SummaryAmino acid transporters (AATs) play indispensable roles in nutrient allocation during plant development. In this study, we demonstrated that inhibiting expression of the rice amino acid transporter OsAAP3 increased grain yield due to a formation of larger numbers of tillers as a result of increased bud outgrowth. Elevated expression of OsAAP3 in transgenic plants resulted in significantly higher amino acid concentrations of Lys, Arg, His, Asp, Ala, Gln, Gly, Thr and Tyr, and inhibited bud outgrowth and rice tillering. However, RNAi of OsAAP3 decreased significantly Arg, Lys, Asp and Thr concentrations to a small extent, and thus promoted bud outgrowth, increased significantly tiller numbers and effective panicle numbers per plant, and further enhanced significantly grain yield and nitrogen use efficiency (NUE). The promoter sequences of OsAAP3 showed some divergence between Japonica and Indica rice, and expression of the gene was higher in Japonica, which produced fewer tillers than Indica. We generated knockout lines of OsAAP3 on Japonica ZH11 and KY131 using CRISPR technology and found that grain yield could be increased significantly. These results suggest that manipulation of OsAAP3 expression could be used to increase grain yield in rice.

BackgroundRice tiller number is one of the most important factors that determine grain yield, while nitrogen is essential for the crop growth and development, especially for tiller formation. Genes involved in nitrogen use efficiency processes have been identified in the previous studies, however, only a small number of these genes have been found to improve grain yield by promoting tillering.ResultsWe constructed over-expression (OX) lines and RNA-interference (Ri) lines, and selected a mutant of OsNPF7.2, a low-affinity nitrate transporter. Our analyses showed that rice tiller number and grain yield were significantly increased in OX lines, whereas Ri lines and mutant osnpf7.2 had fewer tiller number and lower grain yield. Under different nitrate concentrations, tiller buds grew faster in OX lines than in WT, but they grew slower in Ri lines and mutant osnpf7.2. These results indicated that altered expression of OsNPF7.2 plays a significant role in the control of tiller bud growth and regulation of tillering. Elevated expression of OsNPF7.2 also improved root length, root number, fresh weight, and dry weight. However, reduced expression of OsNPF7.2 had the opposite result on these characters. OsNPF7.2 OX lines showed more significantly enhanced influx of nitrate and had a higher nitrate concentration than WT. The levels of gene transcripts related to cytokinin pathway and cell cycle in tiller bud, and cytokinins concentration in tiller basal portion were higher in OX lines than that in WT, suggesting that altered expression of OsNPF7.2 controlled tiller bud growth and root development by regulating cytokinins content and cell cycle in plant cells. Altered expression of OsNPF7.2 also was responsible for the change in expression of the genes involved in strigolactone pathway, such as D27, D17, D10, Os900, Os1400, D14, D3, and OsFC1.ConclusionOur results suggested that OsNPF7.2 is a positive regulator of nitrate influx and concentration, and that it also regulates cell division in tiller bud and alters expression of genes involved in cytokinin and strigolactone pathways, resulting in the control over rice tiller number. Since elevated expression of OsNPF7.2 is capable of improving rice grain yield, this gene might be applied to high-yield rice breeding.Electronic supplementary materialThe online version of this article (10.1186/s12284-018-0205-6) contains supplementary material, which is available to authorized users.

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