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Plant Physiology 149:1621-1622 (2009) © 2009 American Society of Plant Biologists On the InsideA Chloroplast Translocon Involved in Gravitropism The starch-statolith hypothesis of gravitropism posits that the sedimentation of starch-filled amyloplasts provide directional cues to plant organs. Indeed, starch-deficient pgm1 mutants lack a normal response to gravistimulation, whereas the starch excess mutant sex1 displays an increased sensitivity to gravistimulation. The altered response to gravity1 (arg1) mutant, which is distinct from pgm1, is involved in the early phases of gravity signal transduction. The arg1 mutation affects gravitropism without altering starch accumulation, root growth response to phytohormones or polar auxin transport inhibitors, or phototropism. Stanga et al. (pp. 1896–1905) report that they chemically mutagenized arg1 seeds to identify mutants with enhanced root gravitropic defects. They have successfully identified two recessive mutants, modifier of arg1 (mar1) and mar2, both of which affect proteins in the Translocon of Outer Membrane of Chloroplasts, or TOC, complex. The roots of these mutants grow in random directions only when the arg1 mutation is present. Moreover, the mar1 and mar2 mutations do not affect phototropism or sensitivity to phytohormones. Both mutations affect different components of the TOC complex. mar1 possesses a mutation in the TOC75-III gene; mar2 possesses a mutation in the TOC132 gene. Overexpression of TOC132 rescues the random growth phenotype of mar2 arg1 roots. The root cap amyloplasts of mar2 arg1 mutants appear and behave normally. These data indicate a role for the plastidic TOC complex in gravity signal transduction within the statocytes.
During autophagy, a portion of the cytoplasm, including entire organelles, is engulfed in membrane-bound vesicles and delivered to hydrolytic vacuoles. The salvaged nutrients are either mobilized to different parts of the organism or used in the same cell. Recent studies concerning autophagy in plants have been largely based on the knowledge obtained from studies of more than 30 identified Saccharomyces cerevisiae autophagy mutants. Most of the S. cerevisiae autophagy genes have homologs in Arabidopsis (Arabidopsis thaliana) and other eukaryotes, suggesting that autophagy is a well-conserved process. The Yop1 protein of S. cerevisiae regulates vesicular traffic in stressed cells and interacts with Yip1p to mediate membrane trafficking during autophagy. HVA22, a gene originally cloned from barley, is homologous to yeast Yop1. To obtain greater insight into the role of HVA22 in plants, Chen et al. (pp. 1679–1689) have carried out functional analyses of HVA22 homologs in Arabidopsis. Five HVA22 homologs (AtHVA22a–AtHVA22e) were identified. The expression of these genes (except AtHVA22c) is differentially up-regulated by abscisic acid and environmental stress. The expression levels of AtHVA22 genes vary in different organs; generally, fast-growing organs, such as flowers and inflorescence stems, have higher expression levels than slow-growing organs. Among the five, expression of AtHVA22d is most tightly regulated by abscisic acid in vegetative tissues. AtHVA22d RNA interference Arabidopsis plants produced small siliques with reduced seed yield, apparently due to floral defects. The number of autophagosomes in root tips of RNA interference plants was also increased dramatically, and enhanced autophagy was noted in staminal filament cells. The results are consistent with the hypothesis that HVA22 homologs function as suppressors of autophagy in both plants and yeast.
Bromeliads, aroids, and orchids are three of the very few flowering plant lineages that have been able to successfully colonize epiphytic niches. Epiphytic orchids are a particularly species-rich group of plants, making the Orchidaceae an interesting subject for understanding mechanisms of evolutionary radiation and diversification. About 72% of orchid species are estimated to be epiphytic, with the majority of these being restricted to tropical regions. To elucidate the role of Crassulacean acid metabolism (CAM) and orchid epiphytism in the evolutionary expansion of tropical orchids, Silvera et al. (pp. 1838–1847) determined the whole leaf carbon isotopic composition (
The movement protein (MP) of Tobacco mosaic virus (TMV) is required for the cell-to-cell spread of viral RNA. The MP of TMV localizes to plasmodesmata and modifies the size-exclusion limit of the pores, thereby allowing the symplastic transport of viral particles. Whereas the associations of TMV MP with the endoplasmic reticulum (ER) and microtubules have been intensely investigated, much less is known about the role of actin in the MP-mediated transport of viral RNA. Since the ER is tightly associated with the actin network, microfilaments could conceivably play an important role in supporting the ER-mediated targeting of MP and/or viral RNA to plasmodesmata. Consistent with this hypothesis, the treatment of plants with actin polymerization inhibitors reduces both MP particle trafficking and the efficiency by which MP accumulates in plasmodesmata. Moreover, the inhibition of the actin cytoskeleton for several days by either actin silencing or inhibitor treatment reduced the efficiency of TMV movement. Nicotiana benthamiana plants that are transgenic for the actin-binding protein ABD2:GFP exhibit a dynamic ABD2:GFP-labeled actin cytoskeleton and myosin-dependent Golgi trafficking. These plants also support the movement of TMV. To further clarify the role of the actin cytoskeleton in TMV movement, Hofmann et al. (pp. 1810–1823) treated ABD2:GFP-labeled cells with latrunculin B, a marine sponge toxin that inhibits actin polymerization and disrupts microfilament organization. Their observations demonstrate that TMV cell-to-cell movement can continue in the absence of an intact actin cytoskeleton. These findings are consistent with the proposal that the targeting of MP to plasmodesmata occurs by diffusion in the ER membrane and that the actin cytoskeleton is not required for this mechanism. Of course, these results do not rule out the possibility that ER-associated actin filaments and actin-binding proteins may play a more minor role in controlling the efficiency of TMV movement.
