Location: Plant, Soil and Nutrition ResearchTitle: Incomplete transfer of accessory loci influencing SbMATE expression underlies genetic background effects for aluminum tolerance in sorghum) Author
Submitted to: Plant Journal
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/13/2012
Publication Date: 1/4/2013
Publication URL: DOI: 10.1111/tpj.12029
Citation: Melo, J., Lana, U., Pineros, M., Alves, V., Guimaraes, C., Liu, J., Zheng, Y., Zhong, S., Fei, Z., Maron, L., Schaeffert, R., Kochian, L.V., Magalhaes, J. 2013. Incomplete transfer of accessory loci influencing SbMATE expression underlies genetic background effects for aluminum tolerance in sorghum. Plant Journal. 73(2):276-288. Interpretive Summary: Over 20 percent of the U.S. land area and approximately 50 percent of the world’s arable lands are acidic (pH </- 5). On these acid soils, aluminum (Al) toxicity is the primary factor limiting agricultural productivity, as toxic Al results in damaged and stunted plant root systems, ultimately resulting in a reduction of crop yields. Given that a large proportion of the acid soils are found in the tropics/subtropics regions where many developing countries are located, Al toxicity limits agricultural productivity in the very areas where food security is most tenuous. Because of the importance of this problem to agriculture worldwide, there is considerable interest and research effort by researchers at universities, government agencies, and international agriculture organizations in identifying genes that provide tolerance to Al toxicity in order to improve crop Al tolerance via molecular breeding and biotechnology. The release of organic acid from the root apex in response to Al-stress constitutes a widespread Al-tolerance mechanism by which plant roots are able to ameliorate the toxic levels of Al surrounding the growing root. Several Al tolerance genes encoding for membrane proteins involved in the transport of these organic acids out of the root have been identified and cloned. In sorghum, the major Al tolerance gene, SbMATE, encodes an Al-activated root citrate transporter. In this study we expressed tolerant and sensitive versions of SbMATE in sorghum lines differing in their genetic makeup and found that when a very Al tolerant version of SbMATE was transferred via molecular breeding techniques to lines with certain genetic background, we saw a partial loss of the Al tolerance originally seen in the tolerant line from which the tolerant version of SbMATE was transferred. Along with a partial loss of Al tolerance we also see a decreased expression in the SbMATE gene. These findings highlight the importance of understanding the mechanisms governing the expression of genes mediating Al-tolerance response in crops, as a crucial component for future improvements of Al tolerance in cereals via molecular breeding approaches.
Technical Abstract: Impaired root development caused by aluminum (Al) toxicity is a major cause for grain yield reduction for crops cultivated on acid soils which are widespread worldwide. In sorghum, the major Al tolerance locus, AltSB, is due to the function of SbMATE, which is an Al-activated root citrate transporter. Here we conducted a molecular and physiological characterization of different AltSB donors and near-isogenic lines (NILs) harboring different AltSB alleles. We observed partial transfer of Al tolerance from the parents to the NILs that was consistent across donor alleles, emphasizing the occurrence of strong genetic background effects related to AltSB. This reduction in tolerance was variable, with a 20% reduction being observed when highly Al tolerant lines were the AltSB donors and as great as 70% when other AltSB alleles were introgressed. This reduction in Al tolerance was closely correlated with a reduction in SbMATE expression in NILs, suggesting incomplete transfer of regulatory loci acting in trans on SbMATE. Nevertheless, AltSB alleles from the highly Al tolerant sources, SC283 and SC566, were found to retain high SbMATE expression presumably via elements present within or near the AltSB locus, resulting in significant transfer of the Al tolerance phenotype to the derived NILs. Allelic effects could not be explained by coding region polymorphisms, although occasional mutations may affect Al tolerance. Finally, we report on the extensive occurrence of alternative splicing for SbMATE, which by means of the non-sense mediated RNA decay pathway, may be an important component regulating SbMATE expression in sorghum.