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Plant Physiology 143:1081-1082 (2007) © 2007 American Society of Plant Biologists On the InsideMethyl Jasmonate and StomataBoth methyl jasmonate (MeJA) and abscisic acid (ABA) elicit stomatal closing, but it is unclear whether the two compounds use similar or different signaling mechanisms in guard cells. Munemasa et al. (pp. 1398–1407) have investigated the effects of MeJA and ABA on second messenger production and ion channel activation in guard cells of wild-type Arabidopsis (Arabidopsis thaliana) and MeJA-insensitive coronatine-insensitive 1 (coi1) mutants. The coi1 mutation impaired MeJA-induced stomatal closing but not ABA-induced stomatal closing. Both MeJA and ABA induced the production of reactive oxygen species (ROS) and nitric oxide (NO) in wild-type guard cells, but MeJA failed to do so in coi1 guard cells. Pharmacological experiments demonstrated that both ROS and NO are involved in MeJA-induced stomatal closing, as well as ABA-induced stomatal closing. Like ABA, MeJA also activated slow (S-type) anion channels and Ca2+ permeable cation (ICa) channels in the plasma membrane of wild-type guard cell protoplasts. However, in coi1 guard cell protoplasts, MeJA, unlike ABA, elicited neither S-type anion currents nor ICa currents. To further elucidate interactions between ABA and MeJA signaling in guard cells, the authors examined MeJA signaling in the ABA-insensitive mutant ABA-insensitive 2 (abi2-1), whose ABA signal transduction cascade is disrupted downstream of ROS production and NO production. MeJA did not induce stomatal closing in abi2-1, but did stimulate the production of ROS and NO. These results suggest that MeJA triggers stomatal closing via a receptor distinct from the ABA receptor, and that the coi1 mutation disrupts MeJA signaling upstream of the branch point of ABA signaling and MeJA signaling in Arabidopsis guard cells. Based on these new results, the authors present a novel model of hormonal signaling interaction in Arabidopsis guard cells. Proanthocyanidin Synthesis in Grapes Proanthocyanidins (PAs), also known as condensed tannins, are polyphenolic secondary metabolites synthesized via the flavonoid biosynthetic pathway. They are present in many plants and act in defense against plant diseases and in seed dormancy. Dietary PAs are present in many fruits and plant products, like wine, fruit juices, and teas, and contribute to their taste and health benefits. PAs act as potential dietary antioxidants with beneficial effects for human health, including protection against free radical-mediated injury and cardiovascular disease. The PAs found in grapes (Vitis vinifera) are of special interest because of their importance for the taste and astringency of red and white wine (Fig. 1 ). In this issue, Bogs et al. (pp. 1347–1361) report on their isolation and characterization of a grapevine gene (VvMYBPA1) that encodes a MYB transcription factor that is expressed when PAs accumulate during early grape berry development and in seeds. VvMYBPA1 is similar to the Arabidopsis MYB transcription factor TRANSPARENT TESTA2 (TT2) that regulates PA synthesis in the seed coat of Arabidopsis. By complementing the PA-deficient seed phenotype of the Arabidopsis tt2 mutant with VvMYBPA1, the authors confirmed the function of VvMYBPA1 as a transcriptional regulator of PA synthesis. In contrast to the ectopic expression of TT2 in Arabidopsis, the constitutive expression of this transcription factor in Arabidopsis complemented the PA-deficient seed coat phenotype of the Arabidopsis tt2 mutant and also induced ectopic PA accumulation in other tissues, including cotyledons, stems, and roots. This grapevine regulator may provide the potential to alter PA synthesis in grape berries and other fruits and crops by metabolic engineering.
