AcoPMEI acts as a dual-function effector that promotes pollen tube growth and buffers against pistil-derived inhibition in pineapple

Self-incompatibility (SI) severely restricts breeding efficiency and genetic improvement in cultivated pineapple (Ananas comosus (L.) Merr.), yet its molecular basis remains poorly understood. Here, we investigated the molecular processes associated with gametophytic SI (GSI) and compatible pollination using pineapple cultivars 'MD2' and 'BaLi' ('BL') using integrated transcriptomic analyses and functional validation. Our comparative transcriptomic analysis identified AcoPMEI, a gene encoding a pectin methylesterase inhibitor, as a compatibility-associated factor specifically upregulated during compatible pollination in pineapple. Functional assays demonstrated that AcoPMEI promotes pollen tube growth by inhibiting the enzymatic activity of AcoPME14, thereby maintaining a highly methylesterified and extensible apical cell wall. Furthermore, in vitro co-treatment and in vivo heterologous expression assays showed that AcoPMEI partially alleviates AcoRNase-mediated inhibition of pollen tube growth. Yeast two-hybrid (Y2H) and bimolecular fluorescence complementation (BiFC) assays did not support a direct interaction between AcoPMEI and AcoRNase, suggesting that AcoPMEI counteracts pistil-derived inhibition through an indirect, cell wall-mediated mechanism. Baed on SI-responsive gene expression patterns, we propose a transcriptome-informed regulatory model for GSI-associated responses in pineapple. Collectively, these findings identify AcoPMEI as a pollen-derived compatibility-associated factor that promotes pollen tube growth and mitigates RNase-associated inhibition through cell wall modulation, providing insight into the molecular control of pollination compatibility in monocots.