Significance of xylem translocated sulfate in early responses of stomata to drought in poplar plants
Final Report Abstract
Aim of the DFG Project was to test the hypothesis that sulfate is the early root-to-shoot transported signal in response to drought which regulates ABA-dependent stomatal closure. Among potential chemical signals, the phytohormone ABA is probably the most relevant regulator of leaf stomatal conductance. Nevertheless, the origin of ABA involved in early responses of stomata to soil water deficit is still a matter of debate. Early studies support the view that root derived ABA communicates soil water deficit from the root to the shoot and causes a decline in stomatal conductance. More recent reports questioned the role of ABA as primary long-distance signal synthesized by the roots upon drought. The present project showed that upon drought, sulfate concentrations in the xylem sap of poplar increased and caused stomatal closure before an increase in xylem sap ABA contents was observed. Apparently, accumulation of sulfate in the xylem sap during drought was not achieved by enhanced expression of transporters responsible for sulfate uptake by the roots, but jointly by repression of sulfate transporters involved in sulfate retrieval from the xylem sap and by enhanced expression of an anion channel responsible for sulfate release from xylem parenchyma cells. The significance of sulfate transporters for sulfate contents in the xylem sap was also investigated during annual growth in connection with sulfate storage and mobilization in poplar (Populus tremula x P. alba). This study also clearly indicated that the expression of sulfate transporters in the wood controls sulfate contents in the xylem sap. Sulfate fed to the petiole of detached leaves of poplar and Arabidopsis and to peeled epidermis sections of Arabidopsis diminish stomatal conductance and reduced stomata aperture, respectively. It was shown that sulfate reaching guard cells promotes ABA induced stomatal closure by voltage-gating anion channel ALMT12/QUAC1 of the guard cells open. Furthermore, sulfate induced NCED3 expression, a key-step in ABA synthesis. These results support the hypothesis that sulfate triggers guard cell autonomous ABA-synthesis. Altogether, the outcome of this project clearly indicates a pivotal role of sulfate in signaling drought stress from the roots to the shoot and in mediating stomatal closure upon drought.
Publications
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(2013) Differential expression of specific sulphate transporters underlies seasonal and special patterns of sulphate allocation in trees. Plant Cell Environ. 36, 1285-1295
F. Malcheska, A. Honsel, H. Wildhagen, J. Dürr, C. Larich, H. Rennenberg, C. Herschbach
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Drought enhanced xylem sap sulfate closes stomata by affecting ALMT12 and guard cell ABA synthesis. Plant Physiology Jun 2017, 174 (2) 798-814
F. Malcheska, A. Ahmad, S. Batoo, H.M. Müller, J. Ludwig-Müller, J. Kreuzwieser, D. Randewig, R. Hänsch, R.R. Mendel, R. Hell, M. Wirtz, D. Geiger, P. Ache, R. Hedrich, C. Herschbach, H. Rennenberg
