A unique phytochrome B gene in Cuscuta campestris and its responses during the initial stage of haustorium formation

Cuscuta, a genus of stem-parasitic plants, requires specific light and touch stimuli for host recognition and haustorium formation. Blue and far-red light promote parasitism, while red light inhibits it, but the roles played by light-responsive genes in the early stages of Cuscuta parasitism remain largely unexplored. This study utilized an in vitro C. campestris haustorium induction system to investigate this issue. RNA-sequence analysis revealed a group of differentially expressed genes that responded to the initiation of haustorium formation. Genes associated with light-signaling pathways, including phytochromes (PHYs), phytochrome-interacting factors (PIFs), and homeodomain-leucine zipper I/II transcription factors, were specifically up or downregulated in plants exposed to light for 4 h. Phylogenetic analysis demonstrated that two distinct forms of the PHYB gene, CcPHYB1 and CcPHYB2, were present in all sequenced Cuscuta species. CcPHYB2 was not expressed under normal light conditions, but was specifically upregulated under light regimes inducing haustorium formation; by contrast, CcPHYB1 exhibited non-light-dependent expression. Furthermore, the induction of haustorium formation by far-red light was canceled by red light exposure but could be re-activated by subsequent re-exposure to far-red light. The levels of expression of CcPHYB2, CcPIF1, CcPIF4, CcHOMEOBOX FROM ARABIDOPSIS THALIANA 3 (CcHAT3), CcHOMEOBOX 2 (CcHB2) and CcHB13 varied under different haustorium-inducing light conditions. Expression of these genes altered significantly upon exposure to blue and far-red light, but these changes were reversed in plants exposed to red light. These findings highlight the pivotal role of light-responsive signaling during the early stages of parasitism, providing valuable insights into the molecular mechanisms and evolution of light-mediated host recognition and parasitism in Cuscuta species.