral root length (Fig. 5B). However, when grown in soil in a growth chamber, rat4 plants showed no significant differences from wild-type plants in the number of leaves per plant, average plant height, time until flowering and setting of the first siliques, and the average number of siliques per plant (data not shown).



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  		Figure 5. Development of lateral roots in wild-type Ws and rat4 mutant plants. Seeds were surface sterilized and placed on Gamborg's B5 agar in a 150-mm-diameter plate. After a 2-d cold treatment, the plates were placed vertically in a growth chamber (23°C, 16 h of light/8 h of dark), and the plants were observed periodically. The number of lateral roots was counted 2 weeks after germination. A, Wild-type Ws plants showed approximately twice the number of lateral roots as did rat4 mutant plants. B, Typical plate showing the decreased number and rate of growth of lateral roots on rat4 plants compared with wild-type plants.

 


rat4 Plants Are Deficient in A. tumefaciens and Diarrheagenic Escherichia coli Attachment to Roots

Because rat4 mutant plants show decreased expression of a gene putatively involved in cell wall synthesis and because this gene is expressed primarily in root cells most susceptible to Agrobacterium-mediated transformation (Yi et al., 2002Go), we examined the ability of roots of rat4 plants to bind A. tumefaciens cells. When incubated in water using an excess of bacteria and observed in the light microscope, fewer bacteria bound to the cut ends of rat4 roots and to rat4 root hairs than to the corresponding tissues of wild-type Ws plants (Table II). However, when assayed in 0.4% (w/v) sucrose, levels of attachment of A. tumefaciens cells to roots of rat4 and wild-type plants were indistinguishable. In addition, attachment of bacteria to the epidermis of rat4 and wild-type roots was similar in either water or sucrose. The complemented rat4 mutant (RAT4C line 3) recovered the wild-type level of A. tumefaciens attachment in water. When binding of A. tumefaciens cells to roots of the rat4 mutant was measured with an excess of roots (a 1-cm root segment can bind more than 105 bacteria; these experiments were conducted with 103–104 bacteria), the rat4 roots were deficient in binding bacteria in water (4% ± 3% of the bacterial inoculum bound) as compared with the parent strain (26% ± 5% of the bacterial inoculum bound). The complemented rat4 mutant recovered the ability to bind bacteria in water (22% ± 8% of the bacterial inoculum bound). Thus, under certain conditions, the attachment of A. tumefaciens cells to roots was altered in the rat4 mutant. These alterations were reversed when the rat4 mutant was complemented by a wild-type CSLA9 gene.


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  		Table II. Attachment of A. tumefaciens C58 to roots of wild-type, rat4, and complemented rat4 mutant plants

Roots of axenically grown plants were incubated with A. tumefaciens C58 for 2 h. The roots were gently washed, and the no. of bacteria per microscope field was determined.

 

To examine whether the defect in rat4 mutant binding of A. tumefaciens was specific for this bacterium or represented a more general defect in the ability of the plant to bind bacteria, we examined the binding of a diarrheagenic strain of E. coli to roots of the rat4 mutant. Diarrheagenic E. coli bind to root hairs and the root epidermis of wild-type Arabidopsis plants but not to the cut ends of roots (C. Jeter and A.G. Matthysse, unpublished data), suggesting that the binding site is different from that for A. tumefaciens. When examined microscopically, E. coli strain DEC 4A showed very little binding to the rat4 mutant as compared with the parental ecotype Ws (± versus +++ as defined in Table II). The RAT4C complemented line recovered the ability to bind E. coli DEC 4A (+++). This result suggests that the rat4 mutant has a general surface defect that affects binding of diverse bacteria.


rat4 Plants Have No Major Alterations in Cell Wall Linkage Composition

Preliminary examinations of neutral monosaccharide compositions of plants grown in a potting medium showed that the AtCSL9 mutant contained elevated levels of Gal compared with the wild-type plants (data not shown). Because this apparent difference in Gal content indicated a possible alteration in the pectic components, we included quantitation of the uronic acid fraction with neutral sugars in the determination of cell wall monosaccharide analysis of plants during an extended growth period in agar culture. Consistent with the earlier indications, we observed small enrichments in Gal during growth to a greater extent in the mutant than in the wild-type plants.

