Significance of xylem translocated sulfate in early responses of stomata to drought in poplar plants

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Water deficiency, sensed by the root, is supposed to be signaled via xylem transport from the root to the shoot by chemical as well as hydraulic signals. In response to these signals stomatal conductance is reduced to prevent excessive water loss. The chemical signal mostly responsible for stomatal closure in response to drought is thought to be the photohormone abscisic acid (ABA). However, the origin of the ABA involved in this process is still a matter of debate, since it can be synthesized in roots and the shoot. Recent experiments indicated that increasing sulfate concentrations in the xylem constitute an early response to drought; therefore, it has been hypothesized that xylem-borne sulfate accelerates the ABA signal for stomatal closure and that enhanced ABA in the leaves in response to drought does not necessarily originate from synthesis in roots; acceleration of stomatal closure by sulfate is thought to be achieved by activating malate efflux channels of guard cells. To test this hypothesis, (i) mass transport of ABA and sulfate in the xylem, (ii) its sources and the sinks during drought, (iii) drought mediated regulation of expression of sulfate transporters / anion channels, and (iv) the effects of sulfate and ABA on malate efflux from isolated guard cells will be studied. Experiments will be performed with wild type and transgenic poplar lines with altered expression of sulfate transporters, enhanced sulfate use for reduction, and in ABA insensitive mutants.