| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
|
Plant Physiology 141:1161-1162 (2006) © 2006 American Society of Plant Biologists On the InsideDo All Protein Phosphatases 2C Negatively Regulate ABA Signal Transduction?Phosphorylation/dephosphorylation events mediated by a complex network of protein kinases and protein phosphatases play a critical role in abscisic acid (ABA) signaling. Protein phosphatases type 2C (PP2Cs) have been identified as a major component of ABA signaling based on pioneering studies of the ABA-insensitive abi1-1 and abi2-1 mutants. Currently, at least four PP2Cs, ABI1, ABI2, PP2CA, and HAB1, are known to serve as negative regulators of ABA signaling in Arabidopsis (Arabidopsis thaliana). Saez et al. (pp. 1389–1399) have generated double knockout mutants of PP2Cs to determine whether PP2Cs are redundant or additive in their functions. The phenotypic effects observed in single hab1-1, abi1-2, and abi1-3 mutants were notably reinforced in double mutants, which showed both enhanced responsiveness to ABA and drought avoidance. Transpirational water loss under drought conditions was also noticeably reduced in the double mutants as compared to the single parental mutants. These results reveal a cooperative negative regulation of ABA signaling by ABI1 and HAB1. Thus, the combined inactivation of specific PP2Cs involved in ABA signaling could potentially provide an approach for improving crop performance under drought stress conditions. It now appears, however, that not all PP2Cs serve as negative regulators of ABA signaling. Reyes et al. (pp. 1414–1424) present some surprising results concerning an ABA-induced PP2C (FsPP2C2) that had previously been isolated from beech (Fagus sylvatica) seeds. Since transgenic work is not possible in beech, the authors overexpressed FsPP2C2 in Arabidopsis to provide genetic evidence concerning FsPP2C2 function in seed dormancy and other plant responses. Unlike other PP2Cs described to date, the constitutive expression of FsPP2C2 in Arabidopsis, under the cauliflower mosaic virus 35S promoter, produced enhanced sensitivity to ABA and abiotic stress in seeds and vegetative tissues, as well as a dwarf phenotype and delayed flowering. Moreover, all these effects were reverted by GA3 application. In marked contrast to other plant protein phosphatases 2C that have been demonstrated to act as negative regulators of ABA signaling, these results support the hypothesis that FsPP2C2 is a positive regulator of ABA. Moreover, these results indicate the existence of potential cross-talk between ABA signaling and GA biosynthesis. Retinoblastoma-Related Proteins: Developmental Switches As cells exit the shoot apical meristem, the heterotrophic cells of the meristem rapidly gain an autotrophic capability by synthesizing and assembling components of the chloroplast. At the same time, cells undergo enlargement via vacuolization. Despite significant advances in the characterization of the transcriptional networks involved in meristem maintenance and leaf determination, our understanding of the actual mechanism of meristem cell differentiation remains limited. Using a microinduction technique, Wyrzykowska et al. (pp. 1338–1348) show that a local, transient overexpression of a retinoblastoma-related (RBR) protein in the shoot apical meristem is sufficient to trigger cells in the meristem to undergo the initial stages of differentiation in tobacco (Nicotiana tabacum). Interestingly, the cessation of meristem growth and the partial differentiation of the meristem cells persisted long after a short pulse of RBR expression, suggesting that RBR caused an irreversible change in cell behavior. The cytological demonstration that the overexpression of RBR protein induces cell differentiation is complemented by the finding that the overexpression of RBR also up-regulates a photosynthetic gene and down-regulates cell cycle genes as well as at least one gene involved in maintaining meristem identity. Taken together with the recent demonstration that the RBR protein plays a major role in restricting stem cell differentiation in the root apical meristem, these findings contribute to an emerging picture of RBR protein as a central part of the mechanism controlling meristem cell differentiation. Pollen Tube Secretion and Evanescent Wave Microscopy Although the technique of evanescent wave microscopy (EWM) was developed more than two decades ago, it has recently proven to be a very useful tool for the study of secretion. By restricting fluorescence excitation to the vicinity of a dielectric interface and thus suppressing out-of-focus background fluorescence from deeper within the specimen, EWM allows for improved detection and superior depth discrimination of fluorescent structures. Moreover, due to the light confinement of evanescent wave excitation, photobleaching and phototoxic reactions are generally minor compared to conventional epi-excitation or confocal laser scanning microscopy. Wang et al. (pp. 1591–1603) have used EWM to visualize secretory vesicle motions in living pollen tubes of Meyer spruce (Picea meyeri) after labeling the vesicles with the endocytotic/exocytotic tracer FM4-64. This amphiphilic styryl dye has been used previously to investigate endocytosis in living fungal hyphae. Two-dimensional trajectories of individual vesicles were obtained from the resulting time-resolved image stacks and were used to characterize the vesicles in terms of their average fluorescence and mobility. The velocity and direction of vesicle motions, frame-to-frame displacement, and vesicle trajectories were also calculated. Analysis of individual vesicles revealed that two types of motion are present, and that vesicles in living pollen tubes