Mechanisms Of Alkali Tolerance In Chloris Virgata And Rice (Oryza Sativa)

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.