Communication between the AGO protein MEL1 and ubiquitination during sporogenesis

Lian et al. investigated the molecular mechanism of AGO protein MEL1 degradation mediated by a new monocot-specific E3 ligase, XBOS36.

The Plant Cell (2021).

By Jian-Ping Lian, Yu-Chan Zhang and Yue-Qin Chen, Sun Yat-Sen University

Background: Sporogenesis is important for crop seed setting and grain yield. The temporal expression of genes that regulate pollen grain development is important for the progression of sporogenesis. MEIOSIS ARRESTED AT LEPTOTENE1 (MEL1), a meiosis regulatory Argonaut protein in rice, was previously shown to bind phased small RNAs (phasiRNAs) and cleave target genes during early meiosis. Importantly, the accumulation of MEL1 protein is precisely controlled: MEL1 is highly abundant in pollen mother cells (PMCs) and at the early meiotic stages but is eliminated soon after meiosis.

Question: Why must the abundance of MEL1 be precisely controlled? How is MEL1 regulated during pollen grain development?

Findings: We found that the precise regulation of MEL1 homeostasis is important for gametogenesis and seed setting of rice plants. MEL1 is ubiquitinated and subsequently degraded via the proteasome pathway in vivo during late sporogenesis. We identified a monocot-specific E3 ligase, XBOS36, that is responsible for the degradation of MEL1. Inhibition of MEL1 degradation either by knockout of XBOS36 or by overexpression of MEL1 prevented the formation of pollen at the microspore stage, due to mis-regulation of target genes of the phasiRNA-MEL1 complex. Our results indicate that proper temporal regulation of MEL1 is essential for mRNA regulation during pollen development and for normal sporogenesis in rice.

Next steps: We have determined that XBOS36 mediated the ubiquitin-dependent degradation of MEL1. We are working to elucidate the functions of XBOS36 in mediating the decay of the other members of the AGO protein family during sporogenesis.

Jian-Ping Lian, Yu-Wei Yang, Rui-Rui He, Lu Yang, Yan-Fei Zhou, Meng-Qi Lei, Zhi Zhang, Jia-Hui Huang, Yu Cheng, Yu-Wei Liu, Yu-Chan Zhang, and Yue-Qin Chen. (2021). Ubiquitin-dependent Argonaute protein MEL1 degradation is essential for rice sporogenesis and phasiRNA target regulation. Plant Cell.