The results of the monosaccharide composition were unexpected, considering that the mutation was traced to a defect in a CSLA gene. However, the loss of one type of polymer may be compensated by increases in another. Therefore, we performed linkage analyses to account for the major linkages representative of the backbone polymers of the major non-cellulosic polysaccharides (Table III). Again, all backbone polymers typical of Arabidopsis walls were present in both mutant and wild-type plants. There was a small increase in Gal-linked polymers in the rat4 mutant during later stages of development.


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  		Table III. Quantitation of the major linkage groups of backbone polymers in wild-type and rat4 mutant seedlings

Uronic acid residues in purified cell walls were activated by water-soluble diimide and carboxyl-reduced with NaBD4 to convert them to their respective 6,6-dideuteroneutral sugars. GalA and GlcA residues were determined by electron-impact mass spectrometry of the partly methylated alditol acetate derivatives as described by Carpita and Shea, 1989Go. Values are the means of two samples; the variance was equal to or less than 3% of the mean. In standard carbohydrate nomenclature, a 4-Glcp refers to a glucopyranosyl residue with a sugar linked to its O-4 position. Iteration of this linkage with only Glc residues is inferred as a (1->4)-{beta} -glucan. The actual derivative is 2,3,6-tri-O-methyl-(1-deuterio)-1,4,5-tri-O-acetyl-glucitol.

 


   DISCUSSION
TOP
 ABSTRACT
 RESULTS
 DISCUSSION
MATERIALS AND METHODS
 LITERATURE CITED
 
To understand the contribution of the host plant to the process of Agrobacterium-mediated plant transformation, we have identified Arabidopsis mutants with altered transformation susceptibility (Nam et al., 1999Go; Zhu et al., 2003Go). In this study, we further characterize rat4, one of the mutants initially identified (Nam et al., 1999Go). Previous results showed that rat4 is deficient in both stable (crown gall tumorigenesis and the formation of herbicide-resistant calli) and transient (GUS expression) transformation. In this study, we show that rat4 contains two copies of T-DNA inserted, in an inverted repeat configuration, into the 3'-UTR of the CSLA9 gene. Introduction of wild-type copies of CSLA9 genomic DNA into the rat4 mutant reversed the rat phenotype, restoring transformation competence to root segments. This result indicates that disruption of the CSLA9 gene is responsible for the observed rat phenotype of the rat4 mutant. Coincident with the restoration of transformation competence, complemented rat4 lines regained a wild-type level of CSLA9 transcripts compared with the highly reduced level of CSLA9 transcripts found in the rat4 mutant. Our RT-PCR results indicate that insertion of T-DNA into the CSLA9 3'-UTR results in the instability of RAT4 transcripts. Analysis of CSLA9 promoter-gusA fusion genes in transgenic Arabidopsis indicated that the CSLA9 gene is expressed in specific plant tissues. In particular, expression of the CSLA9 promoter in mature roots resembles that of the histone H2A gene HTA1 in that it is restricted to the root elongation zone and newly emerged lateral root primordia. These sites correspond to regions of the root most highly susceptible to Agrobacterium-mediated transformation (Yi et al., 2002Go).

Homozygous rat4 plants grown in soil superficially appear normal. We detected no statistically significant differences from wild-type plants with regard to the number of leaves per plant, time to flowering and setting of the first silique, or the number of siliques per plant. However, rat4 plants grown in vitro had approximately one-half the number of lateral roots as did wild-type plants, and these lateral roots grew more slowly. With regard to their interaction with A. tumefaciens, however, rat4 plants differed greatly from their wild-type progenitors. Attachment assays indicated that A. tumefaciens C58 binds poorly to rat4 roots in water, although attachment in 0.4% (w/v) sucrose was similar with both rat4 and wild-type roots. In wild-type plants, the rat4 promoter expresses strongly in lateral root primordia. Consistent with a potential function of the CSLA9 gene in lateral root generation, the number and rate of growth of lateral roots differed between rat4 and wild-type plants.

