T6P-SnRK1 signaling putatively mediates the crosstalk between embryo and endosperm
Using the new comparison method, our results showed that T6P-SnRK1 signaling pathway was active in the endosperm, consistent with that of the wheat (Martínez-Barajas et al., 2011). As shown in Figure 7, at early stage of 5 to 10 DAF, embryo had a negative effect on sucrose, glucose and fructose levels in endosperm during 5-10 DAF, probably due to its nutrient consumption. The decrease of sucrose content in endosperm was accompanied by the decline in T6P levels, which in return inhibited SnRK1 activity (Figure 7a). SnRK1 promoted catabolic activity in the endosperm, as evidenced by the declining accumulation of starch and storage proteins (prolamin and glutelin), and the up-regulation of genes encoding amylase (AMY3D ), lipase (Os01g0651800 ,Os01g0710700 , and Os05g0574100 ), and protease (OsSAG12 and OsSCP28 ) in endosperm. Consistently, genes participating in synthetic process of starch (OsAGPL4 ,OsSSSIIIa , OsSSIVa , OsSSIVb , andOs02g0807100 ) and storage protein (OsEnS-115 ,Prol-14 , Pro-15 , Prol13b.20 , and Glutelin type-B 2-like ) were all down-regulated at 5-10 DAF (Figure 7b).
Conversely, at middle stage between 20-25 DAF, T6P-SnRK1 signaling showed an opposite trend relative to that between 5-10 DAF. Gene activities of sucrose metabolism (OsINV1 and OsNIN6 ) and T6P synthesis (OsTPS9 and Os08g0414700 ) were up-regulated (Figure 7). SnRK1 activity was inhibited synchronously in endosperm (Figure 7a). Meanwhile, starch synthetic process was promoted and catabolic processes were inhibited by SnRK1, as reflected by the overrepresented starch-synthetic genes (OsAGPL3 and ISA1 ) and underrepresented catabolic genes encoding amylase (AMY3A andAMY3E ), and protease (OsAP1 , Os04g0535200 ,Os08g0267300 , Os05g0403000 , Os04g0330900 , andOsSAG12-1 ) in endosperm (Figure 7b; Table S5).
In addition, this comparison method revealed that the influence of embryo on IAA was similar with T6P but converse with SnRK1 activity across the developmental stages, decreasing at 10 DAF while increasing at 15-25 DAF (Figure 7a). Recently, by modulating T6P content in growing embryos of garden pea (Pisum sativum ), Meitzel et al. (2021) found that auxin acts downstream of T6P to facilitate seed filling. Our finding of the synchronously dynamic pattern of IAA and T6P indicates a similar role of auxin-T6P pathway in grain-filling process of rice.