The THO Complex Non-Cell-Autonomously Represses Female Germline Specification through the TAS3-ARF3 Module

Abstract

Summary In most sexually reproducing plants, a single somatic, sub-epidermal cell in an ovule is selected to differentiate into a megaspore mother cell, which is committed to giving rise to the female germline. However, it remains unclear how intercellular signaling among somatic cells results in only one cell in the sub-epidermal layer differentiating into the megaspore mother cell. Here we uncovered a role of the THO complex in restricting the megaspore mother cell fate to a single cell. Mutations in TEX1, HPR1 and THO6, components of the THO/TREX complex, led to the formation of multiple megaspore mother cells, which were able to initiate gametogenesis. We demonstrated that TEX1 repressed the megaspore mother cell fate by promoting the biogenesis of TAS3-derived ta-siRNA, which represses ARF3 expression. The TEX1 protein was present in epidermal cells but not in the germline, and through TAS3-derived ta-siRNA, restricted ARF3 expression to the medio domain of ovule primordia. Expansion of ARF3 expression into lateral epidermal cells in a TAS3 ta-siRNA-insensitive mutant led to the formation of supernumerary megaspore mother cells, suggesting that TEX1- and TAS3-mediated restriction of ARF3 expression limits excessive megaspore mother cell formation non-cell autonomously. Our findings reveal the role of a small RNA pathway in the regulation of female germline specification in Arabidopsis.

Translating auxin responses into ovules, seeds and yield: Insight from Arabidopsis and the cereals

“…Some ta‐siRNAs target ARF family members for repression, such as ARF3 and ARF4 , through tasiR‐ARF (Fei et al ). Consistent with incorrect regulation of ARF3 in THO/TREX mutants, ectopic expression of ARF3 in wild‐type ovules results in supernumerary MMC‐like cells (Su et al ).…”

The transcription and export complex THO/TREX contributes to transcription termination in plants

“…A. thaliana mutants in either HPR1 , TEX1 , THO2 or THO6 are defective in the synthesis of one or multiple forms of small RNAs, including miRNA, siRNA, and tasiRNAs, although the mode of action behind this defect is currently unknown [14–17]. Some of the phenotypes associated with the tex1 and hpr1 mutants in A. thaliana , such as repression of female germline specification or reduction in scopolin biosynthesis under abiotic stress, are likely caused by a reduction in tasiRNA or miRNA production [37, 38].…”

“…In some cases, extension of 3’UTR may also lead to disruption of the downstream gene by the formation of an antisense RNA with potential to trigger siRNA-mediated gene silencing, or by transcriptional interference via RNAPII collision [46]. The tex1 and hpr1 mutant are known to have multiple phenotypes, ranging from defects in vegetative and reproductive development [18, 38], responses to both biotic and abiotic stress [36, 37] and the expression of genes encoding acid phosphatases [16] or ethylene signaling pathway repressor [20]. It would be of interest to determine if the genes affected by 3’UTR extensions contribute to some extent to these phenotypes.…”

Functional dissection of the ARGONAUTE7 promoter

“…Redundant clusters of activator binding sites are thought to be common, and may contribute to robustness (Frankel et al, 2010;Hong, Hendrix, & Levine, 2008;Levine, 2010;Perry, Boettiger, Bothma, & Levine, 2010). Effects may be larger in other tissues, given the important functions of ARF repressors in fruits and roots (Marin et al, 2010;Sessions & Zambryski, 1995;Simonini, Bencivenga, Trick, & Østergaard, 2017;Simonini et al, 2016;Su et al, 2017). Alternatively, the sites may simply be nonfunctional, at least in A. thaliana.…”