The function of the CSLA9 protein in the Agrobacterium-mediated plant transformation process is unknown. However, the rat4 mutant is blocked in an early step in Agrobacterium-mediated transformation: Transient expression of GUS activity directed by an incoming T-DNA is greatly reduced relative to that shown by wild-type plants, indicating that the lesion occurs before T-DNA integration (Nam et al., 1999Go; Zhu et al., 2003Go). This early block in transformation is consistent with the observed deficiency in the ability of rat4 roots to bind A. tumefaciens cells. Perhaps rat4 plants are altered in their ability to secrete particular polysaccharides necessary for bacterial recognition of the host and subsequent attachment. The synthesis of non-cellulosic, secreted polysaccharides from the root is one of the speculated functions for CSL proteins (Richmond and Somerville, 2001Go). Two other rat mutants (rat1 and rat3) are also deficient in the ability to bind A. tumefaciens cells (Nam et al., 1999Go; Y.M. Gaspar, J. Nam, C.J. Schultz, L.-Y. Lee, P. Gilson, S.B. Gelvin, and A. Bacic, unpublished data). However, rat4 is the first of these mutations to occur in a gene that encodes a CSL. These results suggest the complexity of plant cell wall architecture that must exist for attachment of A. tumefaciens to occur.

The CESA/CSL gene superfamily comprises about 40 members in Arabidopsis. They are defined by the presence of up to eight membrane-spanning sequences surrounding a catalytic domain containing four conserved aspartyl residues and a QxxRW signature motif that is considered necessary for nucleotide sugar binding and catalysis in processive synthases of glycans in which at least one (1->4)-{beta}-linked sugar is turned 180° with respect to the next sugar in the chain (Delmer, 1999Go; Richmond and Somerville, 2001Go). In addition to CESAs of bacteria and plants, this putative catalytic motif is found in synthases as divergent as those of chitin and hyaluronans. True plant CESAs are distinguished from bacterial synthases by two plant-specific sequences inserted into the catalytic domain and two Zn finger domains upstream from the first membrane-spanning sequence. All CSL proteins contain the conserved aspartyl residues and the signature motif but, with the exception of the CSLD group, possess no Zn-fingers and have truncated plant-specific sequences (Delmer, 1999Go; Richmond and Somerville, 2001Go). The CSL genes cluster into at least six distinct groups, with CSLA comprising nine members. The Zn finger domains participate in the dimerization of CESA polypeptides, indicating that these features are responsible for the assembly of particle rosettes involving dozens of CESA molecules needed to make microfibrils (Kurek et al., 2002Go). The lack of Zn fingers indicates that the CSLs are involved in the synthesis of non-cellulose polysaccharides.

Because of the presence of the signature motifs, CSLs are likely to synthesize other (1->4)-{beta}-linked polysaccharides typically found in plant walls, such as xyloglucans, glucurono (arabino) xylans, and (gluco) mannans, although other polysaacharides with iterated linkage structures, such as (1->4)-{alpha}-D-galacturonans, (1->4)-{beta}-D-galactans, arabinogalactans, and (1->5)-{alpha}-L-arabinans may be generated by the CSLs (Carpita and Vergara, 1998Go; Richmond and Somerville, 2001Go). To date, no biosynthetic activity of any CSL has been defined (Richmond and Somerville, 2001Go; Bonetta et al., 2002Go). Insertional mutagenesis has produced knockouts of each CSL class in Arabidopsis, but no biochemical phenotype has been demonstrated unequivocally (Bonetta et al., 2002Go). The disruption of root hair formation in the kojak loss-of-function mutation in AtCSLD3 and the rat4 mutation in AtCSLA9 remain the only mutants of CSLs with obvious phenotypes. With respect to the rat4 mutant, we were unable to detect a deficiency in any major polymer typically found in the cell walls. Linkage analysis showed that all (1->4)-{beta}-linked polysaccharides, namely xyloglucans (4- and 4,6-linked Glc), glucuronoxylans [4- and 2(3),4-Xyl], (gluco) mannan (4- and 4,6-Man), and the linkages expected for (1->4)-{beta}-arabinans, homogalacturonan, rhamnogalacturonan I, and arabinogalactans, are represented.

