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Plant Physiology 147:439-440 (2008) © 2008 American Society of Plant Biologists On the InsideSieve Tube Amino Acid CompositionSamples of pure phloem sap from individual sieve tubes (STs) can be obtained by means of aphid stylectomy. This technique has been used in previous studies to measure ST composition, and in particular amino acids, using HPLC for analysis. Limitations in the sensitivity of the HPLC technique, however, meant that analysis could only be performed where enough phloem sap was obtained. Since severed stylets exude at a range of rates, limitations of analytic techniques have previously restricted study to those stylets with a high exudation rate, thus producing larger sample volumes and leading to potentially unrepresentative data in the literature. Gattolin et al. (pp. 912–921) have used a high-sensitivity capillary electrophoresis coupled to a laser-induced fluorescence detection method to quantify 16 amino acids in wheat (Triticum aestivum) ST samples as small as 2 nL collected by severing the stylets of feeding aphids. This technique is sensitive enough to allow for the observation of the full range of variation that exists in an individual ST. The authors report that some amino acid titers are higher than those previously reported. Moreover, certain amino acids showed striking covariations in their respective concentrations across ST samples, and a distinct diurnal variation in the titer of some amino acids was evident. Finally, an apparent relationship between the exudation rate of ST sap and its total amino acid concentration was observed; samples containing higher total amino acid concentrations exuded from the severed stylet bundles more slowly. Responsiveness of Stomata to Red Light: Not All Plants Are the Same The stomata of the maidenhair fern Adiantum capillus-veneris (Fig. 1 ) are found only on the lower surface of leaves, and their guard cells contain densely arranged chloroplasts. Like many leptosporangiate ferns, the stomata of Adiantum lack a blue light-specific opening response but open in response to red light. In theory, red light-induced stomatal opening could be explained by photosynthetic activity or by phytochrome sensitivity or both. Since stomata open in response to a low concentration of ambient CO2 and close in response to elevated levels of CO2, it is possible that a decrease in internal CO2 brought about by mesophyll photosynthesis may induce the stomatal opening. In accord with this idea, red light applied to a large area of Arabidopsis (Arabidopsis thaliana) leaves, including mesophyll cells, triggered stomatal opening, probably due to mesophyll photosynthesis. However, the responses were not found when a localized beam of red light was applied only to individual guard cells, and thus internal CO2 was proposed to have a role as an intermediate in the red light-induced stomatal response. If, however, Adiantum lacks responsiveness to CO2, how could the mechanism of the red-light sensitivity of its stomata be the same as in Arabidopsis? In this issue, Doi and Shimazaki (pp. 922–930) have investigated the stomatal responses of Adiantum to light and CO2 in the dark. They demonstrate that the guard cells are completely insensitive to CO2. Moreover, the stomatal conductance of Adiantum showed much higher light sensitivity when the light was applied to the lower leaf surface, where stomata exist, than when it was applied to the upper surface. This suggests that guard cells likely sensed the light required for stomatal opening. The photosynthetic inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea inhibited the induction of stomatal opening by red light. These results indicate that red light-induced stomatal opening Adiantum is driven by photosynthetic electron transport in guard cell chloroplasts.
Engineering Faster Growth in a Potential Biofuel Species Sengon (Paraserianthes falcataria) is one of the fastest growing trees in the tropics. The sengon tree typically gains 7 m in height per year and reaches a mean height of 25.5 m and a bole diameter of 17 cm after 6 years. As a woody legume, sengon also thrives on marginal land, where it grows symbiotically with N2-fixing Rhizobium and phosphorus-accumulating mycorrhizal fungi. The tree is useful for timber, pulp, and paper, and might potentially be developed into a biofuel in the future. In this issue, Hartati et al. (pp. 552–561) demonstrate the production of transgenic sengon for the first time. The authors succeeded in overexpressing a poplar (Populus alba) cellulase (PaPopCel1) gene in sengon using Agrobacterium. Previous studies have shown that the overexpression of plant cellulase in plants does not lead to a lack of cellulose, but rather modifies the cell walls by trimming off disordered Glc chains from the microfibrils. For example, transgenic poplar overexpressing Arabidopsis cellulase (cel1) had longer internodes and longer fiber cells. Thus, the impetus for the present study was the hope that the overexpression of poplar cellulase in sengon might increase its remarkable growth rate even more. The authors report that the overexpression of PaPopCel1 does, indeed, lead to an increase in the length and width of sengon stems, as well as to larger and slightly greener leaves. Interestingly, the sleep movements of the leaves at dusk were slower than those on the wild-type plants. MicroRNA and Phosphate Uptake The nutrient demand of shoots and the nutrient supply from roots must be balanced and coordinated to maintain normal growth and development of