Abstract R2R3-MYB transcription factors are key regulators of plant development and secondary cell wall biosynthesis, yet their roles in orchids remain poorly understood. Here, we identify and functionally characterize PaMYB55 from Phalaenopsis Sogo Yukidian V3, an ortholog of Arabidopsis MYB55, predominantly expressed during late flower development. Virus-induced gene silencing (VIGS) of PaMYB55 resulted in smaller sepals and petals due to reduced cell expansion with decreased expression of expansion-related genes (PaEXRA10-like and PaXTH6), accompanied by altered epidermal integrity, sunken striations, and premature floral senescence. Histochemical staining and quantitative assays revealed decreased cellulose and lignin deposition in both floral organs and peduncles, correlating with downregulation of cellulose synthase (PaCESA4, PaCESA7, PaCESA9) and lignin biosynthetic genes (PaPAL1, PaC4H, PaCCR1, PaCCoAMT, PaCOMT1–3, PaF5H, PaCAD2). Furthermore, PaMYB55 silencing compromised pathogen restriction, as shown by enhanced spread of GFP-tagged Cymbidium mosaic virus beyond vascular bundles into mesophyll tissues, and upregulated immune-related genes. Our findings demonstrate that PaMYB55 positively regulates cell expansion and cell wall integrity by promoting lignin and cellulose biosynthesis, thereby contributing to floral morphology, tissue structural stability, and defense against pathogen spread. This work advances our understanding of MYB functions in orchids and highlights PaMYB55 as a potential target for improving both ornamental traits and disease resistance in breeding programs.
Mao et al. (Tue,) studied this question.