Given the large number of members in each CSL class, failure to detect a biochemical phenotype associated with a deficiency may reflect a functional redundancy (Bonetta et al., 2002Go), an idea that is supported by overlapping expression patterns in tandem arrays (Richmond and Somerville, 2001Go). However, A. tumefaciens recognizes a specific feature in cell wall composition and/or architecture that is missing in the rat4 mutant. Given the localized temporal and spatial expression pattern of AtCSLA9, a loss of a specific polysaccharide in a few cells could be masked by competent expression of a functionally redundant CSLA in the majority of the surrounding cells. We currently are exploiting immunocytochemical and Fourier infrared imaging techniques to investigate wall composition in single cells. The latter technique has proven of some use in detecting an alteration in wall architecture in a CSLB6 mutant (Bonetta et al., 2002Go).

The CSLA9 protein joins a growing list of plant proteins that are involved in Agrobacterium-mediated transformation (Zhu et al., 2003Go). Many of these proteins, such as VIP1 (Tzfira et al., 2001Go), importin-{alpha} (Ballas and Citovsky, 1997Go; S. Bhattacharjee and S.B. Gelvin, unpublished data), and F actin (P. Rao and S.B. Gelvin, unpublished data), may directly interact with the T-complex, whereas others may play a more indirect role by, for example, maintaining the correct cell wall architecture to allow attachment of A. tumefaciens and the subsequent transfer of T-complex components. It is likely that CSLA9 is one of these latter groups of proteins that affect transformation by altering cellular constituents important for the transformation process. Further investigations of these genes and proteins will likely reveal their roles not only in Agrobacterium-mediated transformation but also in plant cell wall structure and metabolism.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 RESULTS
 DISCUSSION
 MATERIALS AND METHODS
LITERATURE CITED
 

Plant Lines and Growth Conditions

Seeds of Arabidopsis ecotype Ws or the derived rat4 mutant were surface sterilized with a solution composed of 50% (v/v) commercial bleach and 0.1% (w/v) SDS for 10 min, then rinsed five times with sterile distilled water. Seeds were placed on Gamborg's B5 medium, pH 5.7 (Gibco-BRL, Gaithersburg, MD) solidified with 0.75% (w/v) Bactoagar (Difco, Detroit). Seeds were cold treated at 4°C for 48 h in darkness, then germinated. Plants were grown in an environmental chamber at 23°C and 70% relative humidity with a 16-h-light/8-h-dark cycle. For plants grown in potting mixtures, Metromix 360 (Scotts, Marysville, OH) was used.


Nucleic Acid Manipulations

Bacterial artificial chromosome DNA, plasmid DNA, and plant genomic DNA was isolated according to Reiter et al. (1992Go). Restriction enzyme digestion (New England Biolabs, Beverly, MA), ligation of the resulting fragments, and electroporation into Escherichia coli cells (DH10B, Stratagene, La Jolla, CA) were conducted as described (Sambrook et al., 1989Go). The wild-type CSLA9 gene-containing P1 clone, MED24, was ordered from The Arabidopsis Biological Resource Center (Ohio State University, Columbus).


