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Dual Genetic Pathways Control Nodule Number |
In this issue,
Penmetsa et al. (pp. 998-1008) identify a novel
hyper-nodulation mutant in Medicago truncatula, designated sunn (for supernumeric nodules). In marked contrast to the
parental genotype that develops approximately 8 nodules per root in
response to inoculation with Sinorhizobium meliloti, the
sunn mutant develops roughly 70 nodules. In this respect, it
is similar to the previously described ethylene-insensitive mutant
sickle. Penmetsa et al., however, show that these two
mutants are readily distinguishable. First, in contrast to
sickle, in which insensitivity to ethylene is thought to be
causal to the hyper-nodulation phenotype, nodulation in sunn
shows normal sensitivity to ethylene. (Curiously, the root growth of
sunn mutants is insensitive to ethylene, although other aspects of the ethylene triple response are normal). Second, the
anatomy the two mutants differ in the nodulation zone: Successful infection and nodule development in sunn occur predominantly
opposite xylem poles, similar to wild type. In sickle,
however, both infection and nodulation occur randomly throughout the
circumference of the developing root. Third, genetic analysis indicates
that sunn and sickle correspond to separate and
unlinked loci, whereas the sunn/sickle double mutant
exhibits a novel and additive super-nodulation phenotype. Similar to
super-nodulation mutants described in soybean (Glycine
max) and Lotus japonicus, grafting
experiments demonstrate that the sunn phenotype
is determined by the genotype of the shoot, implicating a mobile signal
(auxin?) in conditioning nodule number. The authors propose a model for
the genetic control of nodule number in M. truncatula,
wherein distinct genetic pathways involving sickle and
sunn, respectively, mediate rhizobial infection and nodule organogenesis.
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Single Nucleotide Polymorphisms (SNPs) and Conservation
Biology |
Eurycoma longifolia (Simaroubaceae) is a small
medicinal tree that grows in the forests of Southeast Asia (Fig.
1). Traditionally, it has been used as a
blood coagulant for complications during childbirth, as a treatment for
dysentery, and as an aphrodisiac, among other applications. Extracts
from E. longifolia also contain biologically active
compounds with antiplasmodial activity. Widespread harvesting of
wild-grown trees has led to rapid thinning of natural populations,
causing a potential decrease in genetic diversity among E.
longifolia. Suitable genetic markers would be very useful for
propagation and for breeding programs to support the conservation of
this species. In this issue, Osman et al. (pp. 1294-1301) have identified a series of SNPs within the genomes of
several E. longifolia accessions that may be useful in
reducing the complexity of genome study in this species. Compared with
other genetic markers, SNPs are more abundant in the genome and are
much more stably inherited. Another advantage of SNP-based genotyping
is that SNP detection does not involve gel electrophoresis, which is
relatively slow and labor intensive. Moreover, many different
strategies have been developed for high throughput detection of SNPs.
In the case of E. longifolia, the occurrence of 51 identified SNPs reflects the geographic origins of individual plants
and can distinguish different natural populations. This work
demonstrates the rapid development of molecular genetic markers in
species for which little or no genomic sequence information is
available. The SNP markers that the authors have developed may also
prove useful in identifying genetic fingerprints that correlate with
other properties of E. longifolia accessions, such as
amenability to regeneration via somatic embryogenesis or the presence
of bioactive metabolites.
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Figure 1.
Eurycoma longifolia, a medicinal
tree of Southeast Asia, has been overharvested. Single nucleotide
polymorphisms have proven useful in assessing the genetic diversity of
this threatened species in the wild.
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Biotin Regulation of Gene Expression |
Biotin is a water-soluble vitamin that is synthesized by
plants and required by all living organisms for normal cellular
function and growth. As a coenzyme, biotin plays a critical role in the regulation of a number of enzymes involved in primary metabolism. In
this issue, Che et al. (pp. 1479-1456) demonstrate a new
non-catalytic function for biotin in plants. Specifically, the authors
report that biotin regulates the expression of the subunit genes of
methylcrotonyl-coenzyme A (CoA) carboxylase (MCCase) by mechanisms
independent of its role as a cofactor. The authors took advantage of
the bio1 mutant of Arabidopsis, which is blocked in
the de novo biosynthesis of biotin, to elucidate the role
of biotin in regulating MCCase expression. In response to the
bio1-associated depletion of biotin, the normally
biotinylated A-subunit of methylcrotonyl-CoA carboxylase (MCCase)
accumulates in its inactive apo-form, and both subunits of MCCase
hyperaccumulate. This induction occurs either because the translation
of each subunit mRNA is enhanced or because the turnover of each
subunit protein is reduced. Evidence is also presented that biotin is
required for the two MCCase subunit genes to respond to metabolic
signals. Under environmental conditions that reduce the carbon status
of seedlings (reduced CO2 or darkness), transcription of the MCCase genes is normally induced. However, this
induction in gene transcription fails to occur in seedlings that are
depleted of biotin. These studies indicate that biotin not only
regulates gene expression by modulating transcription (as occurs in
bacteria and animals), but also mediates regulation of gene expression
at the translational and/or posttranslational level.
