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Plant Physiology 145:291-292 (2007) © 2007 American Society of Plant Biologists On the InsideFlavonoid Movement in ArabidopsisFlavonoids have diverse functions ranging from plant interactions with symbiotic microorganisms to the control of auxin transport. The current view is that flavonoids are synthesized in the cells in which they accumulate and serve local functions. Recently, however, several studies have indicated that flavonoids may move from their site of synthesis. For example, Arabidopsis (Arabidopsis thaliana) roots grown in complete darkness do not accumulate flavonoids since the expression of genes encoding enzymes of flavonoid biosynthesis is light dependent. Flavonoids, however, do accumulate in root tips of plants with light-grown shoots and dark-grown roots, consistent with shoot-to-root flavonoid movement. Fluorescence microscopy of seedlings stained with diphenylboric acid 2-aminoethyl ester (DPBA), a probe that fluoresces upon binding flavonoids, provides a powerful tool for examining the accumulation, localization, and movement of flavonoids in living plants. Arabidopsis transparent testa4 (tt4) mutants lack flavonoids and when treated with DPBA have only dim green fluorescence. To test the hypothesis that flavonoids move long distances in Arabidopsis, Buer et al. (pp. 478–490) applied flavonoids locally to various tissues of tt4 mutants. They used confocal microscopy to examine which tissues harbored the DPBA fluorescence and determined which tissues facilitated long-distance flavonoid movement. To verify that flavonoids were capable of root-to-shoot and shoot-to-root movement, they used HPLC analysis of tissues distal to the application site to show that downstream products of the flavonoid pathway were present in these tissues. Finally, to determine how this movement was facilitated they tested various inhibitors of ATP-binding cassette transporters and H+-ATPase and describe a putative mechanism for this movement. Together their results showed that flavonoids can move in Arabidopsis and suggest that this movement may be mediated by an ATP-binding cassette-type transporter. Can We Further Optimize Photosynthesis? It is often assumed that the distribution of resources between enzymes of photosynthetic carbon metabolism have been optimized by natural selection. However, natural selection for survival and fecundity does not necessarily select for maximal photosynthetic productivity. Moreover, atmospheric CO2 concentration, the key photosynthetic substrate, has changed more over the past 100 years than the past 25 million years with the likelihood that natural selection has had inadequate time to reoptimize resource partitioning for this change. Could photosynthetic rate be increased by altered partitioning of resources among the enzymes of carbon metabolism? To address this question, Zhu et al. (pp. 513–526) have extended existing metabolic models of C3 photosynthesis by including the photorespiratory pathway and metabolism to starch and Suc to develop a complete dynamic model of photosynthetic carbon metabolism. The model that they have developed consists of linked differential equations, each representing the changes of concentration of one metabolite. Initial concentrations of metabolites and maximal activities of enzymes were extracted from the literature. The model was demonstrated to realistically simulate the dynamics of CO2 fixation and metabolite concentrations. Using an evolutionary algorithm, the partitioning of a fixed total amount of protein-nitrogen between enzymes was allowed to vary. The individual with the higher light-saturated photosynthetic rate was selected in each generation and used to seed the next generation. After 1,500 generations, photosynthesis was increased substantially. This suggests that the light-saturated rate of photosynthesis in C3 leaves might currently be suboptimal. The model also indicates that plants currently have an overinvestment in photorespiratory pathway enzymes and an underinvestment in Rubisco, sedoheptulose-1,7-bisphosphatase, and Fru-1,6-bisphosphate aldolase. Moreover, an increase in sink capacity was also indicated to lead to increased photosynthesis. These results suggest that the manipulation of enzyme and carbohydrate partitioning could greatly increase carbon gain without any increase in the total protein-nitrogen investment in the apparatus for photosynthetic carbon metabolism. Pollen Lethality in RNA Interference Plants The generation of loss-of-function mutants by RNA interference (RNAi) approaches and T-DNA insertional mutagenesis have proven to be very successful in determining gene functions. AGAMOUS LIKE18 (AGL18) is a MADS-box gene that is expressed during Arabidopsis pollen development. To reveal a function for AGL18, Xing and Zachgo (pp. 330–333) produced different transgenic AGL18 RNAi populations. Surprisingly, a pollen lethality phenotype found in many RNAi lines was not detectable in an AGL18 T-DNA knockout mutant. To investigate these contradictory results, the authors made a series of transgenic lines using different vectors, promoters, and reporter genes. Analyses revealed that the pollen lethality phenotype observed in AGL18 RNAi plants is not related to the loss of AGL18 function. To test if pollen development is generally affected by the RNAi mechanism, two genes were selected for which no function during pollen development has been reported. Strikingly, over 20% of both of these transgenic T1 plants displayed the pollen lethality phenotype with 20% to 50% of the pollen being aborted. These data demonstrate that all different RNAi T1 populations tested have a large proportion, about 20% to 30%, of their T1 plants that produce 20% to 50% nonviable pollen, irrespective