We have developed specific assays for the levels of mRNA, antigenic protein and catalytic activity under standard conditions of the root plasma membrane H+-ATPase of sunflower (Helianthus annuus L. var. HSE) seedlings. During the first 6 days of root development there is a parallel increase in the levels of mRNA, antigenic protein and activity. No significant changes were induced by alkaline pH or NaCl. Acetic acid at pH 4 reduces the level of mRNA more than the levels of antigenic protein and activity (to 30% and 50-60% of controls, respectively). Illumination of the aerial part of the seedlings reduces the antigenic protein and activity in the roots to 50-60% of controls without affecting the level of mRNA. These results suggest that there is regulation of the expression of root plasma membrane H+-ATPase by development and environmental conditions. © 1991.

M. Roldan was postdoctoral fellow of the Spanish CSIC. She was on leave from Estación Experimen- tal del Zaidin, CSIC, Granada, Spain and she was supported by an EMBO short term fellowship dur- ing part of her stay at EMBL. J.M. Pardo was sup- ported by a long term EMBO fellowship.

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