How does ENB1 contribute to formation of a persistent endosperm in maize?

Wang et al. explore cellulose deposition in maize seeds.

Qun Wang and Rentao Song

China Agricultural University, Beijing 100193, China

Background: Plant seeds are mainly classified into two types: one with a persistent endosperm as in cereals, and the other with a transient endosperm as in dicots. These two types of seeds absorb photo-assimilated sucrose via transfer cells at the maternal/filial interface. Transfer cells of cereal (e.g. maize) and dicot (e.g. legume) seeds are distinctly different, manifesting as a rib-like cell wall ingrowth (CWI) in the former and a papillae-like CWI in the latter. These two types of CWI formation both require cellulose deposition. However, it remains unclear whether there is a specific cellulose synthase (CESA) that directs CWI synthesis.

Question: Which CESA synthesizes CWI of transfer cells and is the CWI key in the formation of the persistent or transient endosperms in plant seeds?

Findings: We identified a maize seed mutant enb1, in which the persistent endosperm was drastically degraded during seed development. We cloned the mutant enb1 genes and found it encodes a CESA5 that directs synthesis of rib-like CWI in transfer cells. We established that limited maternal sucrose import via the defective rib-like CWI is the cause of endosperm breakdown in enb1. We also found that sucrose induces ENB1 through the transfer cell-specific transcription factor MRP1. Therefore, sucrose-induced ENB1 synthesizes rib-like CWI that enhances maternal sucrose import, sustaining a persistent endosperm in the maize seeds. Moreover, we found that overexpression of ENB1 can further enhance sucrose import and increase seed weight, providing a new strategy for yield improvement in cereal crops.

Next steps: We will explore the regulatory mechanisms for the development of rib-like CWI in maize transfer cells, seeking to answer questions such as how do sucrose and hormones serve as signals that regulate rib-like CWI development? We will also attempt to manipulate the development of rib-like CWI to breed new maize lines with high yields.