- © 2004 American Society of Plant Biologists
Green Algae Produce Chemical Mimics of Bacterial Quorum Sensing Signals
In the 1late 1960s, it was observed that cultures of the marine bioluminescent bacteria Vibrio fischeri produced light only when large numbers of bacteria were present. It was later shown that the luminescence was initiated not by the removal of an inhibitor but by the accumulation of an activator molecule or “autoinducer.” This molecule is made by the bacteria and activates luminescence when it has accumulated to a high enough concentration. The bacteria are able to sense their cell density by monitoring the autoinducer concentration. This mechanism of cell density sensing was termed quorum sensing. N-acyl homo-Ser lactones (AHLs) are the best studied of the known bacterial quorum sensing signals and have been detected in over 50 species of bacteria. In this issue, Teplitski et al. (pp. 137–146) report that the unicellular soilfreshwater alga Chlamydomonas reinhardtii secretes a variety of substances that mimic the activity of AHL signal molecules. Colonies of Chlamydomonas and Chlorella spp. stimulated quorum sensing-dependent luminescence in Vibrio harveyi, indicating that these algae may produce compounds that affect the quorum sensing system in Vibrio species. Treatment of the soil bacterium Sinorhizobium meliloti with a partially purified AHL from Chlamydomonas affected the accumulation of 16 of the 25 proteins that were altered in response to the bacterium's own AHL signals. Peptide mass finger-printing identified 32 proteins affected by the bacterium's AHLs or the purified algal mimic. The algal mimic was able to cancel the stimulatory effects of bacterial AHLs on the accumulation of seven of these proteins, suggesting that the secretion of AHL mimics by the alga could be effective in disrupting the quorum sensing of soil bacteria that the algae may encounter in nature, possibly inhibiting colonization or infection of the algae by the bacteria.
Rhamnogalacturonan II in Cell Walls of Lower Plants
Although many of the morphological and biochemical changes that allowed plants to adapt to life on land have been documented, there is only limited information available on the composition and architecture of non-flowering plant cell walls. Unfortunately, this gap in our knowledge makes it difficult to understand the evolutionary origins of cell walls and the changes in wall structure that occurred during the evolution of land plants. In this issue, Matsunaga et al. (pp. 339–351) report on their survey of lower plants for the presence of the cell wall pectic polysaccharide rhamnogalacturonan II (RG-II). Borate ester cross-linking of the cell wall pectic polysaccharide rhamnogalacturonan II (RG-II) is required for the growth and development of angiosperms and gymnosperms. The authors report that the amounts of borate cross-linked RG-II present in the sporophyte primary walls of members of the most primitive extant vascular plant groups (Lycopsida, Filicopsida, Equisetopsida, and Psilopsida) are comparable with the amounts of RG-II in the primary walls of angiosperms. In marked contrast, the primary cell walls of the dominant gametophyte generation of members of the Bryopsida, Hepaticopsida, and Anthocerotopsida have only about 1% of the amounts of RG-II present in angiosperm walls. Their data indicate that the amount of RG-II incorporated into the walls of plants increased during the evolution of vascular plants from their bryophyte-like ancestors. Thus, the acquisition of a boron-dependent growth habit may be correlated with the ability of vascular plants to maintain upright growth and to form lignified secondary walls. The conserved structures of pteridophyte, lycophyte, and angiosperm RG-IIs suggests that the genes and proteins responsible for the biosynthesis of this polysaccharide appeared early in land plant evolution and that RG-II has a fundamental role in wall structure.
Legume Porins
Porins are a diverse group of β-barrel proteins that are located in the outer membranes of bacteria and mitochondria. Voltage-dependent anion channels (VDACs) constitute one of the major families of porins and serve, in mitochondria, as a main pathway for metabolite transport across the outer membrane. Recent evidence has suggested that VDAC porins may not be confined solely to the outer membranes of mitochondria in animal cells. In murine neurons, for example, VDACs are also located in special domains of the plasma membrane called caveolae that form a unique endocytotic and exocytotic compartment at the cell surface. Moreover, proteomic studies of isolated symbiosome membranes from legume nodules have indicated that VDACs might also be involved in the transport of nutrients between plants and rhizobia. Given the transport properties of VDAC porins and the strategic role of the symbiosome membrane in controlling nutrient exchange between the plant and nitrogen-fixing bacteroids in legume root nodules, Wandrey et al. (pp. 182–193) sought to verify these results by immunolocalizing VDAC proteins in root nodule cells. Contrary to expectations, their studies provide no evidence for the occurrence of VDACs on the symbiosome membranes of Lotus japonicus or Glycine max root nodules. They did find VDACs associated with mitochondria and small, unidentified vesicles at the cell periphery. They also characterized five VDAC porins from the model legume, L. japonicus, four of which are functional at the outer mitochondrial membrane when expressed in yeast (Saccharomyces cerevisiae). Although these results cast doubt on a role for VDACs in symbiosome function, they do indicate that VDACs may play more diverse roles in plants than previously suspected.
