DOWN-REGULATION OF PLANT V-TYPE H-ATPASE GENES AFTER LIGHT-INDUCED INHIBITION OF GROWTH()

Cell extension growth in the mesocotyl tip of dark-grown Zea mays L. s eedlings is dependent on vacuole enlargement and massive flux of ER an d Golgi vesicles. Water flow into the expanding vacuole is driven by i on accumulation, which in turn is energized by the vacuolar H+-ATPase (V-ATPase). The V-ATPase energizes the secondary ion transport into th e expanding vacuole. As light exposure leads to a strong inhibition of extension growth, the effect of light on transcript levels for subuni ts A and c of the V-ATPase was analyzed. Partial homologous cDNAs for subunit A and two isoforms of subunit c were cloned by RT-PCR. In dark -grown seedlings transcript levels for both subunits were much higher in the growing mesocotyl tip than in the fully differentiated mesocoty l tissue. Only in the tip region did light exposure lead to a strong a nd coordinate down-regulation of both mRNAs whereas in the differentia ted mesocotyl only a slight decrease was observed. The results indicat e that expression of the 'housekeeping' V-type H+-ATPase is strongly r egulated in response to growth rate.