【作者】 杨春武；

【导师】 石德成；

【作者基本信息】 东北师范大学 ， 植物学， 2010， 博士

【摘要】 土壤碱化与土壤盐化往往相伴发生,大量试验事实已经证明碱胁迫对植物的伤害远大于盐胁迫。然而,碱胁迫这一严重的环境问题却常常被人们所忽视甚至与盐胁迫混淆,至今尚未见对植物抗碱机制系统而深入的研究。据此,本文以探讨尚未明了的植物抗碱机制为目的,选择了抗碱盐生植物虎尾草(Chloris virgata)为主要研究对象,以甜土植物水稻(Oryza sativa)为对比材料。系统探讨并比较了盐、碱两种胁迫对它们的影响以及二者抗碱生理及分子机制。除此之外,还对虎尾草适应天然盐碱生境的生理生态机制进行了初步研究。其主要结果与结论如下:一碱胁迫对植物的作用机制及植物抗碱生理适应机制结果表明,虎尾草和水稻根生长对碱胁迫的响应迥异,意味着根的pH调节能力可能是决定植物抗碱性的关键所在。这一点通过根分泌实验得到了充分的证明。虎尾草可以通过分泌大量乙酸和甲酸来实现根外pH调节,而水稻有机酸分泌量则很少无法有效地降低碱胁迫造成的高pH,因此受害严重。进一步采用无损伤离子扫描微电极技术对碱胁迫下虎尾草根表面pH和H+流进行检测,结果表明其根表面pH基本不变,同时有H+内流现象。综合以上结果,可以作出推断,即虎尾草根外pH调节作用可能主要在根表皮细胞壁内或根皮层质外体空间中进行。由于虎尾草根系对高pH环境具有较强的适应性调节作用,因而在一定强度的碱胁迫下能将高pH危害抵御在体外,使体内环境不受影响,此时虎尾草对碱胁迫的响应与盐胁迫类似。但是当碱胁迫强度超过根系调节能力时,茎叶中Na+浓度急剧增高,造成叶绿体破坏、光合色素含量下降,气孔导度及碳同化能力也急剧下降。水稻在较低碱胁迫强度下就出现了由于体内Na+积累而表现出的碱害现象。因而可以认为,碱胁迫导致茎叶内Na+过度积累可能是碱胁迫对植物伤害大于盐胁迫最主要的原因,茎叶内Na+过度积累可能与碱胁迫的高pH阻遏根系对Na+外排作用有关。在细胞内Na+含量急剧增加的同时,碱胁迫又明显减少茎叶中无机阴离子含量(主要是NO3-),造成茎叶内负电荷亏缺、离子严重失衡和pH不稳定,进而引起一系列链锁的胁变反应。本文实验证实,虎尾草和水稻均通过积累大量有机酸来维持细胞内离子平衡和pH稳定,但二者有机酸代谢调节方式可能有所不同。虎尾草在盐胁迫和非胁迫条件下均积累较多有机酸,而水稻在同样条件下仅合成痕量有机酸。虽然碱胁迫均能诱导二者大量积累有机酸,但水稻的响应似乎比虎尾草更敏感,低强度碱胁迫即可诱导水稻有机酸大量积累,而虎尾草有机酸积累则仅能被较高强度的碱胁迫所诱导。这似乎表明有机酸积累可能是一个被动的适应性调节过程,只有在碱胁迫强度超过根外pH调节限度致使Na+大量涌入造成体内阴离子亏缺时,体内才被动积累有机酸用于细胞内离子平衡和pH调节。二虎尾草适应天然盐碱生境的生理生态机制为验证室内实验的结论,对天然盐碱生境下的虎尾草也进行了野外取样测定。所得结果使上述结论得到了进一步支持。野外实验结果表明:虎尾草在天然盐碱生境下主要依靠吸收大量无机离子和合成大量有机溶质来抵抗渗透胁迫;采取在根中控制Na+促进K+向茎叶运输的策略来减少离子毒害;通过积累以苹果酸和柠檬酸为主的有机酸来维持细胞内离子平衡和pH稳定;通过分泌有机酸调节根际微环境pH以缓解营养胁迫。三碱胁迫下水稻的基因表达调节如上所述,碱胁迫干扰Na+-K+吸收和相关代谢是碱害的生理基础,维持Na+-K+平衡可能就是植物抗碱的最终体现。为此,本文分析了与水稻Na+-K+吸收和控制有关的基因表达包括SOS信号系统基因、NHX基因家族两个成员、HKT基因家族6个成员以及一些钾离子通道基因AKT1、HAK基因家族。结果表明碱胁迫强烈刺激HKT家族、NHX家族、AKT1、HAK基因家族及SOS信号系统在根和茎叶中的表达。我们推测碱胁迫下这些基因在水稻维持Na+-K+平衡过程中可能起至关重要的作用。碱胁迫下SOS途径,HKT家族以及NHX家族过表达可能有助于减轻碱胁迫造成的茎叶高Na+毒害。而AKT1和HAK家族一些成员可能主要在根和茎叶交界处表达,介导根对K+的吸收和向茎叶装载K+以保证茎叶钾营养。更多还原