Bamboos, like many invasive grasses, are aggressive competitors that monopolize space by rapid, clonal growth. In an effort to shed light on the physiological mechanisms underlying the rapid spread and success of certain bamboo species in colonizing gaps, Saha et al. (pp. 1992–1999) have gathered comparative data on the water transport properties of Chusquea ramosissima and Merostachys claussenii. These two monocarpic bamboo species, both native to the subtropical Atlantic forests of Argentina, differ in their growth form and exhibit contrasting strategies of water transport. C. ramosissima is the more aggressive of the two, colonizing canopy gaps and land cleared for agriculture, and attaining culm densities up to approximately 25,000 stems ha–1. C. ramosissima also appears to be the best adapted to growing under conditions in which water is plentiful. The maximum xylem hydraulic conductivity of C. ramosissima culms is twice that of M. claussenii. This high xylem hydraulic conductivity, however, comes at a price: The xylem of C. ramosissima cavitates at relatively high water potentials, while M. claussenii is more drought tolerant. Both species exhibited significant loss of hydraulic conductivity during the day, which was reversed by the nocturnal generation of root pressure. The authors propose that a highly vulnerable vasculature, coupled with diurnal root pressure and an allometry that allows substantial leaf area to be supported on relatively slender culms, are key traits contributing to the ecological success of C. ramosissima.
Plant thermotolerance is enhanced following the synthesis of heat shock proteins (HSPs). In eukaryotes, the expression of HSPs is regulated by heat shock transcription factors that become activated and initiate the transcription of HSPs via binding to heat shock promoter elements in the promoter regions of genes encoding HSPs. Although the downstream components of the heat stress response have been studied in detail, the signal transduction pathway preceding these downstream events is not well understood. Heat shock is known to elicit changes in the levels of intracellular Ca2+, and it has been proposed that calmodulin (CaM) is involved in heat shock signal transduction. Previous studies have shown that the levels of CaM mRNA and protein increased during heat shock in wheat (Triticum aestivum). Moreover, CaM antagonists inhibit the DNA-binding activity of maize (Zea mays) heat shock transcription factors during heat shock. More recently, a CaM-binding protein kinase was identified as an important component in the Ca2+-CaM pathway involved in heat shock signal transduction. To investigate the potential regulatory function of CaM in the heat shock signal transduction pathway, Zhang et al. (pp. 1773–1784) obtained T-DNA knockout Arabidopsis mutants for AtCaM2, AtCaM3, and AtCaM4, and tested their respective tolerances to heat. Previously, the authors had demonstrated that of the nine CaM genes in Arabidopsis, only AtCaM3 is rapidly up-regulated following heat shock. In this study, the authors found that there were no differences between the three knockout mutants and wild-type plants under normal conditions. However, only the AtCaM3 knockout mutant showed a clear reduction in thermotolerance after heat treatment. Moreover, the overexpression of AtCaM3 in either the AtCaM3 knockout or wild-type background significantly rescued or increased the thermotolerance. The authors also found that the accumulation of HSPs and other downstream events were down-regulated in the AtCaM3 knockout mutant and up-regulated in AtCaM3-overexpressing transgenic lines. Taken together, these results suggest that endogenous AtCaM3 is a key component in the Ca2+-CaM-mediated heat shock signal transduction pathway in Arabidopsis.
Division of Health Professions and Natural Sciences FOOTNOTES www.plantphysiol.org/cgi/doi/10.1104/pp.109.900288 Related articles in Plant Physiol.:
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13C) of 1,103 Central American species as a rapid screening method to establish whether CO2 assimilation occurs predominantly by CAM or C3 photosynthesis. These data enabled them to create phylogenetic trees to reconstruct ancestral character states. They report that C3 photosynthesis is the ancestral state and that CAM has evolved at least 10 independent times with several reversals. A large CAM radiation event within the Epidendroideae, the most species-rich epiphytic clade of any known plant group, is linked to a Tertiary species radiation that originated 65 million years ago. Strong CAM, present in 10% of species surveyed, is apparently correlated with climatic variables and the evolution of epiphytism in tropical regions. When all environmental variables were taken into account, altitude was the most important predictor of the CAM photosynthetic pathway: CAM was most prevalent at low altitudes.