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Plant Physiology 141:1-2 (2006) © 2006 American Society of Plant Biologists
On the InsideXanthan Prevents Callose Block to Black Rot DiseaseThe phytopathogenic bacterium Xanthomonas campestris pv campestris (Xcc) is the causal agent of black rot disease in crucifers. Xcc produces xanthan, an extracellular polysaccharide that is essential for pathogenesis, particularly during the early stages of Xcc infection in leaf mesophyll tissue. Other than its appearance during and necessity for bacterial pathogenesis, little is known about the precise functional role of xanthan in Xanthomonas pathogenesis. Yun et al. (pp. 178–187) show that, in contrast to wild-type Xcc, two mutant strains, one lacking xanthan and the other producing a truncated form of xanthan, fail to cause disease in either Nicotiana benthamiana or Arabidopsis plants. Unlike the wild-type Xcc strain, both of the xanthan-deficient mutant strains induce callose deposition in Nicotiana and Arabidopsis plants. Callose deposition appears to be a key factor in plant resistance to Xcc infection. Treatment with an inhibitor of callose deposition prior to infection induces susceptibility to the two xanthan-deficient mutant strains. Moreover, treatment with xanthan but not truncated xanthan suppresses the accumulation of callose and enhances the susceptibility of both Nicotiana and Arabidopsis plants to infection by the two mutant strains. Thus, xanthan's suppressive effect on callose deposition is critical for Xanthomonas infectivity. Calcium Oxalate Crystals Defend against Chewing Insects Because of their known qualities as irritants to humans and their formidable needle-like appearance, raphide crystals of calcium oxalate in plants have long been suggested to be physical deterrents to herbivore feeding. In the barrel medic Medicago truncatula, prismatic calcium oxalate crystals accumulate predominantly in a sheath surrounding secondary veins of leaves. Korth et al. (pp. 188–195) have employed mutants of M. truncatula with decreased levels of calcium oxalate crystals to assess the defensive role of this mineral against insects. They report that caterpillar larvae of the beet armyworm Spodoptera exigua show a clear feeding preference for tissues from mutants with reduced levels of calcium oxalate crystals as compared to wild-type plants. Larvae that fed upon wild-type plants suffer reduced growth and increased mortality compared with those that feed on the mutant plants. The induction of wound-responsive genes is normal in the crystal-poor mutants, indicating that these lines are not deficient in induced insect defenses. Electron micrographs of insect mouthparts indicate that the prismatic crystals in M. truncatula leaves act as physical abrasives during feeding. Food utilization measurements show that calcium oxalate also interferes with the conversion of plant material into insect biomass during digestion. In marked contrast to their detrimental effects on a chewing insect species, calcium oxalate crystals have no effect on the performance of the pea aphid Acyrthosiphon pisum, a sap-feeding insect with piercing-sucking mouthparts. The results confirm a long-held hypothesis for the defensive function of these crystals, and point to the potential value of understanding the genes involved in controlling crystal formation and localization in crop plants. Transcriptome of Desmid Sexual Reproduction Charophycean green algae and land plants share many distinctive characteristics with respect to cellular structures and metabolism. The unicellular desmid Closterium peracerosum-strigosum-littorale complex is the closest unicellular relative of land plants and also is the best-characterized charophycean alga with respect to the process of sexual reproduction (Fig. 1 ). Heterothallic strains of Closterium have two morphologically indistinguishable sexes: mating-type plus (mt+) and mating-type minus (mt–). Sexual reproduction is easily induced when cells of these two sexes are cultured together in nitrogen-depleted medium under light. To elucidate the molecular mechanism of intercellular communication during sexual reproduction, Sekimoto et al. (pp. 271–279) classified 3,236 expressed sequence tags into 1,615 nonredundant groups and generated a Closterium cDNA microarray. Candidate genes for key factors involved in fertilization, such as those that encode putative receptor-like protein kinase, Leu-rich-repeat receptor-like protein, and sex pheromone homologs, were up-regulated during sexual reproduction and/or by the addition of the purified sex pheromones. There also were mating strain-specific differences in the expression of genes coding for aquaporin-related proteins. This first transcriptome profile of Closterium may provide important clues as to the mechanism and evolution of intercellular communication between the egg and sperm cells of land plants. The Closterium microarray resource is also a potentially useful tool for the exploration of genes that are regulated in response to environmental changes. One can easily monitor gene expression changes caused by environmental modifications without influences from other tissues and organs since Closterium is a unicellular plant.
