Exudation of malate is an important mechanism underlying tolerance of wheat to aluminium toxicity. Here we show that ethylene is involved in regulation of ALMT1-dependent malate efflux from wheat roots.
An important mechanism for Al 3+ tolerance in wheat is exudation of malate anions from the root apex through activation of malate-permeable TaALMT1 channels. Here, the effect of ethylene on Al 3+-activated efflux of malate was investigated using Al 3+-tolerant wheat genotype ET8, which has high expression of TaALMT1. Exposure of ET8 plants to Al 3+ enhanced ethylene evolution in root apices. Treatment with the ethylene synthesis precursor 1-aminocyclopropane-1-carboxylic acid (ACC) and ethylene gas suppressed Al 3+-induced malate efflux from root apices, whereas the intracellular malate concentrations in roots were not affected. Malate efflux from root apices was enhanced in the presence of Al 3+ by two antagonists of ethylene biosynthesis, aminoethoxyvinylglycine (AVG) and 2-aminoisobutyric acid (AIB). An increase in Al accumulation in root apices was observed when treated with ACC, whereas AVG and AIB suppressed Al accumulation in root apices. Al 3+-induced inhibition of root elongation was ameliorated by pretreatment with AIB. In addition, ethylene donor (Ethrel) also inhibited Al 3+-induced malate efflux from tobacco cells transformed with TaALMT1. ACC and the anion-channel blocker niflumate had a similar and non-additive effect on Al-induced malate efflux from root apices. Treatment of ET8 plants with ACC enhanced expression of TaALMT1, suggesting that the inhibitory effect of ethylene on Al-induced malate efflux is unlikely to occur at the transcriptional level. These findings indicate that ethylene may behave as a negative regulator of Al 3+-induced malate efflux by targeting TaALMT1-mediated malate efflux by an unknown mechanism.