Gibberellin (GA) biosynthesis and signaling play important roles in seed setting and grain weight; however, how long-distance GA transport contributes to these traits remains poorly understood. Here, we characterized the Nitrate transporter 1/Peptide transporter Family (NPF) protein OsNPF3.5, which mediates GA allocation in rice (Oryza sativa L. var. Nipponbare). OsNPF3.5 was preferentially expressed in the phloem of the leaf blade at the reproductive stage and was responsive to lower temperatures. Ectopic expression of OsNPF3.5 in Xenopus laevis oocytes showed relatively low uptake activity for GA3,4,7 and abscisic acid (ABA), but a significant efflux activity for GA44 across the plasma membrane. Compared to the wild type, pollen fertility, seed setting rate, 1000-grain weight, and grain yield were decreased in osnpf3.5. Moreover, functional disruption of OsNPF3.5 essentially decreased GA44 redistribution from the flag leaf blade and was accompanied by decreased levels of GA3 in anthers and GA1 in caryopses. These results suggest that OsNPF3.5 functions as a GA44 efflux transporter promoting GA44 loading into phloem, thus facilitating GA allocation from flag leaf blade to sink organs including anthers and caryopses, which consequently regulates seed setting, 1000-grain weight and grain yield. This represents a mechanism by which long-distance GA precursor transport gets involved in rice seed setting and grain weight formation under variable environmental conditions.
Wen et al. (Tue,) studied this question.