Calmodulin-Like Proteins in the Symbiosome Space of Root Nodules Signaling between rhizobia and legumes initiates the development of root nodules. During this process, the bacteria are endocytosed by the plant and become surrounded by a plant membrane, thereby forming a symbiosome. Between this membrane and the encased bacteria, there exists a matrix-filled space (the symbiosome space) that is thought to contain a mixture of plant- and bacteria-derived proteins. Changes in intracellular Ca2+ and signaling via Ca2+ are well-documented features of legume-rhizobia interactions and root nodule development. Liu et al. (pp. 167–177) report upon the occurrence in the model legume M. truncatula of a novel family of six calmodulin (CaM)-like proteins (CaMLs) that are expressed specifically in root nodules and that are localized within the symbiosome space. All six nodule-specific CaML genes are clustered in the M. truncatula genome along with two other nodule-specific genes, nodulin-22 and nodulin-25. Sequence comparisons and phylogenetic analysis suggest that an unequal recombination event occurred between nodulin-25 and a nearby CaM, giving rise to the first CaML, and the gene family evolved by tandem duplication and divergence. Thus, an ancestral CaM gene appears to have been co-opted and recruited for root nodule symbiosis. Although the specific functions of nodule-specific CaMLs are not yet known, based upon their location in the symbiosome space and the fact that Ca2+ flux affects anion channel gating, the authors believe they are integrally related to symbiosome function. Abscisic Acid Utilizes Tissue-Specific G-Protein Pathways
G proteins are involved in a multitude of plant processes, including cell division, ion channel regulation, seed germination, biotic and abiotic stress responses, and blue-light-mediated responses. Abscisic acid (ABA) signaling in both seeds and guard cells also involves components of the heterotrimeric G-protein complex. To assess the roles of the Arabidopsis (Arabidopsis thaliana) G Transcriptome Analysis of Cold Acclimation in Chloroplast Mutants Cold acclimation is a complex process characterized by the coordinated regulation of hundreds of genes. The signal transduction pathways leading to the expression of cold-regulated genes involve a regulatory network where only a few regulatory genes control many genes in the cold response. The C-repeat binding factor (Cbf) genes, regulators of the cold-induced transcriptional cascade, have been shown to increase freezing tolerance when overexpressed in transgenic plants. The ability of plants to develop a frost-resistant phenotype, however, also is affected by the presence of light and photosynthetic activity during cold acclimation. An increased PSII excitation pressure (the relative reduction state of Qa, the first stable electron acceptor of PSII) is one of the primary stimuli promoting expression of cold-regulated genes. Consequently, exposure to cold in the absence of light reduces the induction of several cold-regulated genes. Previously, it has been shown that barley (Hordeum vulgare) plants carrying a mutation preventing chloroplast development are impaired in the expression of several cold-regulated genes and completely frost susceptible. The recent commercial availability of a barley microarray provides a powerful new tool for studying the role of the chloroplast in cold acclimation. Svensson et al. (pp. 257–270) investigated four chloroplast barley mutants and the corresponding wild type using a barley DNA microarray to assess the effect of the chloroplast on the expression of cold-regulated genes. Their results highlight the major role of the chloroplast in the molecular adaptation to cold. Their description of the cold response in wild-type barley and in four independent chloroplast mutants allowed the identification of three main pathways containing more than 80% of the wild-type cold-regulated genes: (1) cold-regulated genes unaffected by any mutations, including Cbf genes and many genes known to be under Cbf control; (2) cold-regulated genes constitutively induced, although to different levels, in all mutants, including those activated in response to photooxidative stress; and (3) cold-regulated genes belonging to a signaling pathway(s) disrupted in all mutants, whose expression consequently was not, or was only marginally responsive to cold. Since only a minor portion of cold-regulated genes belongs to the same regulatory pathway as Cbf, the authors conclude that other factors deriving from the chloroplast in addition to Cbf also are required to promote the full suite of molecular changes associated with cold acclimation.
Department of Natural Sciences, Mercy College, Dobbs Ferry, New York 10522 FOOTNOTES www.plantphysiol.org/cgi/doi/10.1104/pp.104.900190. Related articles in Plant Physiol.:
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subunit (GPA1), the G
subunit (AGB1), and a candidate G-protein-coupled receptor (GCR1) in ABA signaling during germination and early seedling development, Pandey et al. (pp.