Role ofTaALMT1 malate-GABAtransporter in alkalinepHtolerance of wheat

Malate exudation through wheat (Triticum aestivumL) aluminium-activated malate transporter 1 (TaALMT1) confers Al(3+)tolerance at low pH, but is also activated by alkaline pH, and is regulated by and facilitates significant transport of gamma-aminobutyric acid (GABA, a zwitterionic buffer). Therefore, TaALMT1 may facilitate acidification of an alkaline rhizosphere by promoting exudation of both malate and GABA. Here, the performance of wheat near isogenic lines ET8 (Al+3-tolerant, highTaALMT1expression) and ES8 (Al+3-sensitive, lowTaALMT1expression) are compared. Root growth (at 5 weeks) was higher for ET8 than ES8 at pH 9. ET8 roots exuded more malate and GABA at high pH and acidified the rhizosphere more rapidly. GABA and malate exudation was enhanced at high pH by the addition of aluminate in both ET8 and transgenic barley expressingTaALMT1.Xenopus laevisoocytes expressingTaALMT1acidified an alkaline media more rapidly than controls corresponding to higher GABA efflux.TaALMT1expression did not change under alkaline conditions but key genes involved in GABA turnover changed in accordance with a high rate of GABA synthesis. We propose that TaALMT1 plays a role in alkaline tolerance by exuding malate and GABA, possibly coupled to proton efflux.