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O-Glucosylation of cis-Zeatin in Maize (Zea
mays) |
In contrast to trans-zeatin, the major and ubiquitous
cytokinin of higher plants, cis-zeatin has traditionally been
viewed as a rare isomer of low biological activity. Recent studies,
however, have indicated that the cis-isomers are the dominant form of
cytokinins at particular stages of development in certain plant species
and have been shown to be associated with male sterility in another. Such observations raise the question of whether cis-zeatin
is of parallel importance to trans-zeatin. Indeed, a recent report suggesting the existence of a maize (Zea mays) gene
(cisZOG1) encoding an O-glucosyltransferase
specific to cis-zeatin lends strong support to this view.
(O-glucosylation changes trans-zeatin to a storage and
transport form that is resistant to degradation by cytokinin oxidases.)
In this issue, Veach et al. (pp. 1374-1380) describe the
isolation of a second maize cisZOG gene, the differential expression of cisZOG1 and cisZOG2, and
the identification of substantial amounts of cis-zeatin isomers in
maize tissues. The expression of cisZOG1 was high in maize
roots and kernels, whereas cisZOG2 expression was high in
roots but low in kernels. The O-glucosides of cis-isomers
were found in roots, young cobs, and kernels, which is compatible with
the expression of cisZOG1 and cisZOG2 in maize. Comparing the two groups of cytokinins, cis-isomers were more prevalent
in roots, stems, and leaves, whereas trans-isomers were more abundant
in the kernels. O-glucosylation of cis-zeatin appears to be
a natural metabolic process in maize. Whether cis-zeatin serves as a
precursor to the active trans-isomer or has unique physiological
functions remains to be demonstrated.
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Secretion of Secondary Metabolites by ATP-Binding Cassette
Transporters |
Many secondary compounds of economic and medicinal
significance occur in rare and exotic plant species, often at extremely low levels. Therefore, there has been much interest in
using cell cultures from such species to produce secondary metabolites.
One way that the efficiency of such in vitro systems might be improved is by engineering the flux of metabolites through the biosynthetic pathways of interest. A potential pitfall of this strategy, however, is
that the hyperaccumulation of certain secondary metabolites may be
autotoxic to the cell culture itself. In this issue, Goossens et
al. (pp. 1161-1164) examine the possibility of genetically engineering ABC transporters into cultured cells as a way of enhancing the extrusion of potentially toxic secondary metabolites. The ABC
protein family, the molecular biology of which is coincidentally reviewed in this issue by Jasinski, Ducos, Martinoia,
and Boutry (pp. 1169-1177) is a large, ubiquitous, and diverse
group of proteins that transport an impressively wide variety of
substrates across biological membranes via the binding and hydrolysis
of ATP. The substrates that can be transported by ABC transporter proteins include peptides, carbohydrates, lipids, heavy metal chelates,
inorganic acids, steroids, and xenobiotics. ABC transporters are
associated with the acquisition of multiple drug resistance by
pathogenic organisms and with detoxification pathways that deal with
either endogenously synthesized or environmental toxic compounds. With
respect to the secretion of plant secondary metabolites, two
subfamilies of ABC proteins, pleiotropic drug resistance (PDR) and
multidrug resistance-associated proteins (MRP), are of particular interest. To identify yeast (Saccharomyces cerevisiae) ABC
transporters with substrate specificity for tropane alkaloids, Goossens
et al. assessed the sensitivity of different yeast strains deficient in
various species of ABC transporters to poisoning by the tropane alkaloids hyoscyamine and scopolamine. Based on this assay, an ABC
transporter (yeast PDR5) with specificity for tropane
alkaloids was identified. When PDR5 was transgenically
introduced into tobacco (Nicotiana tabacum) cv Bright
Yellow-2 (BY-2) cell lines, the secretion of tropane alkaloids from the
plant cells was increased. Because of their diverse substrate
specificities ABC transporters might eventually be useful in enhancing
the production of a wide variety of secondary metabolites in plant cell cultures.
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