XsMAX2 negatively regulates post-fertilization ovule development in Xanthoceras sorbifolium

The Arabidopsis MAX2 gene encodes an F-box protein containing a leucine-rich repeat (LRR) domain, which plays a pivotal role in regulating plant development and stress responses. Although MAX2 has been extensively characterized in these processes, its specific function in post-fertilization ovule development remains elusive. Our previous work identified XsMAX2, a MAX2 homolog in Xanthoceras sorbifolium, as highly expressed in fertilized ovules. Here, we demonstrate that XsMAX2 is a nuclear-localized protein that interacts with SKP1, a core component of the SCF (SKP1-CUL1-F-box) ubiquitin ligase complex. Furthermore, we show that the auxin-responsive transcription factor ARF2 directly binds to the XsMAX2 promoter, regulating its expression in fertilized ovules. Functional studies reveal that upregulation of XsMAX2 leads to increased fruit and ovule abortion, whereas its downregulation enhances their growth and development. Notably, XsMAX2 expression is precisely modulated by nutrient availability (e.g., phosphate and sugars) and the phytohormone pathways (e.g. strigolactone and abscisic acid). Mechanistically, XsMAX2 may suppress cell cycle-related genes and induce aberrant programmed cell death (PCD) in post-fertilization ovules, thereby impairing early seed development. Collectively, our findings establish XsMAX2 as a critical negative regulator of post-fertilization ovule development in X. sorbifolium, integrating environmental and endogenous signals to control reproductive success.