Malate Secretion Research Articles

Secretion of malate and citrate from roots is related to high Al-resistance in Lespedeza bicolor

Lespedeza bicolor (Lespedeza bicolor Turcz. cv. Jiangxi) is a leguminous shrub that is well adapted to acid infertile soils. However, the mechanisms of aluminum resistance in this species have not been established. This study aimed to assess the possible resistance mechanisms of this plant to Al. An Al-sensitive species of Lespedeza, sericea lespedeza [Lespedeza cuneata (Dum.-Cours.) G. Don cv. Zhejiang], was used as a reference. The roots of L. bicolor secreted both malate and citrate after exposure to Al, but roots of L. cuneata did not. The secretion of organic acids from L. bicolor was specific to Al; neither 15-day P starvation nor 50 μM lanthanum induced the secretion of these organic acid anions. Secretion of organic acid anions in L. bicolor was detected after 3–6 h exposure to Al, and the amount increased significantly after 6 h exposure, suggesting that this plant shows a pattern II-type organic acid secretion. This is supported by the finding that the secretion was significantly inhibited by a protein-synthesis inhibitor, cycloheximide. Two kinds of anion-channel inhibitors had different effects on Al-induced secretion of organic acids: 9-anthracene carboxylic acid completely inhibited secretion, phenylglyoxal had no effect. Root elongation in L. bicolor was more severely inhibited by Al in the presence of 9-anthracene carboxylic acid. All these results indicated that the secretion of malate and citrate is a specialized response to Al stress in L. bicolor roots, which might be one of the Al-resistance mechanisms in this species.

Differential Aluminum Resistance and Organic Acid Anions Secretion in Triticale

Triticale (X Triticosecale Wittmack), a hybrid of wheat and rye, shows a high degree of aluminum (Al) tolerance, but variation in Al resistance between cultivars does exist. The mechanisms responsible for differential Al resistance in 10 triticale cultivars were investigated in this study. Triticale roots secreted both malate and citrate in response to Al stress. The amount of organic acid anions secreted was correlated positively to the relative root elongation (an index for Al resistance) and negatively to the Al content in root apices under Al stress, suggesting that the secretion of malate and citrate seems to be involved in the exclusion of Al from root tip. The Al‐induced secretion of malate and citrate was characterized using an Al‐resistant cultivar (ZC 237) and an Al‐sensitive cultivar (OH 1621). Root elongation was significantly inhibited in both ZC 237 and OH 1621 after 24 h of exposure to 30, 50, or 100 µM Al but was more strongly in OH 1621 than in ZC 237 at all Al concentrations tested. A marked lag phase (3 h) between the addition of Al and the secretion of organic acid anions was observed in both triticale cultivars, and the secretion increased with increasing external Al concentration. The two anion‐channel inhibitors, phenylglyoxal and niflumic acid, significantly inhibited the secretion of malate and citrate in ZC 237, with the degree of the inhibition of niflumic acid greater than that of phenylglyoxal. The Al‐induced secretion of malate and citrate decreased to a very low level at low temperature (4°C) in both cultivars. These results indicate that Al‐induced malate and citrate secretion from roots play important roles in excluding Al and thereby detoxifying Al in triticale. The Al‐induced organic acid anions were inhibited by anion‐channel inhibitors and were dependent on temperature.

Ectomycorrhizal Association Enhances Al Tolerance by Inducing Citrate Secretion in Pinus densiflora

To investigate the effects of ectomycorrhizal association on the aluminum (Al) tolerance of Pinus densiflora Sieb. & Zucc., seedlings with or without ectomycorrhizal association with the fungus Pisolithus tinctorius (Pers.) Coker & Couch were exposed to 1 mM Al. Association with P. tinctorius alleviated Al-induced inhibition of root elongation and biomass growth in the mycorrhizal seedlings. Secretion of malate and citrate, both low-molecular-weight organic acids that could detoxify Al by the formation of stable complexes, was investigated in P. tinctorius mycelia and in pine roots with and without P. tinctorius association. Citrate secretion from the P. tinctorius mycelia in vitro was stimulated by Al. Citrate secretion from the roots of the ectomycorrhizal seedlings was also stimulated by Al, but was not detected in the non-mycorrhizal seedlings. These results suggest that citrate secreted from the roots of the ectomycorrhizal seedlings was produced in the hyphae of P. tinctorius. Citrate secretion may play a role in enhancing the Al tolerance of host seedlings.