【Abstract】 Soil alkalization and salinization frequently co-occur. Soil alkalization has become a global environmental problem. However, relatively little attention has been given to this problem, and the insight into mechanisms of alkali tolerance was lacking. In present study, we chosen a glycophyte rice (Oryza sativa) and an alkali-tolerant halophyte (Chloris virgata) as the test organisms, compared the physiological responses of them to salt and alkali stresses, and probed the physiological and molecular mechanisms by which they resist alkali stress. In addition, we also studied eco-physiological adaptive mechanisms of C. virgata to its salt-alkaline habitat. Major conclusions were as follows:1. The effects of alkali stress on plants and the physiological adaptive mechanisms of plants to alkali stressThe results indicated that the sensitiver of rice root to alkali stress was much greater than C. virgata root, revealing that pH adjustment ability may was vital factor decided alkali tolerance, and that pH adjustment outside roots was the central mechanism by which plants resist alkali stress. This point was supported by root exudation experiment. Under alkali stress, rice roots secreted only small concentrations of organic acid (OA), while C. virgata root secreted volume of OA, especially acetic and formic acids. Therefore, C. virgata is able to regulate the pH outside roots and protect roots from high-pH injury. However, OA secretion appeared insufficient to adequately lower the pH of root medium. If OA secretion was localized only at the root surface or the apoplast in cortex, then it could prevent root damage from high pH. Therefor, we detected H+ flux and pH on the surface of C. virgata root using Scanning Ion-selective Electrode Technique (SIET). The results showed that under alkali stress, the pH on the surface of C. virgata root did not lower, and alkali stress induced a H+ influx into the cell wall. According to above results, we propose OA release may be localized only at the root surface or the apoplast in the cortex because this can significantly reduce the metabolic cost of pH regulation. As C. virgata have strong pH regulation ability, under moderate alkali stress, the harmful effect of high pH was resisted by pH adjustment outside the roots and consequently the intracellular environment was not affected, here the response of C. virgata to alkali stress was similar to salt stress. However, when stress intensity exceeded the capacity of root adjustment, the alkali stress led to the sharp increase of Na+ content in shoots, damage the photosynthetic system, and reduce highly net photosynthetic rate and stomatal conductance. For rice, only low alkali stress induced sharp increse of Na+ content in shoots and present high-pH injury action. This indicated that the Na+ excess in shoot caused by alkali stress might be major factor why alkali stress are more destructive to plants than salt stress. In addition, the increased Na+ in shoots under alkali stress might also be related to possible decreased Na+ exclusion.Alkali stress, especially strong alkali stress, caused the massive influx of Na+ and the decrease of inorganic negative charge. The lack of inorganic negative charge and the Na+ excess made together severe deficit of negative charge, breaks the intracellular ionic balance and pH homeostasis, cause a series of strain response. Our results showed that rice and C. virgata both enhanced the synthesis of OAs to compensate for the shortage of inorganic anions, and that OA metabolic regulation might be a key pathway for in vivo pH adjustment. However, their OA metabolic regulation pathway might be different. Under salt stress or non stress, rice only accumulated trace of OA, whereas C. virgata accumulated relative high concentration of OA. Though alkali stress induced the OA accumulation in both rice and C. virgata, the OA accumulation in rice was particular sensitive to alkali stress. Only low alkali stress induced OA accumulation in rice. However, for C. virgata, when stress intensity exceeded the capacity of root adjustment, alkali stress resultd in deficit of negative charge and led to OA accumulation. OA accumulation migh occur only when pH regulation outside roots was failure, suggesting that OA accumulation migh a passive adaptive response to deficit of negative charge.2. Eco-physiological adaptive mechanisms of C. virgata to natural salt-alkaline habitatIn order to test conclusions of controlled experiment, the correlative field experiment also was performed. The conclusions in field experiment was basically accordant with controlled experiment. The field experiment results showed that, under natural salt-alkaline mixed stress, C. virgata absorbs inorganic ions and synthesizes organic solute to resist osmotic stress; controls absorption or transport of Na+ and K+ in roots to reduce the ion injury in shoot; accumulates the OA dominated by malate and citrate to keep intracellular ionic balance and steady pH; secrets OA to lower the pH of root microenvironment and resist nutrient stress.3. Alkali stress induces the alteration of gene expression in riceAbove results have revealed that it is the basis of alkali injury to interfere with the metabolisms of Na+ and K+. Maintaining Na+-K+ balance is the ultimate result of alkali tolerance. In this study, we detected the expressions of several genes involved in Na+-K+ balance, including AKT, SOS signal system, NHX family, HKT family and HAK family. The result showed salt stress only have small effects on these genes in rice, but alkali stress stimulused strongly their expressions in roots and shoots. We hypothesize that rice HKT family, NHX family and SOS pathway might play the important roles in protecting shoots from high-Na+ injury caused by alkali stress, especially in controlling long-distance Na+ transport from roots to shoots. Under alkali stress, the overexpressions of OsHAKs and OsAKT1 might contribute in the release of K+ from roots to shoot, and maintain the potassium nutrition supply of shoots. If so, this will can explain why both K+ content and K+/Na+ in shoot also were much higher than in roots under alkali stress. Although we did not test the functions of these genes in alkali tolerance, their response to alkali stress indicated that they might play important roles in rice alkali tolerance. Therefore, we propose that these genes as candidate genes in alkali tolerance should be investigated in future.更多还原