Aluminium-induced changes in the profiles of both organic acids and phenolic substances underlie Al tolerance in Rumex acetosa L.

The common sorrel, Rumex acetosa L. is well adapted to acid mineral soils with high availability of phytotoxic Al species. The mechanisms of Al resistance in this species are not established. Our goal was to assess the possible implications of organic acids and phenolic substances in Al detoxification in roots and shoots of this plant. R. acetosa plants were exposed in nutrient solution (pH 4.3) to a non-growth reducing Al concentration of 50 μM Al for 5 days. Exclusion of Al from root tips was visualized by haematoxylin staining. Tissue Al and Ca concentrations were analysed by ICP ES. Root and shoot concentrations of organic acids and phenolic substances were analysed by HPLC. A time-dependent (model II type) Al exclusion pattern in root tips was observed. Nonetheless, high Al concentrations accumulated in roots (1170 μg/g) and shoots (275 μg/g). Aluminium supply enhanced root citrate concentrations but decreased shoot organic acid levels. Aluminium induced high levels of anthraquinone in roots and of catechol, catechin and rutin in shoots. Aluminium resistance in R. acetosa implies both exclusion of Al from root tips and tolerance to high Al tissue concentrations. Citrate in roots and phenolics in shoots may bind Al in non-toxic form. Anthraquinones, as strong antioxidants, may play a role in a general defence response to the root stress.     

Recent progress in the research of external Al detoxification in higher plants: a minireview

Aluminum (Al) is highly toxic to plant growth. The toxicity is characterized by rapid inhibition of root elongation. However, some plant species and cultivars have evolved some mechanisms for detoxifying Al both internally and externally. In this review, the recent progress made in the research of external detoxification of Al is described. Accumulating evidence has shown that organic acids play an important role in the detoxification of Al. Some plant species and cultivars respond to Al by secreting citrate, malate or oxalate from the roots. Recently, the anion channel of malate and citrate in the plasma membrane has been characterized and a gene encoding the malate channel has been cloned. The metabolism of organic acids seems to be poorly correlated with the Al-induced secretion of organic acid anions. A number of QTLs (quantitative trait loci) for Al resistance have been identified in rice, Arabidopsis, and other species. Transgenic plants with enhanced resistance to Al have also been reported, but introduction of multiple genes may be required to gain high Al resistance in future.     

OsNRAMP5, a major player for constitutive iron and manganese uptake in rice

Manganese (Mn) and iron (Fe) are essential mineral micronutrients for plants and their deficiency and or toxicity represents a serious agricultural problem. In rice the information about genes involved in Mn uptake from soil is scarce. Recently, we showed that OsNRAMP5 is a plasma membrane protein involved in Mn and Fe transport. The concentration of Mn in roots, shoots and xylem sap of OsNRAMP5 RNAi (OsNRAMP5i) plants was significantly reduced compared with WT plants. The expression of OsNRAMP5 is not controlled by Fe deficiency in root and was also observed in pistil, ovary, lemma and palea. These data show that rice would utilize OsNRAMP5 for constitutive Fe and Mn uptake, while OsNRAMP5 would also play a role in Fe and Mn transport during flowering and seed development.     

Suppression of phospholipase Dγs confers increased aluminum resistance in Arabidopsis thaliana

Aluminum (Al) toxicity is the major stress in acidic soil that comprises about 50% of the world's arable land. The complex molecular mechanisms of Al toxicity have yet to be fully determined. As a barrier to Al entrance, plant cell membranes play essential roles in plant interaction with Al, and lipid composition and membrane integrity change significantly under Al stress. Here, we show that phospholipase Dγs (PLDγs) are induced by Al stress and contribute to Al-induced membrane lipid alterations. RNAi suppression of PLDγ resulted in a decrease in both PLDγ1 and PLDγ2 expression and an increase in Al resistance. Genetic disruption of PLDγ1 also led to an increased tolerance to Al while knockout of PLDγ2 did not. Both RNAi-suppressed and pldγ1-1 mutants displayed better root growth than wild-type under Al stress conditions, and PLDγ1-deficient plants had less accumulation of callose, less oxidative damage, and less lipid peroxidation compared to wild-type plants. Most phospholipids and glycolipids were altered in response to Al treatment of wild-type plants, whereas fewer changes in lipids occurred in response to Al stress in PLDγ mutant lines. Our results suggest that PLDγs play a role in membrane lipid modulation under Al stress and that high activities of PLDγs negatively modulate plant tolerance to Al.     