Growth of Agrobacterium tumefaciens and Inoculation of Arabidopsis Root Segments

We performed root segment inoculation assays as described (Nam et al., 1999Go). All A. tumefaciens strains were cultured in liquid yeast extract, peptone medium (Lichtenstein and Draper, 1986Go) containing the appropriate antibiotics (10 µg mL–1 rifampicin and 25 µg mL–1 kanamycin). Crown gall tumorigenesis assays were conducted using A. tumefaciens A208 (Sciaky et al., 1978Go), which incites large, green teratomas on the roots of Arabidopsis ecotype Ws. A. tumefaciens At872 (Nam et al., 1999Go), containing a plant-active bar gene in the binary vector pCAS1, was used to incite phosphinothricin-resistant calli. A. tumefaciens At849 (Nam et al., 1999Go) contains the binary vector pBISN1 (Narasimhulu et al., 1996Go). pBISN1 contains a plant-active nptII gene and a gusA-intron gene under the control of a "super-promoter" (Ni et al., 1995Go). An intron in the gusA gene prevents expression of GUS activity in bacteria (Liu et al., 1992Go).

To prepare root segments for in vitro infection, individual 7- to 10 d-old seedlings grown on a plate containing Gamborg's B5 medium (Caisson Laboratories, Rexburg, ID) with the appropriate antibiotics were transferred to baby food jars containing solidified B5 medium lacking a selective agent, and the plants were grown for an additional 7 to 10 d. Roots were axenically cut into 3- to 5-mm segments, and bundles of roots from an individual plant were inoculated with A. tumefaciens A208 on a plate containing one-half-strength Murashige and Skoog salts (pH 5.7; Caisson Laboratories). After 2 d of cocultivation at 24°C, the root bundles were separated into individual segments and transferred to solidified Murashige and Skoog medium lacking phytohormones but containing 100 µg mL–1 timentin to kill the bacteria. The plates were incubated at 24°C for 4 to 6 weeks to allow crown gall tumor formation. For GUS activity assays, root segments were placed on solidified callus-inducing medium for 4 to 6 d, after which they were stained with 5-bromo-4-chloro-3-indolyl glucuronide (Jefferson et al., 1987Go).


Attachment of A. tumefaciens C58 to the Roots of rat4 Plants

Roots of axenically grown plants were cut in water, and two to five segments were suspended in 0.2 to 2 mL of the indicated medium in an Eppendorf tube in a small petri dish. Bacteria were grown overnight in Luria broth and diluted in phosphate-buffered saline. For microscopic assays, bacteria were added to cut roots in the indicated medium at a final concentration of 107 bacteria mL–1 and incubated overnight, after which the roots were viewed under a microscope equipped with Nomarski optics. For viable cell count assays with an excess of roots, the bacteria were added to a final concentration of 1 to 5 x 103 bacteria mL–1. Numbers of free and bound bacteria were determined at 0 time and after 120 min of incubation as previously described (Matthysse, 1987Go).


Attachment of Diarrheagenic E. coli DEC 4A to the Roots of rat4 Plants

E. coli DEC 4A was obtained from the DEC collection of Professor T. Whitam (Michigan State University, East Lansing). The plasmid pBCgfp (Matthysse et al., 1996aGo) was introduced into this strain by transformation. Bacteria were grown overnight in Luria broth containing 30 mg L–1 chloramphenicol and diluted in sterile water. Bacteria were added to seedlings in 2 mL of water to a final concentration of 5 x 103 bacteria mL–1. The plants were incubated at room temperature for 3 d, and iospropylthio-{beta}-galactoside was added to induce the expression of the gfp gene carried on the plasmid. Plants were scored the next day using a fluorescence microscope.


