Ontogenetic priming of stomatal control in [i]Arabidopsi[/i]s leaves through gradual exposure to low humidity
Résumé
In plants, stomata form flexible pores at the leaf surface which facilitate CO2 entry for photosynthesis at the expense of water loss. To save water, plants evolved complex signalling mechanisms that close stomata at night when photosynthesis is impossible and in response to abscisic acid (ABA), a major drought hormone. Using reverse genetics, electron microscopy, and physiological assays in isolated epidermis and in planta, we demonstrate that stomata of Arabidopsis thaliana are unexpectedly passive in the young leaves and acquire sensitivity to ABA and darkness during leaf ontogeny. On the other hand, stomata of adult leaves remain
insensitive to ABA when leaves are entertained at high atmospheric humidity, and recover ABA sensitivity when exogenous ABA is applied on the leaves the day prior to the assay. This suggests that lower humidity increases ABA level to a threshold necessary to prime stomatal functioning. Interestingly, an oxygen isotope analysis of leaf water supported the conclusion that adult leaves are exposed to a lower humidity than young leaves. We propose that the microclimate within the Arabidopsis rosette leads to an ontogenetic hardening of stomata, through a gradual exposure of the leaf to low humidity which triggers an activation of ABA metabolism and signalling.