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Plant Physiology 148:1185-1186 (2008) © 2008 American Society of Plant Biologists On the InsideMalic Acid Recruits Beneficial Soil Bacteria Mutualistic associations between roots and plant growth-promoting rhizobacteria (PGPR) help protect plants from soil-borne pathogens. Symbioses with PGPR can also trigger induced systemic resistance in the aerial portions of plants. Despite the importance of PGPR associations in plant defense, little is known about the genetic and biochemical changes responsible for the attraction of PGPR to the rhizosphere. In this issue, Rudrappa et al. (pp. 1547–1556) present evidence that L-malic acid secreted from the roots of Arabidopsis (Arabidopsis thaliana) selectively signals and recruits the beneficial rhizobacterium Bacillus subtilis FB17 in a dose-dependent manner. Elevated levels of L-malic acid also promote the formation of FB17 biofilm on Arabidopsis roots. Underscoring the breadth and sophistication of plant-microbial interactions, it also appears that plants can recruit PGPR in response to foliar infections. For example, root secretions of L-malic acid are induced by the foliar pathogen Pseudomonas syringae. Thus, L-malic acid is involved in the recruitment of beneficial microbes to the rhizosphere.
The shoots of shorter plants within dense stands often undergo rapid shoot elongation. This shade avoidance response improves the chances that these plants will gain access to sufficient sunlight for survival. Shading influences plant growth because it changes the spectral composition of the light impacting these plants. In shaded areas, the preferential absorption of red (R) wavelengths by surrounding vegetation causes the reflected/transmitted light to be enriched in far-red (FR) wavelengths and thus the ratio of R/FR light is low. Within very dense canopies, there are further spectral changes involving not just a lowering of R/FR, but also a reduction of blue light and the total light intensity. Two ecotypes of Stellaria longipes, the alpine and the prairie, are of particular interest in the context of shade avoidance. The dwarf alpine ecotype inhabits an area of sparse vegetation. In contrast, the prairie ecotype grows amongst dense vegetation, where it is more subject to being shaded by surrounding plants. Sasidharan et al. (pp. 1557–1569) have demonstrated that although the alpine Stellaria ecotype is nonresponsive to low R/FR (moderate shading), both ecotypes rapidly elongate under deep shade treatments (green shade and low blue light conditions). Of course, shoot elongation depends upon cell wall loosening, so it is of interest to consider which cell wall-loosening enzymes are involved in shade avoidance responses. The authors report that xyloglucan endotransglucosylase/hydrolase activity was strongly regulated by green shade and low blue light conditions, but not by low R/FR. Expansin activity, on the other hand, was correlated with growth responses to all shading regimens employed, and expansin genes cloned from the internodes of the two ecotypes showed differential regulation in response to the light manipulations. These results indicate that elongation responses to shade involve changes in cell wall extensibility via the transcriptional control of cell wall-loosening enzymes.
Trichomes, in their various forms, serve many functions in plants, including protecting plants from insects, reducing water loss through stomata, reflecting excess light, and synthesizing and storing many secondary compounds. One type of trichome in particular, the fibrous trichomes on cotton (Gossypium hirsutum) seed, is of major importance to the textile industry. Thus, there are many reasons, both basic and applied, why the study of trichomes is of interest. The trichomes of Arabidopsis have been extensively used as a model to address general questions in cell and developmental biology. In this issue, Jakoby et al. (pp. 1583–1602) present gene expression analyses of Arabidopsis trichomes. They have identified 3,231 genes that are up-regulated in mature trichomes relative to leaves without trichomes. They report that cell wall-related transcripts are particularly overrepresented in trichomes. In addition, trichome expression maps revealed high activities of anthocyanin, flavonoid, and glucosinolate pathways, indicative of the roles of trichomes in the biosynthesis of secondary compounds and defense. Arabidopsis trichomes also share many expressed genes with cotton fibers, making them an attractive model to study industrially important fibers. One of the genes that are differentially expressed in cotton fibers is the MYB transcription factor GhMYB25. A combination of transcript profiling and map-based cloning revealed that the NOECK gene of Arabidopsis encodes AtMYB106, a homolog of cotton GhMYB25, and that it acts as a repressor of cell outgrowth in Arabidopsis.