Cytosolic APX knockdown rice plants sustain photosynthesis by regulation of protein expression related to photochemistry,Calvin cycle and photorespiration

The biochemical mechanisms underlying the involvement of cytosolic ascorbate peroxidases (cAPXs) in photosynthesis are still unknown. In this study, rice plants doubly silenced in these genes (APX1/2) were exposed to moderate light (ML) and high light (HL) to assess the role of cAPXs in photosynthetic efficiency. APX1/2 mutants that were exposed to ML overexpressed seven and five proteins involved in photochemical activity and photorespiration, respectively. These plants also increased the pheophytin and chlorophyll levels, but the amount of five proteins that are important for Calvin cycle did not change. These responses in mutants were associated with Rubisco carboxylation rate, photosystem II (PSII) activity and potential photosynthesis, which were similar to non‐transformed plants. The upregulation of photochemical proteins may be part of a compensatory mechanism for APX1/2 deficiency but apparently the finer‐control for photosynthesis efficiency is dependent on Calvin cycle proteins. Conversely, under HL the mutants employed a different strategy, triggering downregulation of proteins related to photochemical activity, Calvin cycle and decreasing the levels of photosynthetic pigments. These changes were associated to strong impairment in PSII activity and Rubisco carboxylation. The upregulation of some photorespiratory proteins was maintained under that stressful condition and this response may have contributed to photoprotection in rice plants deficient in cAPXs. The data reveal that the two cAPXs are not essential for photosynthesis in rice or, alternatively, the deficient plants are able to trigger compensatory mechanisms to photosynthetic acclimation under ML and HL conditions. These mechanisms involve differential regulation in protein expression related to photochemistry, Calvin cycle and photorespiration.     

The role of organic acids in the short- and long-term aluminum tolerance in maize seedlings (<Emphasis Type="Italic">Zea mays</Emphasis> L.)

Aluminum (Al) affects numerous physiological processes in plants. However, Al tolerance mechanisms mediated by increased synthesis of organic acids (OAs) have been outlined recently. In this study, we examined the role of OAs in the short (1–8 h) and long-term (4 days) Al tolerance in maize seedlings. Exposure to Al stress for 4 days results in a rapid inhibition of root growth. Al induced morphological changes in the maize roots, especially at a higher solution of Al concentration (1,000 μM Al). The increase in Al accumulation in roots, including strongly elevated levels of Al accumulated in root cell walls suggests that Al tolerance in maize is mediated in part by higher accumulation of Al in the roots. The enhanced citrate exudation, which was only observed at 1,000 μM Al may lead to detoxification of Al by formation of OA–Al complexes in the root apoplast. This mechanism has been suggested to play a significant role in Al resistance response in maize. The short-term responses underlying internal detoxification via OA-chelators were also investigated. Succinate, malate, citrate and total root OA contents decreased markedly, 2 h after the Al exposure. At 4 and 8 h time points, OA contents increased or remained unchanged, except for that of malate which decreased. The level of OAs in shoots, on the other hand, showed alterations that were less pronounced in response to Al. Specifically, the citrate and total OA concentrations significantly increased at 4 h, but showed a pronounced decrease at the 8 h time point. Based on our findings, we propose that multiple responses, including Al exclusion by Al accumulation in root cells and citrate efflux, may contribute towards higher Al resistance in maize. The rapid OA changes in responses to short-term Al treatment may not be responsible for Al tolerance. However, increased OA synthesis observed in this study may be involved in diminishing the stress triggered by Al. The molecular aspects underlying Al resistance mechanism via Al-induced expression of the enzymes catalyzing OA synthesis and metabolism remain to be elucidated.     

