Yuan Hui-Jun, Ma Qing, Wu Guo-Qiang, Wang Pei, Hu Jing, Wang Suo-Min
State Key Laboratory of Grassland Agro-ecosystems, College of Pastoral Agriculture Science and Technology, Lanzhou University, Lanzhou 730020, PR China.
School of Life Science and Engineering, Lanzhou University of Technology, Lanzhou 730050, PR China.
Ann Bot. 2015 Feb;115(3):495-507. doi: 10.1093/aob/mcu177. Epub 2014 Sep 24.
In order to cope with arid environments, the xerohalophyte Zygophyllum xanthoxylum efficiently compartmentalizes Na(+) into vacuoles, mediated by ZxNHX, and maintains stability of K(+) in its leaves. However, the function of ZxNHX in controlling Na(+) and K(+) homeostasis at the whole-plant level remains unclear. In this study, the role of ZxNHX in regulating the expression of genes involved in Na(+) and K(+) transport and spatial distribution was investigated.
The role of ZxNHX in maintaining Na(+) and K(+) homeostasis in Z. xanthoxylum was studied using post-transcriptional gene silencing via Agrobacterium-mediated transformation. Transformed plants were grown with or without 50 mm NaCl, and expression levels and physiological parameters were measured.
It was found that 50 mm NaCl induced a 620 % increase in transcripts of ZxSOS1 but only an 80 % increase in transcripts of ZxHKT1;1 in roots of wild-type (WT) plants. Consequently, the ability of ZxSOS1 to transport Na(+) exceeded that of ZxHKT1;1, and Na(+) was loaded into the xylem by ZxSOS1 and delivered to the shoots. However, in a ZxNHX-silenced line (L7), the capacity to sequester Na(+) into vacuoles of leaves was weakened, which in turn regulated long-distance Na(+) transport from roots to shoots. In roots of L7, NaCl (50 mm) increased transcripts of ZxSOS1 by only 10 %, whereas transcripts of ZxHKT1;1 increased by 53 %. Thus, in L7, the transport ability of ZxHKT1;1 for Na(+) outweighed that of ZxSOS1. Na(+) was unloaded from the xylem stream, consequently reducing Na(+) accumulation and relative distribution in leaves, but increasing the relative distribution of Na(+) in roots and the net selective transport capacity for K(+) over Na(+) from roots to shoots compared with the WT. Silencing of ZxNHX also triggered a downregulation of ZxAKT1 and ZxSKOR in roots, resulting in a significant decrease in K(+) accumulation in all the tissues in plants grown in 50 mm NaCl. These changes led to a significant reduction in osmotic adjustment, and thus an inhibition of growth in ZxNHX-silenced lines.
The results suggest that ZxNHX is essential for controlling Na(+), K(+) uptake, long-distance transport and their homeostasis at whole-plant level via feedback regulation of the expression of genes involved in Na(+), K(+) transport. The net result is the maintenance of the characteristic salt accumulation observed in Z. xanthoxylum and the regulation of its normal growth. A model is proposed for the role of ZxNHX in regulating the Na(+) transport system in Z. xanthoxylum under saline conditions.
为了应对干旱环境，旱生盐生植物霸王（Zygophyllum xanthoxylum）通过ZxNHX的介导，有效地将Na⁺区室化到液泡中，并维持其叶片中K⁺的稳定性。然而，ZxNHX在整株水平上控制Na⁺和K⁺稳态的功能仍不清楚。在本研究中，研究了ZxNHX在调节参与Na⁺和K⁺运输及空间分布的基因表达中的作用。
通过农杆菌介导的转化利用转录后基因沉默研究了ZxNHX在霸王维持Na⁺和K⁺稳态中的作用。将转化植株在有或无50 mM NaCl的条件下培养，并测定其表达水平和生理参数。
发现50 mM NaCl诱导野生型（WT）植株根中ZxSOS1转录本增加620%，但ZxHKT1;1转录本仅增加80%。因此，ZxSOS1运输Na⁺的能力超过ZxHKT1;1，Na⁺由ZxSOS1装载到木质部并输送到地上部。然而，在ZxNHX沉默株系（L7）中，将Na⁺隔离到叶片液泡中的能力减弱，这反过来又调节了从根到地上部的长距离Na⁺运输。在L7的根中，NaCl（50 mM）仅使ZxSOS1转录本增加10%，而ZxHKT1;1转录本增加53%。因此，在L7中，ZxHKT1;1对Na⁺的运输能力超过ZxSOS1。Na⁺从木质部液流中卸载，从而减少了叶片中Na⁺的积累和相对分布，但与WT相比，增加了根中Na⁺的相对分布以及从根到地上部对K⁺相对于Na⁺的净选择性运输能力。ZxNHX的沉默还引发了根中ZxAKT1和ZxSKOR的下调，导致在50 mM NaCl条件下生长的植株所有组织中K⁺积累显著下降。这些变化导致渗透调节显著降低，从而抑制了ZxNHX沉默株系的生长。
结果表明，ZxNHX通过对参与Na⁺、K⁺运输的基因表达的反馈调节，在整株水平上控制Na⁺、K⁺吸收、长距离运输及其稳态中至关重要。最终结果是维持霸王中观察到的特征性盐积累并调节其正常生长。提出了一个ZxNHX在盐胁迫条件下调节霸王Na⁺运输系统作用的模型。
