Membranes are primary sites of perception of environmental stimuli. Polyunsaturated fatty acids are major structural constituents of membranes that also function as modulators of a multitude of signal transduction pathways evoked by environmental stimuli. Different stresses induce production of a distinct blend of oxygenated polyunsaturated fatty acids, "oxylipins." We employed three Arabidopsis (Arabidopsis thaliana) ecotypes to examine the oxylipin signature in response to specific stresses and determined that wounding and drought differentially alter oxylipin profiles, particularly the allene oxide synthase branch of the oxylipin pathway, responsible for production of jasmonic acid (JA) and its precursor 12-oxo-phytodienoic acid (12-OPDA). Specifically, wounding induced both 12-OPDA and JA levels, whereas drought induced only the precursor 12-OPDA. Levels of the classical stress phytohormone abscisic acid (ABA) were also mainly enhanced by drought and little by wounding. To explore the role of 12-OPDA in plant drought responses, we generated a range of transgenic lines and exploited the existing mutant plants that differ in their levels of stress-inducible 12-OPDA but display similar ABA levels. The plants producing higher 12-OPDA levels exhibited enhanced drought tolerance and reduced stomatal aperture. Furthermore, exogenously applied ABA and 12-OPDA, individually or combined, promote stomatal closure of ABA and allene oxide synthase biosynthetic mutants, albeit most effectively when combined. Using tomato (Solanum lycopersicum) and Brassica napus verified the potency of this combination in inducing stomatal closure in plants other than Arabidopsis. These data have identified drought as a stress signal that uncouples the conversion of 12-OPDA to JA and have revealed 12-OPDA as a drought-responsive regulator of stomatal closure functioning most effectively together with ABA.

The presence of 2 mM bicarbonate in the incubation medium induced stomatal closure in abaxial epidermis of Arabidopsis. Exposure to 2 mM bicarbonate elevated the levels of H(2)O(2) in guard cells within 5 min, as indicated by the fluorescent probe, dichlorofluorescein diacetate (H(2)DCF-DA). Bicarbonate-induced stomatal closure as well as H(2)O(2) production were restricted by exogenous catalase or diphenylene iodonium (DPI, an inhibitor of NAD(P)H oxidase). The reduced sensitivity of stomata to bicarbonate and H(2)O(2) production in homozygous atrbohD/F double mutant of Arabidopsis confirmed that NADP(H) oxidase is involved during bicarbonate induced ROS production in guard cells. The production of H(2)O(2) was quicker and greater with ABA than that with bicarbonate. Such pattern of H(2)O(2) production may be one of the reasons for ABA being more effective than bicarbonate, in promoting stomatal closure. Our results demonstrate that H(2)O(2) is an essential secondary messenger during bicarbonate induced stomatal closure in Arabidopsis.

Phosphatidylinositol-specific phospholipase C (PtdIns-PLC2) plays a central role in the phosphatidylinositolspecific signal transduction pathway. It catalyses the hydrolysis of membrane-bound phosphatidylinositol 4,5-bisphosphate to produce two second messengers, sn-1,2-diacylglycerol and inositol 1,4,5-trisphosphate. The former is a membrane activator of protein kinase C in mammalian systems, and the latter is a Ca 2+ modulator which induces distinctive oscillating bursts of cytosolic Ca 2+ , resulting in regulation of gene expression and activation of proteins. Sustained over-expression of BnPtdIns-PLC2 in transgenic Brassica napus lines brought about an early shift from vegetative to reproductive phases, and shorter maturation periods, accompanied by notable alterations in hormonal distribution patterns in various tissues. The photosynthetic rate increased, while stomata were partly closed. Numerous gene expression changes that included induction of stressrelated genes such as glutathione S-transferase, hormoneregulated and regulatory genes, in addition to a number of kinases, calcium-regulated factors and transcription factors, were observed. Other changes included increased phytic acid levels and phytohormone organization patterns. These results suggest the importance of PtdIns-PLC2 as an elicitor of a battery of events that systematically control hormone regulation, and plant growth and development in what may be a preprogrammed mode.

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The role of nitric oxide (NO) during bicarbonate‐induced stomatal closure was studied in the abaxial epidermis of Pisum sativum. A few experiments were done with 10 μM ABA, for comparison. The presence of 2 mM sodium bicarbonate or 10 μM ABA induced an increase of NO in guard cells. Elevation of NO by sodium nitroprusside induced stomatal closure and enhanced further the closure by bicarbonate. The bicarbonate induced increase in NO of guard cells, or stomatal closure was prevented partially by 2‐phenyl‐4,4,5,5‐tetramethyl imidazoline‐1‐oxyl 3‐oxide, an NO scavenger and N‐nitro‐l‐Arg‐methyl ester, an inhibitor of NO synthase (NOS). These results suggested that guard cells generated NO on exposure to bicarbonate and that NOS was involved at least partially in such NO production. Time course experiments revealed that on exposure to bicarbonate or ABA, the rise in guard cell NO production peaked within 10 min. Experiments using pharmacological compounds like wortmannin/LY294002 (phosphatidylinositol 3 kinase inhibitors), 1H‐(1,2,4)‐oxadiazole‐[4,3a]quinoxalin‐1‐one (guanylyl cyclase inhibitor), nicotinamide (cyclic adenosine diphosphate ribose antagonist), guanosine 5′‐O‐(2‐thiodiphosphate) (G‐protein antagonist) suggested a role of phosphatidylinositol 3‐phosphate or G‐proteins during bicarbonate‐induced stomatal closure.

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