Construction of the Binary Vectors pRAT4C, pRAT4CC, and pRAT4CCC for Genetic Complementation of the rat4 Mutant and pAntir4 for Antisense RAT4 Expression

Three different genomic DNA fragments containing the CSLA9 gene were obtained from bacterial artificial chromosome clone (MED24) DNA or plant genomic DNA and cloned into the plant transformation vector pCB302 (Xiang et al., 1999Go) to form pRAT4C, pRAT4CC, and pRAT4CCC. Plasmids were introduced into A. tumefaciens GV3101 (Koncz and Schell, 1986Go) using a Bio-Rad Genepulser (Bio-Rad Laboratories, Hercules, CA). The resulting A. tumefaciens strains were used to transform homozygous rat4 mutant plants using a vacuum infiltration procedure (Bent and Clough, 1998Go). T1 plants were selected on B5 plates containing phosphinothrincin (20 µg mL–1) or hygromycin (20 µg mL–1), then transferred to soil and allowed to self-pollinate.

CSLA9 cDNA was cloned into the pGPTV-hpt-derived vector (Becker et al., 1992Go) pE1562 under the control of a cauliflower mosaic virus 35S promoter in a reverse orientation to generate antisense RAT4 transcripts. The resulting construction was electroporated into A. tumefaciens GV3101 using a Bio-Rad Genepulser. Transgenic T1 plants were selected on B5 medium containing hygromycin (20 µg mL–1). Tumorigenesis assays were conducted using A. tumefaciens At208 on selected T2 lines.


RT-PCR Assays

Seedlings of wild-type Ws, the rat4 mutant, and complemented lines were grown under the same conditions, and tissues were collected 2.5 weeks after germination. Total RNA was isolated by using Trizol reagent as described by Invitrogen (Carlsbad, CA). The quality and concentration of RNA were determined using a UV spectrophotometer (Molecular Devices Corporation, Sunnyvale, CA) and SOFTmax PRO software, and further verified by agarose gel electrophoresis. First strand cDNA was synthesized in a 25-µL reaction that contained 2 µg of total RNA using a Promega (Madison, WI) RT system. The reverse primer r4rt3 (5'-TTCCCTTCAATGGTTAGGCACA-3') complementary to the sequence at the 3' end of RAT4 gene was used for RT. The resulting cDNA (2.0 µL) was amplified by PCR in a 50-µL reaction by TGO polymerase (Roche, Indianapolis) using r4rt3 and the 5' primer r4rt5 (5'-ATGGAGCTAGGAGATACGACGTCGGTG-3'). Primers directed against the Arabidopsis actin gene act2 (5'-ATGGCTGAGGCTGATGATAT-3' and 5'-TGATCTTGAGAGCTTAGAAA-3') were used as an internal control for equal RNA sample loading and RT-PCR.


Promoter-GUS Expression Analysis of the CSLA9 Gene

A 1.9-kb fragment containing RAT4 5' flanking sequence and sequences encoding the first 24-amino acid residues were amplified from genomic DNA using the forward primer 5'-TTCAGCCGTTGAGCAGTCCTTATTTT-3' and the reverse primer 5'GCATTGTTATGTCGTCTCTGTAT-3' and fused in frame to the gusA gene in the T-DNA binary vector pBI101.3 (CLONTECH, Palo Alto, CA) to generate pRAT4GUS. A. tumefaciens GV3101 containing pRAT4GUS was used to transform Arabidopsis ecotype Ws by a flower vacuum infiltration method (Bent and Clough, 1998Go). Transgenic plants were selected on B5 medium containing 50 µg mL–1 kanamycin. T2 generation transgenic plants were assayed for GUS activity at different developmental stages from 5 d to fully mature. Staining for GUS activity was carried out for 6 to 18 h, and the resulting GUS expression patterns were determined using a dissecting microscope.


Growth and Developmental Phenotypes of the rat4 Mutant

Four to six seeds of wild-type Ws and homozygous rat4 plants were placed on a 150-mm-diameter circular plate containing B5 medium and 100 µg mL–1 timentin. The plates were treated at 4°C for 48 h in darkness, then placed vertically in a growth chamber at 23°C under a 16-h-light/8-h-dark growth cycle. The plants were observed, and lateral roots were counted for each plant 2 weeks after germination. The experiment was repeated three times with 100 plants each for wild-type and rat4 mutant plants. We screened for other phenotypes related to growth and development in rat4 and wild-type plants grown side-by-side in 3-inch square pots. The leaf number, plant height, flowering time, and silique number were scored at different time points.


