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. seedlings is dependent on vacuole enlargement and massive flux of ER and Golgi vesicles. Water flow into the expanding vacuole is driven by ion accumulation, which in turn is energized by the vacuolar H+-ATPase (V-ATPase). The V-ATPase energizes the secondary ion transport into the expanding vacuole. As light exposure leads to a strong inhibition of extension growth, the effect of light on transcript levels for subunits 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 mesocotyl tissue. Only in the tip region did light exposure lead to a strong and coordinate down-regulation of both mRNAs whereas in the differentiated mesocotyl only a slight decrease was observed. The results indicate that expression of the 'housekeeping' V-type H+-ATPase is strongly regulated in response to growth rate.