exhibit complicated behaviors and oscillations that differ from simple Brownian motion. Furthermore, disruption of the actin cytoskeleton had a much more pronounced effect on vesicle mobility than did disruption of the microtubules, suggesting that the actin cytoskeleton plays a primary role in vesicle mobility. A Novel Polyamine Oxidase The polyamines putrescine (Put), spermidine (Spd), and spermine (Spm) are small aliphatic amines commonly found in both prokaryotic and eukaryotic cells. In higher plants, polyamines are key players in a number of plant developmental processes and have been implicated in plant responses to various abiotic stresses and plant-pathogen interactions. In the polyamine back-conversion pathway that has been elucidated in animals, Spm and Spd are first acetylated by Spd/Spm N1-acetyltransferase and then oxidized by polyamine oxidase (PAO) to produce Spd and Put, respectively. To date, the only types of PAOs that have been characterized in plants seem to be involved in the terminal catabolism of polyamines and not in the animal-type polyamine back-conversion pathway. In this issue, Tavladoraki et al. (pp. 1519–1532) present evidence that an animal-like PAO homolog does exist in higher plants, suggesting that a polyamine back-conversion pathway may exist in plants. A database search within the Arabidopsis genome sequence showed the presence of a gene (AtPAO1) encoding for a putative PAO with 45% amino acid sequence identity with maize (Zea mays) PAO. The AtPAO1 cDNA was isolated and cloned in a vector for heterologous expression in Escherichia coli. The purified recombinant protein was shown to be a flavoprotein able to oxidize Spm, norspermine, and N1-acetylspermine. Analysis of the reaction products showed that AtPAO1 produces Spd from Spm, demonstrating a substrate oxidation mode similar to that of animal PAO. To the authors' knowledge, AtPAO1 is the first plant PAO reported to be involved in a polyamine back-conversion pathway. Phloem Loading of Exogenous Salicylic Acid Salicylic acid (SA) plays an important role in plant defense against pathogen attack by functioning as an endogenous signal in the transmission of systemic acquired resistance (SAR). SA has been demonstrated to move from inoculated leaves to other tissues by phloem transport. The mechanism by which SA is transported across membranes, however, is poorly understood. Rocher et al. (pp. 1684–1693) have evaluated the ability of exogenous SA to accumulate in the castor bean (Ricinus communis) phloem by chemical analyses of phloem sap collected from the severed apical part of seedlings. Time-course experiments indicated that SA was transported to the root system via the phloem and redistributed upward in small amounts via the xylem. According to the authors, the involvement of two long-distance transport pathways helps to explain several seeming discrepancies in the literature concerning SA distribution within the plant in response to biotic stress and exogenous SA application. Phloem loading of SA at 1, 10, or 100 µM was dependent on the pH of the cotyledon incubating solution, and accumulation in the phloem sap was the highest (about 10-fold) at the most acidic pH values tested (pH 4.6 and 5.0). Moreover, SA, in terms of its pKa value and octanol/water partitioning coefficient, is nearly ideal for phloem systemicity by way of the ion-trap mechanism. However, SA uptake still occurred at pH values close to neutrality, i.e. when SA is predicted to be only in its dissociated form. Moreover, the analog 3,5-dichlorosalicylic acid, which previous models have predicted to be nonmobile, also moved in the sieve tubes. These discrepancies and other data suggest the possible involvement of a pH-dependent carrier system translocating aromatic monocarboxylic acids, in addition to the ion trap mechanism, in the loading of SA into the phloem. Cytosolic Triacylglycerol Biosynthesis in Oilseeds Vegetable oils are the major source of edible lipids, accounting for more than 75% of the total lipids consumed across the world. The global demand for plant oils has intensified efforts to genetically modify the organism to enhance oil yield. During triacylglycerol (TAG) biosynthesis, acyl-CoA:diacylglycerol acyltransferase (DGAT) catalyzes the final step that acylates diacylglycerol to form TAG. It is well known that TAG biosynthesis occurs in microsomal membranes, but Saha et al. (pp. 1533–1543) report the presence of TAG biosynthetic machinery in the cytosol of developing peanut (Arachis hypogaea) cotyledons. The authors identified and purified a soluble DGAT, the activity of which was NaF insensitive and acyl-CoA dependent, from immature peanuts. The isolated gene shared less than 10% identity with the previously identified DGAT1 and 2 families. Expression of peanut cDNA in E. coli resulted in the formation of labeled TAG and wax ester from [14C]acetate. Several observations indicate that the identified DGAT is cytosolic in nature. (1) The activity is associated with 150,000g supernatant. (2) The enzyme is purified by successive column chromatographic separations without detergent. (3) The isolated gene (AhDGAT) has neither membrane-spanning regions nor signal sequence peptide sequences. These data suggest that the cytosol is an alternative site for TAG biosynthesis in oilseeds. The identified pathway may present opportunities for the bioengineering of oil-yielding plants for increased oil production.
Department of Natural Sciences Mercy College Dobbs Ferry, New York 10522 FOOTNOTES www.plantphysiol.org/cgi/doi/10.1104/pp.104.900199. Related articles in Plant Physiol.:
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| ASPB Publications | PLANT PHYSIOLOGY | THE PLANT CELL | |
|---|---|---|---|
