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Plant Physiology 146:1-2 (2008) © 2008 American Society of Plant Biologists On the InsideThe Role of an Undesirable Seed Protein in Polyamine BiosynthesisPea albumin 2 (PA2) is a legume seed protein that resists digestion by livestock and is a potential human allergen. The biological relevance of PA2 is unclear: It is neither a classic storage protein nor is it degraded upon germination. Vigeolas et al. (pp. 74–82) provide insights into the role of PA2 in planta based on their study of a naturally occurring pea (Pisum sativum) mutant, which they identified through germplasm screening and which lacks this protein. The mutation, which is in a wild uncultivated line, was shown to be due to a deletion of most of the structural genes encoding PA2. Despite having lower albumin levels, the mutant's seeds, which are smaller than normal, have an elevated nitrogen content under a range of growth conditions. Thus, this mutation may potentially have value for crop improvement. Genetic crosses involving the mutant line were established, and metabolomic approaches were employed to identify biochemical changes during seed development that may be linked directly or indirectly to the mutation. The results indicate that metabolic changes occur in the polyamine pathway with altered amounts of spermidine in the PA2-deficient parent and backcross mutant lines. Analysis of enzyme activities in the pathways of polyamine synthesis revealed that the differences in spermidine content were attributable to changes in the overall activities of spermidine synthase and Arg decarboxylase. Thus, the results provide evidence that PA2 plays an important role in regulating polyamine metabolism. Trehalose: A Determinant of Cell Shape?
In plants, most cells are more or less cylindrical. More complex shapes, however, are found and include lobed pavement cells in leaf epidermises and the trichomes of leaves and stems. In this study, Chary et al. (pp. 97–107) isolated cell shape mutants using a screen involving fluorescently labeled vacuolar membrane proteins. Since the vacuole occupies most of the volume of plant cells, the tonoplast marker
Plant Defense against Powdery Mildew Infection Powdery mildew (PM), a disease caused by the fungus Erysiphe necator, infects grape vines and causes significant losses in yield and quality. Most grape cultivars are highly susceptible to E. necator because they are derived from Vitis vinifera, a species that was not exposed to this pathogen during its evolution in the Old World. In contrast, grapevine species such as Vitis aestivalis, which co-evolved with E. necator on the North American continent, possess various levels of resistance to the pathogen. At the microscopic level, the germination rate of E. necator conidiospores does not differ on PM-susceptible and -resistant grapevines. During subsequent stages, however, susceptible and resistant grapevine cultivars differ significantly in their ability to limit the growth of hyphae and restrict the formation and development of PM colonies. To test the hypothesis that there would be gene expression differences between V. aestivalis and V. vinifera when mounting a response to PM, Fung et al. (pp. 236–249) used the Vitis GeneChip to compare PM-responsive gene expression patterns in disease-resistant V. aestivalis and in disease-susceptible V. vinifera. Their hypothesis was dramatically confirmed; they found only three PM-responsive transcripts in V. aestivalis as compared to 625 in V. vinifera. There was a significant increase in the abundance of plant defense-related transcripts in PM-infected V. vinifera, suggesting an induction of the basal defense response. These results suggest that resistance to PM in V. aestivalis is not associated with overall reprogramming of the transcriptome. Conceivably, resistance to PM may be related to the higher levels of endogenous salicylic acid levels that the authors measured in V. aestivalis compared to V. vinifera in the absence of the fungus. Biotin Biosynthesis: A New Type of Gene Clustering Biotin is a vitamin that functions as an enzyme cofactor in cellular metabolism to facilitate CO2 transfer during carboxylation and decarboxylation reactions. The biosynthesis of biotin, first elucidated in bacteria more than 40 years ago, occurs through four reactions. In Escherichia coli, the four genes that encode these enzymes are clustered into an operon. The biosynthesis of biotin in plants occurs through a similar pathway but is divided between two compartments. Two auxotrophic mutants of Arabidopsis have played an important role in the analysis of biotin biosynthesis in plants. The bio1-1 mutant was isolated following a forward genetic screen designed to identify embryo-defective (emb) mutants in which arrested embryos were rescued on an enriched nutrient medium. Embryo rescue experiments and subsequent complementation with an ortholog from E. coli demonstrated that mutant embryos are defective in the second reaction