Natural variation in a molybdate transporter controls grain molybdenum concentration in rice

Authors: HUANG, XIN-YUAN - Nanjing Agricultural University; LIU, HUAN - Nanjing Agricultural University; Pinson, Shannon; LIN, HONG-XUAN - Shanghai Institutes For Biological Sciences; GUERINOT, MARY LOU - Dartmouth College; ZHAO, FANG-JIE - Nanjing Agricultural University; SALT, DAVID - University Of Nottingham

Submitted to: New Phytologist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/8/2018
Publication Date: 2/7/2019
Citation: Huang, X., Liu, H., Pinson, S.R., Lin, H., Guerinot, M., Zhao, F., Salt, D.E. 2019. Natural variation in a molybdate transporter controls grain molybdenum concentration in rice. New Phytologist. https://doi.org/10.1111/nph.15546.
DOI: https://doi.org/10.1111/nph.15546

Interpretive Summary: The objective of this comprehensive study was to identify the gene underlying a previously identified QTL region on rice chromosome 8 associated with large differences in concentrations of the element molybdenum(Mo) in rice grain. Mo is an essential micronutrient for both plants and animals, largely impacting organism biochemistry and health by being necessary within Mo-containing coenzymes (MoCo). Although cases of Mo deficiency in humans is rare, Mo deficiency in plants is fairly common. Plant Mo deficiency is associated with acid soils, and approximately two-thirds of the world’s land area suitable for growing crops is moderately to highly acidic. Rice is one of the most important staple crops in the world, not only providing about one fifth of daily calories for more than half of the world’s population but also providing a critical source of essential mineral nutrients. In two previous QTL mapping studies, involving both bi-parental and a diverse mapping population , we discovered and verified the existence of a grain-Mo QTL on rice chromosome 8. In this study, we first fine mapped the QTL, then used sequence information to identify genes along this portion of chromosome 8 that might be underlying, or causing, the QTL detected association with grain-Mo. In order to learn which of the candidate genes was most likely to be the causative gene, we conducted several more studies including evaluation of the effect of alterations in various portions of the genes, e.g., mutations that changed the protein structure and effectiveness as opposed to mutations that altered the amount of protein made, and yet other changes that altered where the protein went within the cells, which informed us as to how the gene and protein were likely functioning to cause a change in grain-Mo. We ultimately determined that the causal gene for this QTL is OsMOT1;1, and we learned that it affects the amount of Mo that accumulates in both leaves and grains, not by altering how much Mo enters the plant, but by affecting how much Mo is retained in mitochondria, which in turn affects how much MoCo is produced. We further determined that differences in Mo transport were not due to structural differences in the OsMOT1;1 protein product, but instead due to how much protein was produced, caused by differences in the gene promoter region. The identification of OsMOT1;1 provides an important insight into the regulation of Mo homeostasis in plant cells and a useful gene to breed rice varieties resistant to Mo deficiency in soils.

Technical Abstract: Molybdenum (Mo) is an essential micronutrient for most living organisms, including humans. Cereals such as rice are the major dietary source of Mo. However, little is known about the genetic basis of the variation in Mo content in rice grain. Here, we identified a molybdate transporter, OsMOT1;1, as the causal gene for a quantitative trait locus qGMo8 that controls Mo accumulation in rice grain. OsMOT1;1 is mainly expressed in roots and the OsMOT1;1 protein localizes to mitochondria. OsMOT1;1 exhibits transport activity for molybdate, but not sulfate, when heterogeneously expressed in yeast cells. OsMOT1;1 is involved in the uptake and translocation of molybdate under the condition of limited molybdate supply. Knock-down of OsMOT1;1 results in less Mo being translocated to shoots, lower Mo concentration in grains and higher sensitivity to Mo deficiency. We reveal that the natural variation of Mo concentration in rice grains is attributed to the variable expression of OsMOT1;1 due to sequence variation in its promoter. Identification of natural allelic variation in OsMOT1;1 may facilitate the development of rice varieties with Mo enriched grain for dietary needs and improve Mo nutrition of rice on Mo-deficient soils.