Endogenous ^• NO accumulation in soybean is associated with initial stomatal response to water deficit

Drought is the main cause of productivity losses in soybean plants, triggering physiological and biochemical responses, stomatal closure being essential to prevent water losses and thus mitigate the negative effects of drought. Abscisic acid (ABA) is the main molecule involved in stomatal closure under drought conditions along with nitric oxide (^• NO). However, the role of ^• NO in this process is not yet fully understood and contrasting findings about its role have been reported. Most of the assays in the literature have been carried out under in vitro conditions using ^• NO donors or scavengers, but little is known about the effects of endogenously produced ^• NO under drought conditions. This study is aimed to determine the pattern of endogenous ^• NO accumulation from the establishment of water stress and how this relates to stomatal closure and other biochemical and physiological responses. The analysis of soybean plant responses to drought revealed no correlation between whole-leaf ^• NO accumulation and typical water-deficit stress markers. Moreover, ^• NO accumulation did not explain oxidative damage induced by drought. However, endogenous ^• NO content correlated with the early stomatal closure. Analysis of stomatal behavior and endogenous ^• NO content in guard cells through epidermal peel technique showed a stomatal population with high variation in stomatal opening and ^• NO content under the initial stages of water stress, even when ABA responses are activated. Our data suggest that upon early stress perception, soybean plants respond by accumulating ^• NO in the guard cells to inhibit stomatal closure, potentially through the inhibition of ABA responses.