of the biotin biosynthesis pathway, whereas the bio2-1 mutant is disrupted in the final reaction. In this issue, Muralla et al. (pp. 60–73) identify the Arabidopsis gene encoding the third enzyme in the biotin biosynthetic pathway, dethiobiotin synthetase (BIO3). Reverse genetic analyses demonstrated that bio3 insertion mutants have a similar phenotype to the bio1 and bio2 auxotrophs. Unexpectedly, bio3 and bio1 mutants define a single genetic complementation group. Reverse transcription-PCR analyses demonstrated that separate BIO3 and BIO1 transcripts and two different types of chimeric BIO3-BIO1 transcripts are produced. One of the fused transcripts is monocistronic and encodes a bifunctional fusion protein. A splice variant is bicistronic with distinct but overlapping reading frames. Clusters of genes with related metabolic functions are a defining feature of prokaryotic genomes. The eukaryotic orthologs of these genes, on the other hand, tend to be dispersed throughout the genome and do not typically produce a polycistronic transcript. Thus, the BIO1-BIO3 locus provides a fascinating example in Arabidopsis of two adjacent genes involved in sequential reactions of the same pathway that produce a combination of separate and chimeric transcripts. The Pericycle: A Heterogenous Cell Layer The outer tissues of dicotyledonous plant roots—the epidermis, cortex, and endodermis—are clearly organized in distinct concentric layers in contrast to the diarch to polyarch vascular tissues of the central stele. The outermost layer of the stele, the pericycle, has always been regarded, in accordance with the outer tissue layers, as one uniform concentric layer. However, many studies in different species have emphasized the differences between pericycle cells according to their position adjacent to the xylem or the phloem poles. These studies have reported difference in xylem-associated and phloem-associated pericycle cells in terms of cell division competence, cell cycle progression, cell size, cell wall thickening, plasmodesmatal connectivity, microtubular content, and marker gene expression. In this issue, Parizot et al. (pp. 140–148) demonstrate, using cytological approaches, that there are, in fact, two distinct types of pericycle cells. They describe a new enhancer-trap line marker that visualizes this distinction even in the root meristem. A genetic screen resulted in the isolation of mutants perturbed in pericycle differentiation. Detailed phenotypical analyses of two of these mutants combined with observations made in known vascular mutants revealed an intimate correlation between vascular organization, pericycle fate, and lateral root initiation potency, and illustrated the independence of pericycle differentiation and lateral root initiation from protoxylem differentiation. Taken together, the data show that the pericycle is a heterogeneous cell layer with two groups of cells set up in the root meristem by the same genetic pathway controlling the diarch organization of the vasculature. More Evidence for C4 Photosynthesis in Diatoms Accumulating evidence strongly suggests that the CO2-concentrating mechanism (CCM) needed for optimal photosynthesis by diatoms is biochemically similar to the CCM of C4 plants. By means of quantitative-PCR, McGinn and Morel (pp. 300–309) have examined the expression in several diatom species of the two enzymes phosphoenolpyruvate carboxylase (PEPC) and phosphoenolpyruvate carboxykinase (PCKase) thought to be responsible for the carboxylation and decarboxylation of the C4 intermediate. They report the occurrence of a 2- to 4-fold up-regulation of PEPC gene transcripts in Thalassiosira pseudonana cells adapted to low pCO2 but did not detect such regulation in Phaeodactylum tricornutum grown under similar conditions. Transcripts encoding PCKase did not appear to be regulated by pCO2 in either diatom. With the aid of a membrane inlet mass spectrometer system, they also explored the effects of inhibitors that are known to block these enzymes in other organisms have on photosynthetic gas exchange in diatoms. In T. pseudonana and Thalassiosira weissflogii, net CO2 fixation was blocked by DCDP, a specific inhibitor of PEPC. In the presence of 3-mercaptopicolinic acid, a classic inhibitor of PCKase, photosynthetic O2 evolution was almost totally eliminated in three species of diatoms. In contrast, 3-mercaptopicolinic acid had no significant inhibitory effect on photosynthetic O2 evolution or CO2 uptake in several marine chlorophytes tested. These findings support the idea that C4-like CCMs are generally distributed in diatoms.
Department of Natural Sciences FOOTNOTES www.plantphysiol.org/cgi/doi/10.1104/pp.104.900245 Related articles in Plant Physiol.:
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-TIP-GFP effectively delineates cell shape, thereby permitting the rapid identification of mutants. Using this strategy, the authors have identified the cell shape phenotype-1 (csp-1) mutant in Arabidopsis (Arabidopsis thaliana; 
