ABA triggers maize grain filling by inducing phosphorylation of the hub coordinators

Yang et al. explore the mechanism of grain filling in maize.


Tao Yang and Yongrui Wu, National Key Laboratory of Plant Molecular Genetics, CAS Center for Excellence in Molecular Plant Sciences, Shanghai Institute of Plant Physiology and Ecology, Chinese Academy of Sciences, Shanghai 200032, China.

Background: Early seed development and grain filling in maize are two tightly linked developmental stages. The spatial and temporal regulation of the two stages requires the concerted action of several signaling pathways that integrate information from genetic programs, and both hormonal and metabolic signals. Our pioneering work identified a hub coordinator of maize seed development and grain filling, ZmABI19, that directly recognizes and transactivates the central regulator O2 and other critical TFs in the endosperm and Vp1 in the embryo. However, the overall picture of the regulatory networks involved in these processes is far from complete.

Question: Besides ZmABI19, are there more TFs required to coordinate early seed development and grain filling? What are the mechanisms to initiate grain filling by these TFs?

Findings: We report here that a basic leucine zipper TF, ZmbZIP29, with abundant expression during early endosperm development, interacts with ZmABI19 to regulate the central endosperm-filling regulator O2 gene, and their transactivation capacities are enhanced by ABA-induced phosphorylation. This work established a mechanistic framework in which ABA-mediated phosphorylation of two hub coordinators ZmABI19 and ZmbZIP29 by SnRK2.2 enhances gene transactivation for endosperm filling. 

 Next steps: First, besides ABA, what are other signals required to initiate grain filling? Soluble sugars that suddenly increase at the onset of endosperm filling probably function as a signal; second, the transition of early endosperm development to endosperm filling is a critical phase of endosperm development. ZmABI19 and ZmbZIP29 are abundantly expressed during early endosperm development and play crucial roles at this stage. When early endosperm development ends, how do the encoded proteins shift their binding from early targets to genes required for grain filling? We will address these questions in the following years.

Tao Yang, Haonan Wang, Liangxing Guo, Xingguo Wu, Qiao Xiao, Jiechen Wang, Qiong Wang, Guangjin Ma, Wenqin Wang, Yongrui Wu. (2022). ABA-induced Phosphorylation of basic Leucine Zipper 29, ABSCISIC ACID INSENSITIVE 19 and Opaque2 by SnRK2.2 Enhances Gene Transactivation for Endosperm Filling in Maize. https://doi.org/10.1093/plcell/koac044