Abstract
Water availability stands as one of the most important limiting factor influencing crop productivity. Unraveling the mechanisms governing plant perception of water deficit has become crucial for agriculture, particularly in the context of frequent drought events due to climate change. Plant water potential is primarily composed of two biophysical parameters, the turgor potential (P) and the osmotic potential (Π), that will determine water movements in the soil-plant-atmosphere continuum. Here, decline in external Π and P reduction caused by mild water deficit (MWD, where cortical cells in the elongation zone of the primary root maintained turgidity) were considered as input signals, while early changes in gene expression was taken as a readout. To study the whole-genome transcriptional responses to variations in Π and P, water deficit was induced by treatments of the root with various osmolytes: NaCl, sorbitol, polyethylene glycol 8000 and ethylene glycol (EG), into the hydroponic solution. To further explore the regulatory mechanism of gene expression, we observed the effects caused by the transcriptional inhibitor cordycepin and genetic knockouts of components of mRNA decay pathways. Finally, we aimed at developing Π- and P- reporters, for which we used the regulatory elements (promoter and 3'UTR) of Π- and P- correlated genes to drive the expression of reporters.Transcriptomic analysis revealed a few Π- and P- correlated genes. Under MWD treatments, the diffusing solute EG provoked less P reduction, thereby allowing us to distinguish quantitative correlations to Π from correlations to P. A P-correlated At1G64640 (ENODL8) and Π-correlated gene At3G14440 (NCED3) were studied in more details. Extending the correlations to more severe treatments and over time showed that plasmolysis of cortical cells altered their response pattern, and that ENODL8 was probably a true P-correlated gene under MWD treatments, while NCED3 only exhibited a quantitative response to changes in P in the first 30 min. Manipulating their promoter activity and mRNA degradation showed that both ENODL8 and NCED3 expressions were regulated by water deficit at both transcriptional and post-transcriptional levels.Studying Sluc signals in transgenic plants expressing a pNCED3::Sluc-3'UTR construct in response to water deficit demonstrated that this construction could be used as an osmo-reporter in Arabidopsis. Analyzing correlations between Sluc signal and Π or P suggested that, during a long-term treatment, Sluc signal in plants report changes in the external Π, while at the early stage of treatments (30-60 min), Sluc signal report variations in P caused by MWD. Combined with the promoter activity of ENODL8, our findings suggest that the perception site may be located in the elongation and meristem zones under MWD, and in the hypocotyl and cotyledons under SWD. Treating plants expressing the osmo-reporter with fluridone, an inhibitor of ABA synthesis and ISX, a compound impairing cell wall integrity showed that initiation of its response was independent of those.In conclusion, our work showed that reduction of P and Π caused by water deficit governs gene expression at both transcriptional and post-transcriptional levels. We also provide two reliable P- and Π- marker genes, and developped an “osmo-reporter”.