Al-induced secretion of organic acid, gene expression and root elongation in soybean roots

Al-activated organic acid transporter genes (ALMT and MATE) and plasma membrane H+-ATPase gene (PHA) are known to contribute to the regulation of organic acid secretion in several crops. However, it remains unclear how these genes interact to modulate organic acid exudation in the same plant species. In this study, Al-induced expression of genes (GmALMT1, GmMATE1 and GmPHA1), secretion of organic acid and root elongation were characterized in soybean roots. Results indicated that treatment with 50 μM Al activated the expression of GmALMT1, GmMATE1 and GmPHA1, and the exudation of citrate and malate significantly in apical 5 mm region of soybean seedlings, but inhibited root elongation by 57.8 %. The highest malate exudation rate and the maximal expression of GmALMT1 and GmPHA1 were observed after 2 h of 50 μM Al treatment, while the corresponding values for citrate exudation rate and GmMATE1 expression occurred at 8 h. The exudation of malate and citrate contributed to but could not recover Al-triggered root elongation. A root-split experiment indicated that Al-activated gene expression, organic acid secretion and root growth inhibition required the direct contact of Al3+. The removal of shoots in soybean seedlings decreased Al-activated gene expression by 26.1–40.5 %, and secretion of organic acid by 14.7–40.2 %. Furthermore, shoot excision aggravated Al-inhibited root elongation, indicating the existence of other Al tolerance mechanism except the exudation of organic acid. These results suggested that Al-activated expression of GmPHA1-, GmMATE1- and GmALMT1-mediated exudation of malate and citrate, and shoots played an important role in Al toxicity resistance in soybean roots.