Root phosphate exudation and pH shift in the rhizosphere are not responsible for aluminum resistance in rice

A series of hydroponic experiments and an agar culture experiment were carried out to investigate aluminum (Al) accumulation and translocation in two rice (Oryza sativa L.) cultivars (Kasalath and Koshihikari) that differ in Al resistance. Al-resistance mechanisms, including Pi exudation under Al stress and pH shifts in the rhizosphere, were also studied. Al content in rice shoots was 41 mg kg⁻¹ on average and did not differ between the two cultivars, which demonstrated that the rice cultivars were not Al accumulators. The majority of Al (95–97%) accumulated in roots. Al content in roots in the resistant cultivar (Koshihikari) was lower than that in the sensitive cultivar (Kasalath), which indicated that Al-exclusion mechanisms were mainly acting in rice. However, the rate of Pi exudation from the whole root or root tips was very low in both cultivars and was not significantly influenced by Al exposure, and thus seemed not to be the main Al-resistance mechanism. On the other hand, experiments with pH-buffered solution and color changes following culture in agar medium containing bromocresol purple revealed that the Al-induced pH increase could not explain the high Al resistance of rice. In addition, the Al content in shoots of Koshihikari was lower after the formation of iron plaque on the root surface, whereas that of Kasalath was not lower. These results suggested that rice roots cell wall components or root surfaces such as iron plaque, rather than pH changes and/or root exudates including organic acids and phosphate, play important roles in Al resistance in rice.     

Organ-specific analysis of the anaerobic primary metabolism in rice and wheat seedlings. I: Dark ethanol production is dominated by the shoots

During anaerobiosis in darkness the main route for ATP production in plants is through glycolysis in combination with fermentation. We compared the organ-specific anaerobic fermentation of flooding-tolerant rice (Oryza sativa) and sensitive wheat (Triticum aestivum) seedlings. A sensitive laser-based photoacoustic trace gas detection system was used to monitor emission of ethanol and acetaldehyde by roots and shoots of intact seedlings. Dark-incubated rice seedlings released 3 times more acetaldehyde and 14 times more ethanol than wheat seedlings during anaerobiosis. Ninety percent of acetaldehyde originated from shoots of both species. In comparison to wheat shoots, the high ethanol production of rice shoots correlated with larger amounts of soluble carbohydrates, and higher activities of fermentative enzymes. After 24 h of anaerobiosis in darkness rice shoots still contained 30% of aerated ATP level, which enabled seedlings to survive this period. In contrast, ATP content declined almost to zero in wheat shoots and roots, which were irreversibly damaged after a 24-h anaerobic period. When plants were anaerobically and dark incubated for 4 h and subsequently transferred back to aeration, shoots showed a transient peak of acetaldehyde release indicating prompt re-oxidation of ethanol. Post-anoxic acetaldehyde production was lower in rice seedlings than in wheat. This observation accounts for a more effective acetaldehyde detoxification system in rice. Compared to wheat the greater tolerance of rice seedlings to transient anaerobic periods is explained by a faster fermentation rate of their shoots allowing a sufficient ATP production and an efficient suppression of toxic acetaldehyde formation in the early re-aeration period.Angelika Mustroph and Elena I. Boamfa contributed equally to the paper.     

Physiological and genetic analyses of aluminium tolerance in rice, focusing on root growth during germination

Aluminium (Al) ion limits root growth of plants in acidic soils, and rice exhibits the highest level of Al-tolerance among graminous crops. To elucidate Al-tolerance mechanisms in rice, response to Al was compared between rice (Oryza sativa L., cv. Nipponbare) and wheat (Triticum aestivum L., cv. ET8), focusing on seminal root growth at seedling stage and germination stage. At seedling stage, rice and wheat were similarly sensitive to Al in both dose- and time-dependent manner during a 24-h Al exposure. On the contrary, at germination stage, rice was more tolerant to Al than wheat, and wheat roots displayed the loss of plasma membrane integrity more extensively than rice. A rice mutant exhibiting Al hypersensitivity at germination stage was obtained by screening 42,840 R2 progeny derived from the regenerated R0 plants of Nipponbare and thereafter confirmation of the mutant phenotype in R3 progeny. At germination stage, root growth of the mutant was strongly inhibited in the presence of Al but not in the absence of Al. However, at seedling stage, root growth of the mutant and wild type was similarly tolerant to Al. Taken together, we conclude that rice possesses Al-tolerant function that is under genetic control and specifically operates for root growth at germination stage, making rice more tolerant to Al than wheat.