DISCUSSION
Embryo-endosperm relationship is one of the determining factors contributing to seed development and subsequently the yield and quality of cereal grains. However, signals or regulators that coordinate the developmental processes of the two compartments remain largely unknown. This study took advantages of the NB mutant to evolve a novel comparison method for quantifying the influence of embryo on endosperm development, revealing that the embryo has a dragging effect on the developmental transition of the endosperm. To our knowledge, it is the first attempt to directly disclose the evidence of embryo-endosperm interaction during rice seed development. The findings reveal new aspects of the role of embryo in the formation process of rice quality, and help to draw an integrative picture of seed development from the perspective of agronomy and crop physiology.
1. The dragging effect of embryo on endosperm development
Seed development is an orchestrated progression through a series of stages, which can be described by two types of ‘time’: the chronological time and the developmental time defined by the sequence of stages (Ebisuya & Briscoe, 2018). There is evidence of bidirectional interaction between embryo and endosperm throughout development (An et al., 2020). Notably, two recent studies in Arabidopsis suggest an independent relationship between the two tissues. By analyzing mutants with defective endosperm cellularization, O’Neill, Colon, and Jenik (2019) found that this endosperm process is not required for the onset of embryo maturation. Using single-fertilization mutants, Xiong, Wang, and Sun (2021) demonstrated that in the absence of embryo, endosperm develops the same way as in the wild type, suggesting that endosperm development is an autonomously programmed process independent of embryogenesis. These two studies in combination indicate that in terms of developmental time, the mechanisms controlling endosperm and embryo development act independently of each other (O’Neill et al., 2019). On the contrary, in terms of chronological time, the current study show that duration of endosperm filling in the bottom part of the NB grains was prolonged by the embryo (Figure 2j), suggesting a substantial interaction between embryo and endosperm. Similarly, Xiong et al. (2021) showed that rapid embryo expansion can significantly accelerate endosperm breakdown, thus shortening the lifespan of the transient endosperm. Further, it should be noted that cereal seeds like rice have a persistent endosperm, whereas dicots like Arabidopsis have an ephemeral endosperm that degrades as the embryo grows. As a result, whether the independence of endosperm development from embryo in Arabidopsis is applicable to rice awaits further investigation.
Plants have evolved a variety of timing mechanisms that integrate chronological time with developmental time to ensure proper development (O’Neill et al., 2019). For the endosperm, these include internal timers of molecular oscillators based on hormones or metabolites, and external timers dependent on environmental signals or emanating from a different tissue like the embryo. This study reveals a dragging effect of embryo on endosperm development in chronological time, i.e. extending the storage accumulating stage whereas delaying the maturation stage. This finding provides direct evidence for the role of embryo as the external timer controlling endosperm development. In addition, hormones like GA, auxins, and ABA were unevenly distributed in rice seed, with the embryo generally having higher contents at the early and middle stages, as also reported by Zhang et al. (2020a). Interestingly, GA20ox(GA20ox2 and GA20ox3 ), GA3ox (OsGA3ox1 andOsGA3ox2 ), KS (OsKS1 and OsKS4 ) were predominantly expressed in embryo at 5-20 DAF, indicating that embryo may be the GA-synthetic site whereas endosperm is the GA-acting site (Figure 4f). It is well established that embryo-derived GA modulates the secretion of starch-degrading enzymes like α-amylase from the aleurone and scutellum upon germination. But it is still uncertain whether the degradation of starch during seed development is analogous to the germinating process. Our results imply that one of the external timers coordinating rice grain development might be the hormone GA released from endosperm, which needs further investigation.
For internal timers modifying endosperm development, the T6P-SnRK1 signaling pathway may be the key component. At early stage of 5 to 10 DAF, sucrose in endosperm was lower, probably due to the deprivation by the growing embryo. In response to the reduced sucrose content, T6P was decreased simultaneously in endosperm, thus relieving the inhibition of SnRK1 activity. The increased SnRK1 activities promoted the catabolism or suppressed the anabolism of starch and proteins, as reflected by the lower content of starch and prolamins as well as the enhanced gene activity of amylase, lipase, and protease in endosperm (Figure 7). Conversely, at middle stage between 20 and 25 DAF, the T6P-SnRK1 signaling showed an opposite trend relative to that between 5 and 10 DAF, indicating it may be involved in the transition of developmental stages in endosperm (Figure 7). Taken together, the anticorrelation between T6P and SnRK1 activity opens the possibility that the T6P-SnRK1 pathway may be a master regulator coordinating the communication between embryo and endosperm during rice grain formation.