Cell cycle repressor promotes pollen mitosis

Abstract

During male germline development, microspores divide asymmetrically into a larger vegetative cell and a smaller generative cell. This pollen mitosis I is followed by a second, symmetric division of the generative cell to form two sperm nuclei in which the cell cycle is arrested in the G1 phase until fertilization. It is known that the R2R3 MYB transcription factor DUO1 is required for PMII, and that duo1 mutants arrest in the S phase before division. DUO1 is transcriptionally controlled by AT-RICH INTERACTING DOMAIN-CONTAINING PROTEIN 1 (ARID1) and can additionally be influenced by microRNAs at the mRNA level. In contrast to the control of PMII entry by DUO1, the mechanism by which divisions after PMII are prevented is not well understood.

The researchers focus on RBR1 as its animal homologue is known to associate with ARID transcription factors. Indeed, Li and colleagues used reciprocal co-immunoprecipitation assays to confirm that RBR1 interacts with ARID1. As rbr1 knockout mutants are gametophytic lethal, the authors isolated the hypomorphic allele rbr1-4, which produces a 3ʹ truncated transcript that leads to an unstable protein lacking part of the C terminus. Although rbr1-4 mutants clearly have reproductive defects, this phenotype does not correlate with overproliferation but rather underproliferation of germ cells, resulting in a higher proportion of pollen containing only a single sperm nucleus. The authors show that RBR1 interacts with DUO1 in an ARID1-dependent way, and that this interaction stabilizes DUO1. As DUO1 protein levels are reduced in rbr1-4 mutants despite higher transcript levels, it can be assumed that the extreme C terminus of RBR1 is required to stabilize DUO1. These findings suggest that RBR1 is required to promote PMII together with ARID1 and DUO1. Interestingly, it has previously been reported that a stronger rbr1 mutant allele causes overproliferation of germ cells, suggesting that RBR1 might also be required to repress further divisions after PMII. Therefore, it is tempting to speculate that DUO1 requires stabilization by RBR1 to coordinate PMII with a mechanism that prevents additional divisions thereafter.