Phytochrome-mediated light perception in dodders drives haustorium development through epigenetic mechanisms.

Cuscuta species, parasitic plants with minimal photosynthetic capacity, rely on light cues to locate hosts and initiate infection. Unlike nonparasitic plants, they exhibit a reversed shade response, growing towards low red:far-red (R:FR) light typical of dense vegetation. We investigated how R and FR light modulate haustorium development, gene expression, and epigenetic reprogramming in Cuscuta campestris. FR-enriched conditions promoted coiling and haustorium initiation, while red light suppressed parasitic behaviour. Phytochromes B1 and B2 displayed opposing transcriptional responses to light quality, suggesting a modified light perception mechanism. Transcriptome analyses revealed further that FR light triggered the differential expression of over 5000 genes, including those linked to auxin and cytokinin signalling, cell wall remodelling, and organogenesis. Gene co-expression networks identified phytochrome B2 and a Fhy1/Fhl regulator of phytochrome A as possible central hubs associated with chromatin remodellers, histone modifiers, and RNA-directed DNA methylation components. Small RNA profiling indicated stable global sRNA populations across treatments, with shifts in the expression of specific miRNA families, affecting a subset of light-responsive genes. Our findings demonstrate that FR light perception in C. campestris engages both transcriptional and epigenetic regulation to drive haustorium development, reflecting adaptations in light signalling cascades that underpin its parasitic lifestyle.