Purification of Cell Walls

Frozen plant materials were homogenized in 1% (w/v) SDS in 50 mM Tris[HCl] (pH 7.2) in a motorized Duall sintered glass tissue grinder (Thomas Scientific, Swedesboro, NJ) and incubated at 65°C for 30 min to extract protein. Cell walls were collected on a disc of nylon mesh (47-µm square pores, Nitex, Tetko, Briarcliff Manor, NY), washed sequentially with water, ethanol, acetone, and water, and suspended as a slurry in water. Purified wall material was either analyzed directly or stored at 4°C.


Carbohydrate and Linkage Analysis

Uronic acid residues in the purified cell walls were reduced with sodium borodeuteride to their corresponding 6,6-dideutero neutral sugars after activation with a water-soluble diimide (Carpita and McCann, 1996Go). The cell wall materials were subjected to monosaccharide and linkage analysis. The cell walls were hydrolyzed with 2 M trifluoroacetic acid (TFA) containing 1 µmol myo-inositol (internal standard) for 90 min at 120°C. The TFA was evaporated under a stream of nitrogen, and the sugars were converted to alditol acetates (Gibeaut and Carpita, 1991Go). The alditol acetates were separated by gas liquid chromatography on a 0.25-mm x 30-m vitreous silica capillary column of SP-2330 (Supelco, Bellefonte, PA). Temperature was programmed from an initial 80°C hold for 1 min during injection to 170°C at 25°C min–1 and then to 240°C at 5°C min–1 with a 6-min hold at the upper temperature.

The samples were per-O-methylated with Li+ methylsulfinylmethanide and methyl iodide according to Gibeaut and Carpita (1991Go). The per-O-methylated polymers were recovered after addition of water to the mixture and partitioning into chloroform. The chloroform extracts were washed five times with at least 3-fold excess of water, and the chloroform was evaporated under a stream of nitrogen. The methylated polymers purified by chloroform partitioning were hydrolyzed in 2 M TFA for 90 min at 120°C. The sugars were then reduced with NaBD4 and acetylated. The partly methylated alditol acetates were separated by gas liquid chromatography on a 0.25-mm x 30-m vitreous silica capillary column of SP-2330 (Supelco). Temperature was programmed from an initial 80°C hold for 1 min during injection, to 160°C at 25°C min–1, then to 210°C at 2°C min–1, and then to 240°C at 5°C min–1, with a 6-min hold at the upper temperature. All linkages were deduced by a combination of retention time and electron-impact mass spectrometry of the derivatives (Carpita and Shea, 1989Go).


   ACKNOWLEDGMENTS
 
The authors would like to thank Ms. Jacqueline Johnson for technical assistance with the monosaccharide and linkage analyses. We thank Nicholas Cole Wilson for assistance with some of the assays of bacterial binding.

Received July 24, 2003; returned for revision September 1, 2003; accepted September 1, 2003.


   FOOTNOTES
 
Article, publication date, and citation information can be found at http://www.plantphysiol.org/cgi/doi/10.1104/pp.103.030726.

1 This work was supported by the National Science Foundation (Plant Genome grant nos. 99–75715 to S.B.G. and DBI–0217552 to N.C.C.) and by the Department of Energy (grant no. DE–FG02–00ER15074 to A.G.M.). This is paper no. 17,208 of the Purdue Agricultural Experiment Station. Back

2 Present address: Faculty of Natural Resources and Life Sciences, Dong-A University, Pusan 604–714, South Korea. Back

* Corresponding author; e-mail gelvin{at}bilbo.bio.purdue.edu; fax 765–496–1496.


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TOP
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 DISCUSSION
 MATERIALS AND METHODS
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