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

缺水，由根感知，被认为是通过木质部运输从根到茎的化学信号和水力信号的信号。响应这些信号，气孔导度降低以防止水分过多流失。干旱条件下气孔关闭的主要化学信号是光激素脱落酸（ABA）。然而，参与这一过程的ABA的来源仍然是一个有争议的问题，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是对干旱的早期反应；因此，假设木质部传播的硫酸盐加速了ABA信号，导致气孔关闭，叶片中ABA响应干旱的增强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护细胞的苹果酸外排通道来实现的。为了验证这一假设，将研究(i)木质部中ABA和硫酸盐的质量运输，（ii）干旱期间ABA的来源和汇，（iii）干旱介导的硫酸盐转运体/阴离子通道的表达调节，以及（iv）硫酸盐和ABA对孤立保卫细胞苹果酸外排的影响。实验将在硫酸盐转运蛋白表达改变的野生型和转基因杨树系以及ABA不敏感突变体中进行。

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.