Role Of Nitric Oxide And Root Exudates In Aluminum Toxicity And Resistance In Plants

Aluminum (Al) toxicity is the most significant-deleterious factor limiting crop growth on acid soils. Al can rapidly inhibit the elongation of plant roots, but the underlying principle is still unclear. Al resistance mechanisms in plant have been classified into external exclusion and internal tolerance. Among them, secretion of organic acid anions is one of the most important mechanisms of external exclusion. Nitric oxide (NO) is involved in regulation of many physiological processes, including plant response to Al stress.In the present study, we investigated the role of NO in root growth responses to Al stress in rice bean. We compared the organic acid anions secretion, Al-induced oxidative stress, cell wall composition, and proposed a reverse view to that exogenous NO can alleviate Al-induced inhibition of root elongation and increase aluminum tolerance of plant. To further clarify the mechanism of Al toxicity resistant in rice bean, we use semi-quantitative RT-PCR method to study the MATE gene expression in four legume plants. On the other hand, phosphorus (P) deficiency and Al toxicity often co-exist in acid soils. Therefore, the study of plant physiological mechanisms to Al-P interaction and the response to double stresses is meaning to future research. The study also examined the effect of P deficiency and Al toxicity on root exudates in amaranth. The results were as follows:1. Role of NO for Al resistance in rice beanHere we found that exogenous application of the NO donor sodium nitroprusside (SNP) exacerbated the inhibition of Al-induced root growth in rice bean. This was accompanied by an increased accumulation of Al in the root apex. However, Al treatments had no effect on endogenous NO concentrations in root apices. These results indicate that a change in NO concentration is not the cause of Al-induced root growth inhibition and the adverse effect of SNP on Al-induced root growth inhibition should result from increased Al accumulation. Al could significantly induce citrate efflux but SNP had no effects on citrate efflux either in the absence or presence of Al. On the other hand, SNP pretreatment significantly increased Al-induced malondialdehyde accumulation and Evans Blue staining, indicating an intensification of the disruption of plasma membrane integrity. Furthermore, SNP pretreatment also caused greater induction of pectin methylesterase activity by Al, which could be the cause of the increased Al accumulation. Taken together, it is concluded that NO exacerbates Al-induced root growth inhibition by affecting cell wall and plasma membrane properties.2. The relationship between MATE gene and Al-tolerance in four legume plantsFour legume plants different in Al resistance were selected in this study. We found that Liandu rice bean, Pingyang adsuki bean and Wencheng adsuki bean three species have higher Al-resistant than Quzhou rice bean, and Al-induced root growth inhibition may be due to the Al accumulation in root. On the other side, the citrate efflux significantly increased in roots of Quzhou rice bean rather than Liandu rice bean, Pingyang adsuki bean and Wencheng adsuki bean three species; while the citrate secretion in Pingyang adsuki bean root was lower than others. The semi-quantitative RT-PCR expression analysis revealed that the MATE gene expression in Pingyang adsuki bean root is very weak, far lower than the other three kinds of beans. By contrast, the MATE gene expression of Quzhou rice bean was increased significantly under 50μM Al treatment. This implies that the mechanism of root organic acid secretion chelating with aluminum can not completely explain the differences between four kinds of leguminous plants in Al-tolerance. We believe that the Al tolerance of Pingyang adsuki bean may be caused by other Al resistance mechanism.3. The effect of P deficiency and Al toxicity on root secretion in amaranthIn acid soils, phosphorus deficiency and aluminum toxicity are two inseparable factors limiting crop production, while organic acid secretion is considered to be the common mechanism of P deficiency and Al toxicity. Our study showed that there are different mechanisms in response to Al-induced organic acid secretion and P deficiency-induced organic acid secretion. P deficiency and Al toxicity can both significantly induce increase of phenolic compounds secretion and accumulation of endogenous phenolic compounds. Moreover, double stresses induced higher secretion than either treatment alone, indicating that phenolie compounds may play an important role in mechanisms of P deficiency and Al tolerance in amaranth.