Back to Acid Soil Fields: The Citrate Transporter SbMATE Is a Major Asset for Sustainable Grain Yield for Sorghum Cultivated on Acid Soils

Abstract

Aluminum (Al) toxicity damages plant roots and limits crop production on acid soils, which comprise up to 50% of the world's arable lands. A major Al tolerance locus on chromosome 3, AltSB, controls aluminum tolerance in sorghum [Sorghum bicolor (L.) Moench] via SbMATE, an Al-activated plasma membrane transporter that mediates Al exclusion from sensitive regions in the root apex. As is the case with other known Al tolerance genes, SbMATE was cloned based on studies conducted under controlled environmental conditions, in nutrient solution. Therefore, its impact on grain yield on acid soils remains undetermined. To determine the real world impact of SbMATE, multi-trait quantitative trait loci (QTL) mapping in hydroponics, and, in the field, revealed a large-effect QTL colocalized with the Al tolerance locus AltSB, where SbMATE lies, conferring a 0.6 ton ha(-1) grain yield increase on acid soils. A second QTL for Al tolerance in hydroponics, where the positive allele was also donated by the Al tolerant parent, SC283, was found on chromosome 9, indicating the presence of distinct Al tolerance genes in the sorghum genome, or genes acting in the SbMATE pathway leading to Al-activated citrate release. There was no yield penalty for AltSB, consistent with the highly localized Al regulated SbMATE expression in the root tip, and Al-dependent transport activity. A female effect of 0.5 ton ha(-1) independently demonstrated the effectiveness of AltSB in hybrids. Al tolerance conferred by AltSB is thus an indispensable asset for sorghum production and food security on acid soils, many of which are located in developing countries.

Keywords: Al tolerance; AltSB; QTL mapping; Sorghum bicolor; field trials.

Figure 1

Field phenotyping sites. Satellite image (A) and respective soil sampling points (B). Al saturation (%) spatial variation in the superficial (0–20 cm) (C), and subsuperficial (20–40 cm) (D) soil layers. Adapted from Menezes et al. (2014). x- and y-axis in panels B, C and D correspond to spatial coordinates in Universal Transverse Mercator (UTM) 23k format. Satellite images were obtained on March 5, 2014.

Figure 2

Phenotypic means for grain yield (ton ha^–1) under control (2% Al saturation) and Al stress (56% Al saturation) conditions in the field for the parental lines BR007 and SC283. Means are based on 2-year trials. Least significant difference (LSD) = 0.59 (P = 0.05). Error bars are shown.

Figure 3

Graphical display of the QTL detected by multi-trait QTL analysis for relative net root growth (RNRG) after 5 d at {27} µM Al³⁺ in nutrient solution, and grain yield (ton ha^–1) under 56% Al saturation stress in the field. The associated tail probability of the Wald statistics, P, is expressed as –log₁₀(p), analogous to the usual LOD score profile. The red horizontal line indicates the significant threshold obtained using a correction for multiple testing following Li and Ji (2005), with a genome wide test level α = 0.05.

Figure 4

Detailed graphical display of the QTL detected by a multi-trait QTL analysis on chromosome 3 for relative net root growth (RNRG) after 5 d at {27} µM Al³⁺ in nutrient solution and grain yield (ton ha^–1) under 56% Al saturation stress in the field. The associated tail probability of the Wald statistics, P, is expressed as –log₁₀(p), analogous to the usual LOD score profile. The red horizontal line indicates the significant threshold obtained using the Li and Ji (2005) correction with α = 5%.