Long-distance transport of the phytohormone abscisic acid (ABA) has been studied for ~50 years, yet its mechanistic basis and biological significance remain very poorly understood. Here, we show that leaf-derived ABA controls rice seed development in a temperature-dependent manner and is regulated by defective grain-filling 1 (DG1), a multidrug and toxic compound extrusion transporter that effluxes ABA at nodes and rachilla. Specifically, ABA is biosynthesized in both WT and dg1 leaves, but only WT caryopses accumulate leaf-derived ABA. Our demonstration that leaf-derived ABA activates starch synthesis genes explains the incompletely filled and floury seed phenotypes in dg1. Both the DG1-mediated long-distance ABA transport efficiency and grain-filling phenotypes are temperature sensitive. Moreover, we extended these mechanistic insights to other cereals by observing similar grain-filling defects in a maize DG1 ortholog mutant. Our study demonstrates that rice uses a leaf-to-caryopsis ABA transport–based mechanism to ensure normal seed development in response to variable temperatures.
(A) ABA efflux activity analysis using Xenopus oocytes. The complementary RNA (cRNA) for DG1 or for the known ABA transporter AtDTX50 (positive control), or water as a negative control, were injected into oocyte cells, which were then injected with ABA. The ABA concentration in the incubation buffer was measured before injection and after 10, 20, and 30 min of incubation. Data are means ±SD (n = 3). (B) ABA efflux transport activity analysis using 3H-ABA and protoplasts of WT (Nip), a knockout line (ko-1), and an overexpression line: DG1-OE(35S::DG1cDNA-eGFP). The radioactivity in the protoplast incubation buffer was measured before loading and after 10, 20, and 30 min of incubation (presented as the proportional increases in the 3H-ABA signal for the three incubation intervals). Data are means ± SD (n = 3; **P < 0.01, Student’s t test). (C to E) Plasma membrane localization of a DG1-EGFP fusion protein (C) in the root tip of the overexpression line transgenic plant. (F to H) The root tip of WT Nipponbare was used as the negative control. FM4-64 dye (red) stains plasma membranes. Scale bar, 20 μm. Photo credit: Guohua Zhang.