Although mitochondria are often portrayed as static, rod-shaped organelles, studies of living cells have demonstrated that they are among the most dynamic of organelles in terms of form and distribution. Changes in their architecture and their ability to translocate rapidly throughout the cytoplasm appear to be of critical importance for executing their cellular functions. In addition, mitochondria constantly undergo fission, fusion, and branching changes. All of these changes can occur within minutes. Plant mitochondria also undergo a great deal of DNA recombination. This implies that before cytokinesis, the different mitochondrial compartments must fuse to allow for mitochondrial DNA (mtDNA) intermixing. When and how the conditions for mtDNA intermixing are established are largely unknown. The "discontinuous whole" hypothesis postulates that the individual, small mitochondria must transiently fuse to transfer their mtDNA into a common physical space. Seguí-Simarro et al. (pp. 1380–1393) have investigated the cell cycle-dependent changes in mitochondrial architecture in different Arabidopsis cell types using confocal microscopy, conventional, and three-dimensional electron microscopy techniques. Whereas the mitochondria of cells from most plant organs are typically small and dispersed, shoot apical and leaf primordial meristematic cells contain small, discrete mitochondria in the cell periphery and one large, mitochondrial mass in the perinuclear region. During mitosis, approximately 60% of the smaller mitochondria fuse with the large mitochondrion, whose volume increases to 80% of the total mitochondrial volume, and reorganizes into a cage-like structure encompassing first the mitotic spindle and then the entire cytokinetic apparatus. During cytokinesis, the cage-like mitochondrion divides into two independent tentacle-like mitochondria from which new, small mitochondria arise by fission. These cell cycle-dependent changes in mitochondrial architecture shed light on how meristematic cells may achieve such a high rate of mtDNA recombination and ensure the even partitioning of mitochondria between daughter cells.
Many dynamic cellular events occur during pollen germination and pollen tube growth, including calcium oscillations, vesicle transport, ion fluxes, cell wall biosynthesis, and cytoskeletal changes. This raises the question of whether transcriptional changes also occur during pollen germination and tube growth. Although it was previously proposed that the mature pollen grains of some plant species may already contain most of the transcripts needed for germination and tube growth, pollen germination and growth in other species is blocked by inhibitors of transcription. Wang et al. (pp. 1201–1211) have examined the changes in the Arabidopsis pollen transcriptome during germination and tube growth. They report that the number of expressed genes increased significantly from desiccated mature pollen to hydrated pollen and again to growing pollen tubes—findings consistent with the observation that pollen germination and tube growth are significantly inhibited in vitro by transcriptional inhibition. Overall, transcription is increased during pollen germination and tube growth both in terms of the total number of transcribed genes and the transcriptional levels of some genes. A number of genes were significantly up-regulated, and, more importantly, some new groups of genes were preferentially transcribed during pollen germination and/or pollen tube growth. A gene ontology analysis of these up-regulated genes revealed an emphasis toward cell rescue, transcription, signal transduction, and cellular transport during pollen germination and tube growth. Most of the functional categories correlate well with the known cellular dynamic activities in germinating pollen and/or growing pollen tubes. Members of the CaM/CML (calmodulin/calmodulin-like proteins), CHX (cation/hydrogen exchanger), and Hsp (heat shock protein) gene families, in particular, were markedly up-regulated.
Synthesis and degradation of the
Division of Health Professions and Natural Sciences FOOTNOTES www.plantphysiol.org/cgi/doi/10.1104/pp.104.900275 Related articles in Plant Physiol.:
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-glucan polymers that compose starch granules are of great importance in the physiology of plants. The essential catalytic mechanism for 
6) glycoside bonds. Three of these enzymes are similar to the prokaryotic isoamylases and one is closely related to the prokaryotic pullulanases. Collectively, this group of plant enzymes is referred to as starch debranching enzymes (DBEs), and the members are further classified as three isoamylase-type DBEs (ISA1, ISA2, and ISA3) and one pullulanase-type DBE (PU1). Amylopectin catabolism, in particular, is influenced by the activity of DBEs because in this polymer about 5% to 6% of the glycoside bonds are in the "branched"