Arabidopsis Gene Expression at Low Atmospheric Pressure
Plants will be integral components of any advanced life support systems designed for long-term space travel or colonization. Currently, there are severe limitations to producing Earth-orbital, lunar, or Martian plant growth facilities that contain Earth-normal atmospheric pressures within light, transparent structures. However, some of these engineering difficulties can be avoided by growing plants in reduced atmospheric pressures. As a step toward understanding how plants adapt to low atmospheric pressures, Paul et al. (pp. 215–223) have identified genes central to the initial response of Arabidopsis to hypobaric conditions. Exposure of plants to an atmosphere of 10 kPa compared with the sea-level pressure of 101 kPa resulted in the differential expression of more than 200 genes between the two treatments. Less than one-half of the genes induced by hypobaria are similarly affected by hypoxia, suggesting that response to hypobaria is unique and is more complex than an adaptation to the reduced partial pressure of oxygen inherent to hypobaric environments. In addition, the suites of genes induced by hypobaria confirm that water movement is a paramount issue at low atmospheric pressures, because many of gene products intersect abscisic acid-related, drought-induced pathways. A curious finding was that although hypobaria switched on drought stress pathways, Arabidopsis plants under hypobaric conditions exhibited no signs of wilting. This observation begs the question as to whether successful adaptation to a 10-kPa environment actually requires the activation of desiccation-related pathways. Conceivably, if the desiccation response is not necessary for adaptation to low pressure, then the induction of these pathways could represent a drain on metabolism with a resultant cost to production.
Plasticity of Root Hair Development in Arabidopsis
The formation of extra root hairs in response to sub-optimal availability of Fe or P is a well-documented example of developmental plasticity aiding in the uptake of limiting nutrients from the soil. Müller and Schmidt (pp. 409–419) have extended these findings by investigating the plasticity of root hair patterning in 19 Arabidopsis mutants carrying lesions in various parts of the root hair developmental pathway. An increase in root hair density in P-deficient plants was achieved mainly by the formation of extra hairs in positions normally occupied by non-hair cells. P-deficient conditions also evoked a significant increase in root hair length, which is likely to improve P acquisition. Roots of Fedeficient plants were characterized by a high percentage of extra hairs with bifurcated tips. The data presented suggests that there are separate developmental pathways induced in Fe- and P-deficient plants. Divergence in root hair patterning was most pronounced among mutants with defects in genes that affect the first stages of differentiation, suggesting that nutritional signals are perceived at an early stage of epidermal cell development.
Arabidopsis Root Transcriptome
Fizames et al. (pp. 67–80) performed a large-scale identification of genes expressed in roots of the model plant Arabidopsis by Serial Analysis of Gene Expression (SAGE). For tag-to-gene assignment, they developed a computational approach based on 26,620 genes annotated from the complete sequence of the genome. This new resource allowed them to characterize the expression of more than 3,000 genes, for which there is no EST or cDNA in the databases. They report the identification of 270 genes differentially expressed between roots of plants grown either with NO3- or NH4NO3 as N source. Among the various genes differentially expressed between NO3- and NH4NO3 libraries, several are known to be affected by the nature of the N source. For instance, NO3- uptake and assimilation is known to be markedly repressed in the presence of NH4+, and the observation that both nitrate reductase and the high-affinity nitrate transporter NRT2.1 were down-regulated at the transcript level in NH4NO3-grown plants confirms previous studies. Also, genes encoding enzymes involved in metabolism of carboxylic acids such as malate dehydrogenase, malic enzyme or isocitrate dehydrogenase were found to be strongly regulated by nitrogen. This strong stimulation of carboxylic acid synthesis in response to increased N assimilation may be due to the need to create carbon skeletons for the synthesis of amino acids. The fact that several aquaporin genes were up-regulated in NH4NO3-grown plants is a novel finding. It is known that supply of N in the external medium markedly modifies the hydraulic conductivity of the root system. The effect of N source on aquaporin gene expression observed in this study suggests the existence of interactions between N and H2O transport in root cells.
