Boron Toxicity Reduces Water Transport from Root to Shoot in Arabidopsis Plants. Evidence for a Reduced Transpiration Rate and Expression of Major PIP Aquaporin Genes

Toxic boron (B) concentrations cause impairments in several plant metabolic and physiological processes. Recently we reported that B toxicity led to a decrease in the transpiration rate of Arabidopsis plants in an ABA-dependent process within 24 h, which could indicate the occurrence of an adjustment of whole-plant water relations in response to this stress. Since plasma membrane intrinsic protein (PIP) aquaporins are key components influencing the water balance of plants because of their involvement in root water uptake and tissue hydraulic conductance, the aim of the present work was to study the effects of B toxicity on these important parameters affecting plant water status over a longer period of time. For this purpose, transpiration rate, water transport to the shoot and transcript levels of genes encoding four major PIP aquaporins were measured in Arabidopsis plants treated or not with a toxic B concentration. Our results indicate that, during the first 24 h of B toxicity, increased shoot ABA content would play a key role in reducing stomatal conductance, transpiration rate and, consequently, the water transport to the shoot. These physiological responses to B toxicity were maintained for up to 48 h of B toxicity despite shoot ABA content returning to control levels. In addition, B toxicity also caused the down-regulation of several genes encoding root and shoot aquaporins, which could reduce the cell to cell movement of water in plant tissues and, consequently, the water flux to shoot. All these changes in the water balance of plants under B toxicity could be a mechanism to prevent excess B accumulation in plant tissues.

有毒硼（B）浓度会导致植物的一些代谢和生理过程受损。最近我们报道，在24小时内，硼中毒会使拟南芥植株的蒸腾速率在一种依赖脱落酸（ABA）的过程中降低，这可能表明植株整体水分关系会针对这种胁迫进行调整。由于质膜内在蛋白（PIP）水通道蛋白因其参与根系吸水和组织水力传导而成为影响植物水分平衡的关键成分，本研究的目的是在更长时间段内研究硼中毒对这些影响植物水分状况的重要参数的影响。为此，我们测量了用或未用有毒硼浓度处理的拟南芥植株的蒸腾速率、向地上部分的水分运输以及编码四种主要PIP水通道蛋白的基因的转录水平。我们的结果表明，在硼中毒的最初24小时内，地上部分脱落酸含量的增加在降低气孔导度、蒸腾速率以及进而向地上部分的水分运输方面起着关键作用。尽管地上部分脱落酸含量恢复到对照水平，但这些对硼中毒的生理反应在硼中毒长达48小时内仍能维持。此外，硼中毒还导致编码根和地上部分水通道蛋白的几个基因下调，这可能会减少植物组织中细胞间的水分移动，进而减少向地上部分的水分通量。硼中毒条件下植物水分平衡的所有这些变化可能是一种防止植物组织中硼过度积累的机制。