Ameriorative Effect of Excreted Organic Acids from Plant Roots on Aluminum Toxicity in Acid Soils.
Hideaki Matsumoto1, Hong Shen2, Zheng Ming Yang3, Hiroki Osawa4, Takayuki Sasaki1, and Yoko Yamamoto1. (1) Research Institute for Bioresources, Okayama University, Chuo 2-20-1, Kurashiki, Japan, (2) College of Natural Resources and Environment, South China Agricultural University, Guangzhou 510642, Guangzhou, China, (3) Agricultural Division Council of Jilian University, 5333 Xi'an Road Changchum, Jilian, China, (4) Graduate School of Agricultural and Life Science, University of Tokyo, Chuo 2-20-1, Jilian, Japan
It is estimated that approximately 70% of the soils are problem soils in the world. Acid soils are the largest problem soil. Acid soils occupy approximately 30% or 3950 billion ha of the world's ice free land area. Thus attention has been paid to improving the agricultural production in acid soils. The main factor in producing acid soils is protons(H+ ions)in rain water which removing the more soluble nutrients and gradually dissolving most primary and secondary minerals. Al, the most abundant metal in the earth's crust, has been implicated as early as 1918 as a cause of root growth retardation in barley and rye plants grown on acid soils. The primary target of Al is the root apex and inhibition of root elongation is induced primarily by Al toxicity. Several strategies have been presented to manage acid soils. The primary method has been the application of large amount of lime to raise soil pH. However, liming does not remedy soil acidity below the plow layer. Therefore effort in solution of Al toxicity has been directed to development of Al-tolerant crop cultivars. Many recent studies provided the strong evidence that Al-tolerant genotypes exclude Al from root tips by exudation of organic acids from root which chelate toxic Al rendering less toxic in the rhizosphere (Exclusion mechanism). Al tolerance caused by secretion of citrate in soybean: Al tolerant (cv Suzunari) and sensitive soybean(cv Shishio) were screened. Marked relationship between Al tolerance and secretion of citrate was observed. Citrate secretion required the direct contact of root with Al. Higher activity of plasma membrane H+-ATPase coincided with more citrate secretion in Suzunari than Shishio. RT-PCR(real-time reverse transcription) and immunodetection indicated that the increase of plasma membrane H+-ATPase activity by Al is caused by transcriptional and translational regulation. Furthermore plasma membrane H+-ATPase activity and expression were higher in Suzunari than in Shishio. Al activates the threonine-oriented phosphorylation of plasma membrane H+-ATPase in a dose- and time-dependent manner. Malate secretion of Al tolerant wheat: Near isogenic Al tolerant (ET8) and sensitive wheat line(ES8) were used. The dramatic and instant secretion of malate from root tip of ET8 but not ES8 was observed under Al stress. The constitutively and strongly expressed cDNA in root tip of ET8 and weakly in ES8 was detected. The function of this gene product was Al activated malate transporter and named as ALMT1. The transgenic barley introduced ALMT1 could secrete malate under Al stress and grow well in cultural solution containing Al and in acid soil. But non-transgenic wild barley was failed. These results suggest that ALMT1 is capable of conferring Al tolerance to plant cells.