RGB1 regulates expression of OsYUC11 probably throughOsNF-YB1
The results above showed that RGB1 regulates grain development and sucrose metabolism and starch accumulation probably through stimulating the expression of endosperm-specific OsYUC11 , which enhances auxin biosynthesis in endosperm cells. However, how RGB1 regulates the expression of OsYUC11 is still elusive. In the present experiment, we first analyzed the promoter of OsYUC11 that might bind and regulate OsYUC11 expression using PlantPAN 2.0 database. The results showed that there were 305 putative transcript factors which might bind to the promoter of OsYUC11 (Figure 5a; Table S2). Among them, four transcript factors, Os08g45110 , OsNF-YB1 ,OsABI5 and OsEIL4 , were both co-expressed withOsYUC11 and down-regulated in RGB1Ri line on 5 and 10 DAF (Figure 5a and 5b), implying that the expression of OsYUC11 might be regulated by these four transcript factors. We further compared the temporal expression patterns of these four transcript factors in grains of wildtype and RGB1Ri lines and the results showed that only the transcript levels of OsNF-YB1 increased rapidly and were higher in the wildtype grains than those in RGB1Ri lines, especially at the rapid stage of grain filling (Figure 5c), which is well consistent with those of starch accumulation and grain IAA content (Figure 1i and 3b and 3c). Furthermore, the transcript levels of OsNF-YB1 were positively related to those of OsYUC11 (Figure 5d). It is reasonable to assume that OsNF-YB1 play a key role in controllingOsYUC11 expression, and finally the auxin content in grains. To verify this assumption, we also conducted an EMSA assay and the results clearly showed that OsNF-YB1 was able to bind the promoter ofOsYUC11 (Figure 5e). Based on above results, we proposed that the expression of OsYUC11 in rice grains was under the control ofOsNF-YB1 .