Donald Danforth Plant Science Center, 975 North Warson Rd, St Louis, MO 63132, USA.
J Exp Bot. 2010 Jul;61(12):3395-405. doi: 10.1093/jxb/erq160. Epub 2010 Jun 21.
Recent reports suggest that early sensing of soil water stress by plant roots and the concomitant reduction in stomatal conductance may not be mediated by root-sourced abscisic acid (ABA), but that other xylem-borne chemicals may be the primary stress signal(s). To gain more insight into the role of root-sourced ABA, the timing and location of the expression of genes for key enzymes involved in ABA biosynthesis in Zea mays roots was measured and a comprehensive analysis of root xylem sap constituents from the early to the later stages of water stress was conducted. Xylem sap and roots were sampled from plants at an early stage of water stress when only a reduction in leaf conductance was measured, as well as at later stages when leaf xylem pressure potential decreased. It was found that the majority of ABA biosynthetic genes examined were only significantly expressed in the elongation region of roots at a later stage of water stress. Apart from ABA, sulphate was the only xylem-borne chemical that consistently showed significantly higher concentrations from the early to the later stages of stress. Moreover, there was an interactive effect of ABA and sulphate in decreasing maize transpiration rate and Vicia faba stomatal aperture, as compared to ABA alone. The expression of a sulphate transporter gene was also analysed and it was found that it had increased in the elongation region of roots from the early to the later stages of water stress. Our results support the suggestion that in the early stage of water stress, increased levels of ABA in xylem sap may not be due to root biosynthesis, ABA glucose ester catabolism or pH-mediated redistribution, but may be due to shoot biosynthesis and translocation to the roots. The analysis of xylem sap mineral content and bioassays indicate that the anti-transpirant effect of the ABA reaching the stomata at the early stages of water stress may be enhanced by the increased concentrations of sulphate in the xylem which is also transported from the roots to the leaves.
最近的报告表明，植物根系对土壤水分胁迫的早期感知以及随之而来的气孔导度降低，可能不是由根源脱落酸（ABA）介导的，而是其他木质部携带的化学物质可能是主要的胁迫信号。为了更深入地了解根源 ABA 的作用，我们测量了玉米根系中参与 ABA 生物合成的关键酶基因的表达时间和位置，并对水分胁迫早期到后期根木质部汁液成分进行了全面分析。在水分胁迫的早期阶段，当仅测量到叶片导度降低时，以及在叶片木质部压力势降低的后期阶段，从植物中采集木质部汁液和根。结果发现，在所检查的大多数 ABA 生物合成基因中，只有在水分胁迫的后期阶段，根的伸长区才会显著表达。除了 ABA，硫酸盐是唯一一种在胁迫的早期到后期阶段都表现出浓度明显升高的木质部携带化学物质。此外，与单独使用 ABA 相比，ABA 和硫酸盐在降低玉米蒸腾速率和蚕豆气孔开度方面存在交互作用。还分析了硫酸盐转运体基因的表达，发现它在水分胁迫的早期到后期阶段，在根的伸长区表达增加。我们的结果支持这样的观点，即在水分胁迫的早期阶段，木质部汁液中 ABA 水平的升高可能不是由于根系生物合成、ABA 葡萄糖酯的分解代谢或 pH 介导的再分配，而是可能由于地上部分的生物合成和向根系的转运。木质部汁液矿质含量分析和生物测定表明，在水分胁迫的早期阶段到达气孔的 ABA 的抗蒸腾作用可能会因木质部中硫酸盐浓度的增加而增强，硫酸盐也从根部运输到叶片。
