Aluminum Tolerance in Sorghum and Maize - Translational Genomics for Crop Breeding: Abiotic Stress, Yield and Quality - page 83

Chapter 6

Aluminum Tolerance inSorghum and Maize

JurandirV.Magalhaes,LyzaG.Maron,MiguelA.Pineros,ClaudiaT.Guimaraes,and
LeonV.Kochian

Abstract

The soils of the tropics and subtropics are highly weathered, leading to poor soil fertility and low soil
pH. Root growth and function on these acid soils is impaired by aluminium (Al) toxicity, leading to a
yield instability that jeopardizes food security worldwide. A wealth of physiological evidence exists
for an Al-tolerance mechanism based on Al exclusion from the growing root tip. This is facilitated
by the release of Al-binding organic acids such as malate and citrate, which keeps rhizotoxic Al
away from sensitive sites in the root apex. More recently, Al-activated organic acid transporters in
the ALMT and MATE (multidrug and toxic compound extrusion) protein families have been cloned
and provide the molecular support for this Al-tolerance mechanism. Here a historical review of Al
tolerance in maize and sorghum is presented, followed by an analysis of the more recent research on the
molecular determinants of Al tolerance. We show that Al tolerance provided by MATE proteins spans
the genetic divergence between sorghum and maize, and is a conserved physiological mechanism in
both species. Some features of this mechanism are strikingly common in sorghum and maize, such
as the close relationship between phenotypic variation and MATE gene expression. However, while
the genetic basis for maize aluminium tolerance is quantitative, in sorghum, SbMATE underlies a
major Al-tolerance locus. More subtle features of this Al-tolerance trait are now emerging, such as
the importance of trans -acting factors in sorghum, whereas Al-tolerance gene expression in maize
appears to be predominantly controlled in cis. Knowledge of the molecular basis of Al tolerance is
now providing the framework to address pivotal historical questions in the field, such as the occurrence
of genetic background effects for Al tolerance. We advocate the point of view that the answer to such
questions will inevitably form the basis for modern molecular breeding strategies designed to explore
in full the potential for genetic solutions to the Al-tolerance problem for crops grown on acid soils.