A plasma membrane-localized transporter remobilizes aleurone layer magnesium for seed germination in rice

The aleurone layer in cereal grains acts as a major reservoir of essential mineral nutrients, significantly influencing seed germination. However, the molecular mechanism underlying the redistribution of nutrients from the aleurone layer in the germinating seed is still not well understood. Here, in rice, we identified a plasma membrane (PM) localized magnesium transporter, MAGNESIUM RELEASE TRANSPORTER 3 (MGR3), is critical for seed germination. OsMGR3 is predominantly expressed in the aleurone layer cells of endosperm, facilitating magnesium remobilization during germination. Non-invasive Micro-test Technology assay data demonstrated that the loss-of-function of OsMGR3 restrained magnesium efflux from the aleurone layer. In the embryo/endosperm grafting experiment, we observed that the mutation of OsMGR3 in the aleurone layer suppressed the growth and differentiation of the embryo during germination. Furthermore, magnesium fluorescence imaging revealed the osmgr3 mutant seeds showed impaired exportation of aleurone layer-stored magnesium to the embryo, consequently delaying germination. Importantly, we discovered that disrupting OsMGR3 could inhibit pre-harvest sprouting without affecting rice yield and quality. Therefore, the magnesium efflux transporter OsMGR3 in the aleurone layer represents a promising genetic target for future agronomic trait improvement.