of the gene and binary vector type used for RNAi vector construction. Next, they examined whether not only RNAi constructs, but transgenes in general, such as antisense constructs, reporter gene constructs, as well as overexpression constructs or even empty vectors also disturb normal pollen development. They observed that in all different T1 populations, over 10% of the transgenic plants showed the pollen lethality phenotype. Thus, the occurrence of the pollen lethality phenotype seems to be a general, common phenomenon among transgenic plant populations but the number of plants with this phenotype was significantly increased in all of the investigated transgenic RNAi populations. The authors also determined that the onset of abnormal pollen development occurred at a late uninucleate microspore stage, possibly through sporophytic effects. Finally, evidence is presented that the observed pollen lethality phenotype in the different transgenic plants is inheritable and transmitted through the female. The Predicted Arabidopsis Interactome The complex cellular functions of an organism frequently rely on physical interactions between proteins. Interactomes are genome scale maps of protein-protein interactions. Interactomics is quickly becoming a valuable new area of systems biology by comprehensively deducing the networks of protein-protein interactions that form the basis for much of signaling and regulatory control as well as the machinery of cellular function. Where the cost of a high-throughput experimental approach is prohibitive, a computational alternative is often a useful preliminary step, especially when combined with literature extraction of all published protein interactions. Predicted interactomes are deduced from experimental interactomes of other species. A pair of interacting orthologs (interologs) in the reference species predicts an interaction in the test species. Interactome predictions from interologs are best suited for studies of highly conserved proteins. In this issue, Geisler-Lee et al. (pp. 317–329) present a predicted Arabidopsis protein interactome based on the interolog method. The authors report that interacting proteins were significantly more likely to be found within the same subcellular location, and significantly less likely to be found in conflicting localizations than randomly paired proteins. A notable exception was that proteins located in the Golgi were more likely to interact with Golgi, vacuolar, or endoplasmic reticulum sorted proteins, indicating possible docking or trafficking interactions. In addition the authors have predicted interactions for many previously unknown proteins in known pathways and complexes. S-Adenosylmethionine Import into the Golgi Pectins are complex polysaccharides present in the primary cell wall in plants. S-adenosylmethionine (SAM) is the substrate used in the methylation of the pectin homogalacturonan in the Golgi apparatus. SAM is synthesized in the cytosol but it is not known how it is subsequently transported into the lumen of the Golgi apparatus. To establish whether SAM is indeed transported into the Golgi and used in the methylation of homogalacturonan, Ibar and Orellana (pp. 504–512) analyzed the incorporation of SAM into Golgi vesicles obtained from etiolated pea (Pisum sativum) epicotyls. Their results suggest that SAM is indeed transported into the lumen of the Golgi and used in the methylation of pectins. Moreover, this importation process shows features that differ from other Golgi-localized transporters, such as the nucleotide sugar transporters, suggesting that a new class of Golgi transporter is involved in pectin biosynthesis. Ethylene and Cell Death Control in a Lesion Mimic Mutant Lesion mimic mutants that constitutively express defense responses and display spontaneous programmed cell death are useful models for deciphering cell death signaling pathways. LMMs are classified as either initiation or propagation mutants. Propagation mutants are unable to control the rate and extent of the lesions, whereas initiation mutants are unable to initiate the process. Although ethylene and other plant hormones, particularly salicylic acid (SA), are involved in the complex cross talk regulating plant defense responses to microbial attack, ethylene's functions remain unclear. The Arabidopsis mutant vascular associated death1 (vad1) displays necrotic hypersensitive response-like lesions propagating along the vascular system, an enhanced expression of defense genes, an accumulation of high levels of SA, and an increased resistance to virulent and avirulent strains of Pseudomonas syringae. Previous analyses have revealed the vad1-1 cell death phenotype to be dependent on SA biosynthesis, but independent of NPR, a gene that is essential in activating systemic, inducible plant defense responses. Now, Bouchez et al. (pp. 465–477) examine the role of ethylene in the cell death and resistance phenotypes of vad1-1, and the transcriptional regulation of this negative regulator of cell death by ethylene. By using the vad1-1 mutant in combination with ethylene signal transduction mutants, the authors show that ethylene plays a major role in the cell death program controlled by VAD1. Moreover, the transcriptional activation of VAD1, which was previously shown to occur late during plant-pathogen interactions, appears to be dependent on ethylene production and signaling. These results suggest that VAD1 acts as an integrative node in hormonal signaling, with ethylene acting in concert with SA as a positive regulator of cell death propagation.
Department of Natural Sciences, Mercy College, Dobbs Ferry, New York 10522 FOOTNOTES www.plantphysiol.org/cgi/doi/10.1104/pp.104.900239 Related articles in Plant Physiol.:
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