Wang Yan, Kelley Seamus, Zentella Rodolfo, Hu Jianhong, Wei Hua, Wang Lei, Shabanowitz Jeffrey, Hunt Donald F, Sun Tai-Ping
Department of Biology, Duke University, Durham, NC, 27708, USA.
Department of Chemistry, University of Virginia, Charlottesville, VA, 22904, USA.
Nat Commun. 2025 Apr 28;16(1):3965. doi: 10.1038/s41467-025-59095-5.
The phytohormone auxin plays a pivotal role in promoting fruit initiation and growth upon fertilization in flowering plants. Upregulation of auxin signaling by genetic mutations or exogenous auxin treatment can induce seedless fruit formation from unpollinated ovaries, termed parthenocarpy. Recent studies suggested that the class A AUXIN RESPONSE FACTOR6 (ARF6) and ARF8 in Arabidopsis play dual functions by first inhibiting fruit initiation when complexed with unidentified corepressor IAA protein(s) before pollination, and later promoting fruit growth after fertilization as ARF dimers. However, whether and how posttranslational modification(s) regulate ARF6- and ARF8-mediated fruit growth were unknown. In this study, we reveal that both ARF6 and ARF8 are O-fucosylated in their middle region (MR) by SPINDLY (SPY), a unique nucleocytoplasmic protein O-fucosyltransferase, which catalyzes the addition of a fucose moiety to specific Ser/Thr residues of target proteins. Epistasis, biochemical and transcriptome analyses indicate that ARF6 and ARF8 are downstream of SPY, but ARF8 plays a more predominant role in parthenocarpic fruit growth. Intriguingly, two ARF6/8-interacting proteins, the co-repressor IAA9 and MED8, a subunit of the coactivator Mediator complex, are also O-fucosylated by SPY. Biochemical assays demonstrate that SPY-mediated O-fucosylation of these proteins reduces ARF-MED8 interaction, which leads to enhanced transcription repression activity of the ARF6/8-IAA9 complex but impaired transactivation activities of ARF6/8. Our study unveils the role of protein O-fucosylation by SPY in attenuating auxin-triggered fruit growth through modulation of activities of key transcription factors, a co-repressor and the coactivator MED complex.
植物激素生长素在开花植物受精后促进果实起始和生长过程中起关键作用。通过基因突变或外源生长素处理上调生长素信号传导，可诱导未授粉子房形成无籽果实，即单性结实。最近的研究表明，拟南芥中的A类生长素响应因子6（ARF6）和ARF8具有双重功能，在授粉前与未知的共抑制因子IAA蛋白复合时首先抑制果实起始，而在受精后作为ARF二聚体促进果实生长。然而，翻译后修饰是否以及如何调节ARF6和ARF8介导的果实生长尚不清楚。在本研究中，我们发现ARF6和ARF8在其中间区域（MR）被SPINDLY（SPY）进行O-岩藻糖基化，SPY是一种独特的核质蛋白O-岩藻糖基转移酶，它催化将岩藻糖部分添加到靶蛋白的特定丝氨酸/苏氨酸残基上。上位性、生化和转录组分析表明，ARF6和ARF8是SPY的下游，但ARF8在单性结实果实生长中起更主要的作用。有趣的是，两个与ARF6/8相互作用的蛋白，共抑制因子IAA9和共激活因子中介体复合物的一个亚基MED8，也被SPY进行O-岩藻糖基化。生化分析表明，SPY介导的这些蛋白的O-岩藻糖基化降低了ARF-MED8相互作用，这导致ARF6/8-IAA9复合物的转录抑制活性增强，但ARF6/8的反式激活活性受损。我们的研究揭示了SPY介导的蛋白O-岩藻糖基化通过调节关键转录因子、共抑制因子和共激活因子MED复合物的活性来减弱生长素触发的果实生长的作用。
