Heterochromatic silencing is reinforced by ARID1‐mediated small RNA movement in Arabidopsis pollen

Abstract

Summary In the plant male germline, transposable elements (TEs) are reactivated in the companion vegetative nucleus, resulting in siRNA production and the intercellular movement of these siRNAs to reinforce TE silencing in sperm. However, the mechanism by which siRNA movement is regulated remains unexplored. Here we show that ARID1, a transcription factor which is constitutively expressed in the vegetative nucleus but dynamically accumulates in the generative cell (the progenitor of sperm) to promote the second pollen mitosis, mediates siRNA movement to reinforce heterochromatic silencing in the male germline. We looked for regulators involved in the accumulation of ARID1 in the generative cell, and found that AGO9, a germline‐specific AGO in Arabidopsis, is required for the accumulation of ARID1 in the generative cell. Mutations in either ARID1 or AGO9 lead to the interruption of not only the second pollen mitosis but also the movement of siRNA from the vegetative nucleus to the male germline, resulting in the release of heterochromatic silencing in the male germline. Moreover, conditional knockdown of ARID1 in the generative cell causes reduced heterochromatic silencing in both bicellular and mature pollen. This study provides insights into how a spatiotemporal transcription factor coordinates heterochromatic silencing and male germline maturation.

Pseudouridine guides germline small RNA transport and epigenetic inheritance

“…4j). This localization is consistent with a role for AGO9 before, and after siRNA transport as previously proposed 48,51 . We therefore performed immunoprecipitation of small RNA from WT and psd pollen using antibodies to AGO9.…”

“…AGO9 has a similar localization (Fig. 2d,e) and has previously been associated with siRNA movement in pollen 48,49 . We therefore examined AGO9 localization using an mCherry N-terminal fusion 50 and found that AGO9 was first localized in the VN at the bicellular stage, but then moved to the sperm cells by the tricellular stage (Fig.…”

Epigenetic Regulation of Megaspore Mother Cell Formation

“…Moreover, ARID1 (ARID domain-containing 1), a transcription factor that is required for heterochromatic silencing and sperm cell formation (Zheng et al, 2014), regulates MET1 reciprocally in the gamete cells, and also inhibit MMC formation (Li et al, 2017). In addition, ARID1 acts with AGO9 together to mediate siRNA movement in male gametes (Wu et al, 2021). The fact that multiple heterochromatin regulators, for example, RdDM factors, H1, MET1, and ARID1, even TRAF Mediated Gametogenesis Progression (TRAMGaP), an AGO9-interacting protein (Singh et al, 2017), negatively regulate MMC specification and differentiation, indicates that heterochromatin silencing restricts the potential germline identity of the surrounding somatic cells.…”

Targeted suppression of siRNA biogenesis in Arabidopsis pollen reveals distinct Pol IV activities in the sperm and vegetative cell lineages

“…In contrast, the generative cell undergoes a second mitosis and gives rise to twin sperm cells that slowly progress through S-phase prior to double fertilization, when RdDM pathways are transiently switched off 10,15 . For this reason, epigenetic reprogramming in the male germline has been largely associated to the activity of siRNAs originated from the vegetative cell 12,16,17 and anther tissues during meiosis 18 , but the origins, dynamics and functions of siRNAs accumulated in mature pollen are still poorly understood.…”