Phytohormones
GAs showed a gradual decrease over seed developmental course. Compared with endosperm, embryo had higher content at 5 DAF, while exhibited an opposite trend at 10 DAF (Figures 4f, S6f). GA-synthetic genes,KS (OsKS1 and OsKS4 ), GA20ox(OsGA20ox2 and OsGA20ox3 ), and GA3ox(OsGA3ox1 and OsGA3ox2 ), were uniquely expressed in embryo at 5-20 DAF, implying that the embryo may be main site for GAs synthesis (Xue, Zhang, & Xue, 2012; Zhang et al., 2020a). In addition, two GA-deactivating genes, EUI and OsGA2ox3 , were specifically enriched in endosperm at 5 DAF and 15-20 DAF, respectively.
Auxin gradually increased until 10-15 DAF in embryo and endosperm, then decreased thereafter (Figures 4g, S6g). Compared with endosperm, embryo had higher content at 5-20 DAF. Comparison of gene expression revealed differential auxin biosynthesis in embryo and endosperm. The step catalyzed by YUCCAs and TARs occurred in both embryo and endosperm at 5-20 DAF, whereas that catalyzed by TAA1 (OsTAA1;4 ) occurred mainly in endosperm at 5 DAF.
ABA content in embryo and endosperm showed two peaks at 15 DAF and 25 DAF in embryo and endosperm (Figures 4h, S6h), in line with the biphasic pattern of the embryonic development and endorsed the findings of previous study (Qin, Tang, Pan, & He, 1990). The expression profiles of the ABA-synthesis genes in embryo and endosperm also displayed two peaks. In embryo, NCED1 , OsZEP1 and ABA2 were highly expressed at 5 DAF, while NCED3 and NCED5 at 25 DAF. In endosperm, NCED1 , OsZEP1 and ABA2 were highly expressed at 5 DAF, while NCED1 and OsZEP1 at 20 DAF. In addition, ABA content in endosperm was half of that in embryo.
Isopentenyl transferase (IPT) family members are the rate‐limiting enzymes of cytokinins (CKs) synthesis. OsIPT4 were specifically expressed in endosperm, especially at 5-10 DAF (Figures 4i, S6i), suggesting that CKs have more crucial roles in endosperm than in embryo, in particular at the early stage to initiate and maintain cell division or syncytium formation. In embryo, CKs levels gradually increased, peaked at 10 DAF and then decreased thereafter, consistent with the expression patterns of CKs-synthetic genes. The CKs in endosperm was lower than those in embryo at 5-30 DAF and peaked at 5 DAF.