plants. Long-distance signals are required to report the nutrient status in these tissues. The recent identification of a diverse and dynamic population of small RNAs in the phloem sap of many plant species suggested a potential function of these small RNAs as information molecules. In this issue, Lin et al. (pp. 732–746) present more evidence supporting the idea that a small RNA serves as a long-distance signal for the regulation of inorganic phosphate (Pi) homeostasis. Their studies employ a mutant (pho2) that overaccumulates Pi in shoots as a result of enhanced Pi uptake and translocation. Recently, PHO2 was identified as a target gene of microRNA399 (miR399), which encodes a ubiquitin-conjugating E2 enzyme. The expression of miR399 was up-regulated by Pi deprivation in accordance with reduced PHO2 mRNA level. Transgenic plants overexpressing miR399, in which PHO2 is suppressed, display Pi toxicity in the shoots as a consequence of enhanced Pi uptake. In the present study, the authors have performed reciprocal grafting between wild-type and miR399-overexpressing Arabidopsis and tobacco (Nicotiana sativum) plants to address the systemic regulation of PHO2 by miR399. The differential expression of miR399 precursors and mature miR399 in shoots and roots of pho1 mutant plants and in wild-type plants during Pi deficiency provides further evidence that the long-distance movement of miR399 from shoots to regulate the expression of PHO2 in roots. The authors have also identified small interfering RNAs that may provide another means of regulating for PHO expression. Phytochrome's Role in Systemic Acquired Resistance Upon attempted infection by microbial pathogens, plants induce a multitude of defenses against the attacking intruders. A localized contact of leaf tissue with microbes can lead to systemic acquired resistance (SAR), a state of enhanced, broad-spectrum resistance at the whole plant level that protects against subsequent pathogen attack. Plant salicylic acid (SA) levels rise systemically during SAR, and this increase is required for induced expression of SA-dependent pathogenesis-related (PR) genes and systemic enhancement of disease resistance. An appropriate light environment is required for the establishment of a complete set of resistance responses in several plant-pathogen interactions. For example, Arabidopsis plants inoculated in the dark with an avirulent strain of Pseudomonas syringae are not able to substantially accumulate SA and fail to induce expression of the key phenylpropanoid pathway enzyme Phe ammonia lyase. Light not only is required for SA biosynthesis, but also controls SA perception, because treatment of Arabidopsis leaves with exogenous SA in dim light or in the dark results in strongly reduced expression of the SA-induced defense gene PR-1. Both impaired production and perception of SA therefore account for the observation that PR-1 expression in P. syringae-treated Arabidopsis leaves is completely suppressed in dark-situated plants. In the present work, Griebel and Zeier (pp. 790–801) examine the light dependency of inducible plant defenses in the Arabidopsis-P. syringae model interaction at the molecular level. Employing Arabidopsis photoreceptor double mutants, they show that inducible defense responses at inoculation sites are not or are only moderately altered when cryptochrome, phototropin, or phytochrome photoperception is impaired. Induction of SAR and SA-dependent systemic defense reactions, however, are compromised in phyAphyB mutants. In contrast to the strong dependence of SAR on phytochrome photoperception, SAR can be established without functional cryptochrome or phototropin signaling pathways. The Response of Alternative Oxidase Mutants to Stress Alternative oxidase (AOX) is a terminal oxidase found in the mitochondria of all plants. Although it is expressed during normal growth and development, it is often dramatically induced at the transcript level by a variety of stresses and poisons. Although AOX is rapidly induced by a variety of stress treatments in various plants, there is little evidence that an absence of AOX has a dramatic affect on plant function even under stressful conditions. Giraud et al. (pp. 595–610) have used moderate combined stress conditions with Arabidopsis lines lacking AOX1a in order to gain further insight into the role of AOX in plants. They report that the treatment of aox1a mutant plants with moderate light under drought conditions resulted in a marked phenotypic difference compared to the wild type, as evident from a 10-fold increase in the accumulation of anthocyanins in leaves, alterations in photosynthetic efficiency, increased O2– production, and reduced root growth at the early stages of seedling growth. Transcripts encoding proteins involved in the synthesis of anthocyanins, transcription factors, chloroplastic and mitochondrial components, cell wall synthesis, and Suc and starch metabolism changed, indicating that effects were not confined to mitochondria where AOX1a is located. Microarray and other analyses revealed that the transcripts typically induced upon stress treatment had altered basal levels in untreated aox1a plants. Taken together, these results indicate that aox1a plants have a greatly altered stress response even when mitochondria or the mitochondrial electron transport chain is not the primary target of the stress.
Division of Health Professions and Natural Sciences FOOTNOTES www.plantphysiol.org/cgi/doi/10.1104/pp.104.900263 Related articles in Plant Physiol.:
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