Al-induced efflux of organic acid anions is poorly associated with internal organic acid metabolism in triticale roots.

The secretion of organic acid anions from roots has been identified as a mechanism of resistance to Al. However, the process leading to the secretion of organic acid anions is poorly understood. The effect of Al on organic acid metabolism was investigated in two lines of triticale (xTriticosecale Wittmark) differing in Al-induced secretion of malate and citrate and in Al resistance. The site of Al-induced secretion of citrate and malate from a resistant line was localized to the root apices (terminal 5 mm). The levels of citrate (root apices and mature root segments) and malate (mature segments only) in roots increased during exposure to Al, but similar changes were observed in both triticale genotypes. The in vitro activities of four enzymes involved in malate and citrate metabolism (citrate synthase, phosphoenolpyruvate carboxylase, malate dehydrogenase, and NADP-isocitrate dehydrogenase) were similar for sensitive and resistant lines in both root apices and mature root segments. The response of these enzymes to pH did not differ between tolerant and sensitive lines or in the presence and absence of Al. Moreover, cytoplasmic and vacuolar pH were not affected by exposure to Al in either line. Together, these results indicate that the Al-dependent efflux of organic acid anions from the roots of triticale is not regulated by their internal levels in the roots or by the capacity of the root cells to synthesize malate and citrate.

Magnesium ameliorates aluminum rhizotoxicity in soybean by increasing citric acid production and exudation by roots.

Superior effectiveness of Mg over Ca in alleviating Al rhizotoxicity cannot be accounted for by predicted changes in plasma membrane Al3+ activity. The influence of Ca and Mg on the production and secretion of citrate and malate, and on Al accumulation by roots was investigated with soybean genotypes Young and PI 416937 which differ in Al tolerance. In the presence of a solution Al3+ activity of 4.6 microM, citrate and malate concentrations of tap root tips of both genotypes increased with additions of either Ca up to 3 mM or Mg up to 50 microM. Citrate efflux rate from roots exposed to Al was only enhanced with Mg additions and exceeded malate efflux rates by as much as 50-fold. Maximum citrate release occurred within 12 h after adding Mg to solution treatments. Adding 50 microM Mg to 0.8 mM CaSO4 solutions containing Al3+ activities up to 4.6 microM increased citrate concentration of tap root tips by 3- to 5-fold and root exudation of citrate by 6- to 9-fold. Plants treated with either 50 microM Mg or 3 mM Ca had similar reductions in Al accumulation at tap root tips, which coincided with the respective ability of these ions to relieve Al rhizotoxicity. Amelioration of Al inhibition of soybean root elongation by low concentrations of Mg in solution involved Mg-stimulated production and efflux of citrate by roots.

Pattern of aluminum-induced secretion of organic acids differs between rye and wheat.

Al-Induced secretion of organic acids from the roots has been considered as a mechanism of Al tolerance, but the processes leading to the secretion of organic acids are still unknown. In this study, the secretion pattern and alteration in the metabolism of organic acids under Al stress were examined in rye (Secale cereale L. cv King) and wheat (Triticum aestivum L. cv Atlas 66). Al induced rapid secretion of malate in the wheat, but a lag (6 and 10 h for malic and citric acids, respectively) between the exposure to Al and the secretion of organic acids was observed in the rye. The activities of isocitrate dehydrogenase, phosphoenolpyruvate carboxylase, and malate dehydrogenase were not affected by Al in either plant. The activity of citrate synthase was increased by the exposure to Al in the rye, but not in the wheat. The secretion of malate was not suppressed at low temperature in the wheat, but that of citrate was stopped in the rye. The Al-induced secretion of citrate from roots of the rye was inhibited by the inhibitors of a citrate carrier, which transports citrate from the mitochondria to the cytoplasm. All of these results suggest that alteration in the metabolism of organic acids is involved in the Al-induced secretion of organic acids in rye, but only activation of an anion channel seems to be responsible for the rapid secretion of malate in the wheat.