2 PLASMA-MEMBRANE H-ATPASE GENES EXPRESSED IN GUARD-CELLS OF VICIA-FABA ARE ALSO EXPRESSED THROUGHOUT THE PLANT()

Guard cells modulate stomatal apertures in response to hormones, metab olic demands and environmental stimuli. The guard cell PM H+-ATPases p lay a critical role in this process by generating the electrochemical gradient to drive solute transport and concomitant water flux. The PM H+-ATPase activity is specifically regulated by red and blue light, fu ngal toxins and auxin. To determine if the unique responsiveness of th e guard cell PM H+-ATPase is due to the expression of a cell-specific isoform, we amplified by PCR, and cloned portions of PM H+-ATPase gene s VHA1 and VHA2, which are expressed in guard cell protoplasts (GCP). In situ hybridization to leaf tissue sections indicated that VHA1 and VHA2 genes were expressed in guard cells and mesophyll cells but not i n epidermal cells or vascular tissues. Furthermore, a gene-specific qu antitative reverse transcription (RT)-PCR detected VHA1 and VHA2 mRNAs in both GCP and mesophyll cell protoplast mRNA as well as in mRNA iso lated from roots, leaves, stems and flowers. Thus, two PM H+-ATPase ge nes expressed in guard cells are also expressed in many other tissues and cell types, This suggests that the unique responsiveness of the gu ard cell PM H+-ATPases to environmental stimuli results from cell-spec ific signal transduction pathways rather than the expression of a cell -specific PM H+-ATPase.