Development and line dependent variations of Petunia plasma membrane H +-ATPase sensitivity to auxin

Abstract

The response to auxin of the plasma membrane H +-ATPase from Petunia leaves was investigated in order (i) to check the general significance of sensitivity changes recently evidenced during development of tobacco and (ii) to test the intraspecific variability of the response to auxine. Two lines of Petunia hybrida, Tlvl and St40, were selected for their differential sensitivity to hormones in organogenesis experiments. Using a large variety of criteria, membrane fractions obtained were shown to be greatly enriched in plasma membrane vesicles containing an active H +-ATPase able to generate a proton gradient. The proton translocation activity was stimulated by indoleacetic acid (IAA) on a biphasic way depending on the hormone concentration. In addition, the IAA concentration inducing the highest stimulation was found to change during plant development. A transient but dramatic decrease of the optimal auxin concentration (4 orders of magnitude, from 10 −5 M to 10 −9 M IAA) was observed at a time period which was assessed to correspond to floral induction. Both lines exhibited the same overall behavior towards auxin up to the end of floral induction. However, in the line Tlvl, the responsiveness to the hormone disappeared after floral induction, whereas a constant stimulation by 10 −5 M IAA was observed up to flowering in the line St40. These results are basically identical to those already obtained with tobacco. It is therefore concluded that a transient increase of the sensitivity to auxin of the plasma membrane H +-ATPase could be of general significance during floral induction, but in addition that some genetic variability could also be found between genotypes of a given species.