Integration of Transpiration Rate, Photosynthesis, Abscisic-Acid Signaling, and Gaurd-Cell Gene Expression

Guard cells in the leaf epidermis regulate transpiration rate (E) and carbon dioxide uptake by adjusting stomatal aperture size. Sucrose (suc) is a recent product of photosynthesis that accumulates around guard cells during transpiration. Similarly, abscisic acid (ABA) is a potent regulator of guard cell physiology that can be transported to guard cells via the transpiration stream. I tested the hypothesis that suc that accumulates around guard cells as a result of transpiration affects levels of messenger ribonucleic acids (mRNAs) in guard cells. I present preliminary investigations of a similar hypothesis for ABA.The hypothesis was tested with three approaches. One, sucrose was fed to leaves via the petiole. Two, guard cells were incubated in solutions of suc and/or ABA. Three, relative humidity (RH) was increased to decrease E. As expected, E and the rate of suc delivery to guard cells were around 4 fold and 3 7 fold lower, respectively, under high RH (90%) conditions than under control (60% RH) conditions. I measured mRNA levels in guard cells and leaves of Vicia faba of genes encoding protein products involved in sugar transport or metabolism and one mRNA induced in guard cells by ABA. Selected mRNAs encode a plasma membrane localized H+ suc symporter, a plasma membrane localized H+ monosaccharide symporter, a small subunit of ribulose 1, 5 bisphosphate carboxylase/oxygenase, a suc phosphate synthase, a suc synthase, a cell wall invertase, or a small subunit of adenosine diphosphate glucose pyrophosphorylase. I found that E linked accumulation of solutes in the guard cell apoplast affects mRNA levels of some sugar and ABA regulated genes. Our laboratory surmises that suc, ABA, and other signals found in the transpiration stream that are present at sub threshold levels in the transpiration stream, accumulate to threshold levels around guard cells, where they induce signal transduction. Signal transduction in these experiments was measured by changes in mRNA levels. Interestingly, E induced changes in mRNA levels measured during the first day following a switch to high RH growth conditions returned to control levels after several days in high humidity, possibly indicating acclimation, at the molecular level, to changes in E.