The Aleurone Is the Primary Determinant of Seed Dormancy in Arabidopsis Both the embryo and the seed coat have been implicated in the control of Arabidopsis dormancy and germination. Dormancy is genetically determined, and the seeds of some genotypes are dormant after months or years of dry storage, whereas seeds with other genotypes lose dormancy within weeks. This process of dormancy loss can be hastened or slowed by environmental conditions. For example, stratification under cold, damp conditions often removes dormancy. Genetic evidence indicates that ABA is central to the establishment and maintenance of seed dormancy and that GA is important for germination. Recent data have also shown that NO is involved in a signaling pathway that promotes the loss of dormancy. Bethke et al. (pp. 1173–1188) report that Arabidopsis seeds remain dormant after their nonliving testas are removed, but lose their dormancy when the living aleurone is damaged or removed. Regardless of the dormancy status of the seed, no seeds exhibited true embryo dormancy. All isolated embryos grew and greened within 3 to 4 d of removal from the seed coats (living aleurone with adhering dead testa). Their data indicate that the aleurone layer is the primary determinant of seed dormancy. Embryos from dormant seeds, however, had a lesser growth potential than those from nondormant seeds. Further experiments revealed that Arabidopsis aleurone cells responded to NO, GA, and ABA, and that NO was upstream of GA in a signaling pathway that leads to the swelling of protein storage vacuoles. Cell Cycle Genes and the Inhibition of Maize Leaf Growth during Cold Nights The growth of maize (Zea mays) seedlings is sensitive to low temperature, particularly during early spring. Delayed sowing to avoid this problem reduces the length of the growth season and the potential yield. Cold in early spring has several distinct effects on the establishment of maize seedlings in the field, including reduced photosynthesis and cell death. It is interesting, however, that even before visible symptoms of chilling injury occur, suboptimal temperatures inhibit leaf growth. The activity of A-type cyclin-dependent kinase (CDKA), one of the central regulators of cell cycle progression, is associated with the decrease in leaf growth rate in other species under stress conditions. Rymen et al. (pp. 1429–1438) were curious whether CDKA might also play a role in inhibiting early leaf growth of maize, under cold regimens that inhibit growth prior to the visible manifestations of chilling injury. To do so, they compared the daytime growth of seedlings exposed to control (18°C) and cold (4°C) nights and studied the cellular growth mechanisms that inhibit growth. They found a specific inhibition of cell cycle activity in the leaf basal meristem. This discovery led them to identify 43 putative maize homologs of cell cycle regulatory genes based on sequence database information. They analyzed the transcription of these genes by constructing expression profiles along the leaf growth zone using real-time PCR. Their results suggest that long-term exposure to low night temperature causes specific changes in the expression of cell cycle genes in maize leaves. Although the regulation of cell cycle progression involves several posttranslational mechanisms, these results highlight that transcriptional changes also play a role. A combination of opposite effects on positive and negative cell cycle regulators apparently leads to prolonged cell cycle duration involving all cell cycle phases and results in decreased cell production. In turn, this decreased cell production leads to slower growth rates and shorter final leaf size. Proteome of the Maize Endosperm The development of cereal endosperm is a complex process involving five key steps: a coenocytic phase involving numerous nuclear divisions, cellularization, cell differentiation, food reserve synthesis, and cell maturation. Although the morphological steps of endosperm development are well described, the underlying molecular mechanisms are not. In this issue, Méchin et al. (pp. 1203–1219) present a proteomic study of maize endosperm development. They examined the accumulation pattern of 409 proteins at seven developmental stages. Hierarchical clustering analysis allowed four main developmental profiles to be recognized. Early stages, devoted to cellularization, cell division, and cell wall deposition, corresponded to a period of maximum expression of actin, tubulin, cell organization proteins, and proteins involved in respiration and in the protection of cells against ROS. A finding consistent with the recent demonstration that anoxic conditions predominate during starch accumulation in the endosperm was that an increase in glycolytic enzymes relative to Kreb's cycle enzymes was apparent during the starch accumulation phase. Another significant finding was the specific late-stage accumulation of pyruvate orthophosphate dikinase, which suggests a critical role for this enzyme in the starch/protein balance through an inorganic pyrophosphate-dependent restriction of ADP-Glc synthesis.
Department of Natural Sciences, Mercy College, Dobbs Ferry, New York 10522 FOOTNOTES www.plantphysiol.org/cgi/doi/10.1104/pp.104.900219 Related articles in Plant Physiol.:
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