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

根感受到的水分缺乏，应该是通过化学信号和水力信号通过木质部从根到地上部的运输来发出信号。作为对这些信号的响应，气孔导度被降低，以防止过度失水。光激素脱落酸(ABA)被认为是导致气孔关闭响应干旱的主要化学信号。然而，参与这一过程的ABA的来源仍然存在争议，因为它可以在根和茎中合成。最近的实验表明，木质部中硫酸盐浓度的增加构成了对干旱的早期反应；因此，人们假设木质部携带的硫酸盐加速了气孔关闭的ABA信号，而叶片中增强的ABA对干旱的响应并不一定来自根的合成；硫酸盐加速气孔关闭被认为是通过激活保卫细胞的苹果酸外流通道实现的。为了验证这一假说，我们将研究(1)ABA和硫酸盐在木质部中的质量运输；(2)干旱期间ABA的源和库；(3)干旱对硫酸盐转运体/阴离子通道表达的调节；(4)硫酸盐和ABA对分离保卫细胞苹果酸外流的影响。实验将在野生型和转基因杨树品系中进行，这些品系的硫酸盐转运蛋白表达发生了变化，硫酸盐还原利用增强，以及ABA不敏感突变体。

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.