-ATPase Isoform Impairs Sucrose Translocation, Stomatal Opening, Plant Growth, and Male Fertility

The plasma membrane H 1 -ATPase builds up a pH and potential gradient across the plasma membrane, thus activating a series of secondary ion and metabolite transporters. pma4 (for plasma membrane H 1 -ATPase 4), the most widely expressed H 1 -ATPase isogene in Nicotiana plumbaginifolia , was overexpressed in tobacco. Plants that overexpressed PMA4 showed no major changes in plant growth under normal conditions. However, two transformants were identified by their stunted growth, slow leaf initiation, delayed stem bolting and flowering, and male sterility. Protein gel blot analysis showed that expression of the endogenous and transgenic pma4 was cosuppressed. Cosuppression was developmentally regulated because PMA4 was still present in developing leaves but was not detected in mature leaves. The glucose and fructose content increased threefold, whereas the sucrose content remained unchanged. The rate of sucrose exudation from mature leaves was reduced threefold and the sugar content of apical buds was reduced twofold, suggesting failure of sucrose loading and translocation to the sink tissues. Cosuppression of PMA4 also affected the guard cells, stomatal opening, and photosynthesis in mature leaves. These results show that a single H 1 -ATPase isoform plays a major role in several transport-dependent physiological processes.