Ethylene negatively regulates aluminium-induced malate efflux from wheat roots and tobacco cells transformed with TaALMT1

Summary

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.

Abstract

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.