Aluminum toxicity effect on phosphate nutrition and expression of phosphate transporter genes from ryegrass plants
The beef and dairy market in Southern Chile is based on grassland production and the main forage species is perennial ryegrass (Lolium perenne L) Large portions of soil used for this activity are acidic, with low phosphorus (P) availability and high levels of soluble aluminum (Al) affecting not only yield but also the quality and persistence of pastures. Previous studies have reported that P addition could alleviate Al toxicity in plants, but the influence of P nutrition on Al phytotoxicity is still controversial. Therefore, obtaining genotypes more P efficient (uptake and distribution) in this species is likely to result in considerable economic and ecological benefits. Moreover, P efficient genotypes could ameliorate the effects of Al toxicity. However, P uptake and transport mechanism underlying Al stress remain unknown in the pasture grass. In chapter I, we present a general introduction of this doctoral thesis, showing the hypothesis and aims of this study. In chapter II we explore the general background of the response mechanisms of plants to phosphorus deficiency, focusing on phosphate (Pi) transporters PTH1s and their regulation in crops. This chapter aims to improve our knowledge on Pi uptake and transport by plants and how plants have developed strategies to cope with P deficiency mostly in acid soils. In chapter III, we present functional characterization and expression patterns of two PHT1 members from ryegrass plants (Lolium perenne) to determine their roles in the specificity of Pi transport. Yeast mutant complementation assays showed that LpPHT1;4 can complement the growth defect of the yeast mutant Dpho84 under Pi deficient conditions, whereas the yeast mutant expressing LpPHT1;1 was not able growth under Pi deficiency. These results suggest that LpPHT1;1 functions as a low affinity Pi transporter, whereas LpPHT1;4 acts as a high affinity Pi transporter to maintain Pi homeostasis under stress conditions in ryegrass plants. In chapter IV, we evaluated P nutrition and Al toxicity interaction on P uptake, antioxidant responses and the gene expression of phosphate transporters from ryegrass plants. Two ryegrass cultivars with different Al resistances, the Al tolerant Nui cultivar and the Al sensitive Expo cultivar were hydroponically grown under low (16 uM) and optimal (100 uM) P doses for 16 days. After P treatments, plants were exposed to Al doses (0 and 200 uM) under acidic conditions (pH 4.8) for 24 h. Al and P accumulation were higher in the roots of Nui than in those of Expo. Moreover, lower Al accumulation was found in shoots of Nui independently of the P supply. Oxidative stress induced by Al toxicity and P deficiency was more severe in the Al sensitive Expo. Expression levels of the L. perenne phosphate transporters were higher in Nui than in Expo. While LpPHT1 expression was up regulated by P deficiency and Al toxicity in both cultivars, LpPHT4 expression only increased in the Al tolerant cultivar. These data suggest that the Al tolerance of Nui is a consequence of Al immobilization by P mediated by the high expression of phosphate transporters. All these results are discussed in the final chapter (V) In addition, studies about an Al transporter from ryegrass also were discussed and shown in annex 2. We conclude that: (1) Ryegrass Pi transporters show highly conserved characteristics among Pi transporters from Poaceae family (2) The functional analysis suggests that LpPHT1;1 (low affinity Pi transporter) and LpPHT1;4 (high affinity Pi transporter) play important roles in Pi uptake and translocation in ryegrass (3) Al tolerance in ryegrass is mainly due to its high efficiency to transport P from growth solution and to its greater antioxidant activity allowing an increased alleviation of Al toxicity.