Guard-cell expression of abscisic acid receptors for engineering water-use-efficient plants without trade-offs in growth.

Abscisic acid (ABA) induces stomatal closure, reducing transpirational water loss, a critical adaptation for agriculture under drought. However, ABA is often viewed cautiously as stomatal closure limits CO uptake for photosynthesis and growth. We have demonstrated that ectopic expression of selected ABA receptors in Arabidopsis reduces transpiration without growth penalty, thus achieving high water use efficiency (WUE). The tissues and mechanisms underlying this trait remain unclear, though prior evidence suggests a significant contribution from shoot ABA responsiveness. We hypothesize that guard-cell ABA signaling contributes to the trait of high WUE without growth penalty. We generated lines overexpressing 14 individual ABA receptors under the guard cell-specific promoter and examined leaf growth, transpiration, water potential, and net carbon assimilation rate (A). We found that guard-cell overexpression of specific ABA receptors reduced transpiration and A compared to the wild-type under well-watered conditions, but leaf growth was not adversely affected. The mechanism involved receptor-mediated reduction in transpiration and resultant improved leaf water potential for efficient intermediate-aged leaf growth. Under drought, these lines showed enhanced WUE without compromising biomass accumulation. Our findings highlight activation of ABA signaling in guard cells as a promising strategy for developing water-saving crops without growth trade-offs.