Agrobacterium-mediated transformation for virus-induced gene silencing (VIGS) in Castilleja tenuiflora: Cte-chlH and Cte-PDS.

Key message: Agrobacterium-mediated transformation allowed the establishment of the VIGS system based on the TRV for specific marker genes, Cte-PDS and Cte-chlH, in C. tenuiflora, a hemiparasitic plant. Castilleja tenuiflora Benth. (Orobanchaceae) is a medicinal and hemiparasitic plant recognized for its specialized metabolism. Genetic transformation systems are valuable for gene function analysis and understanding the regulation of the biosynthetic pathways of bioactive molecules. Here, we present two efficient Agrobacterium-mediated transformation protocols, defined as "transformation by injection" (Ti-AI) and "transformation by cocultivation" (Tc-AII). For both methods, two strains of A. tumefaciens were used: PCH32 and C58C1. These strains harbored a binary expression vector containing β-glucuronidase (GUS) as a reporter gene. The putative transformants tested positive in the GUS histochemical staining assay, which was confirmed via PCR. Tc-AII showed a higher transformation efficiency (Ti-AI: 23 vs. Tc-AII: 65%) although Ti-AI generated the highest survival rate of the putative transformants (Ti-AI: 75 vs. Tc-AII: 37%). Tc-AII-C58C1 was used to evaluate a VIGS system based on pTRV. Vectors were constructed with phytoene desaturase (Cte-PDS) and protoporphyrin magnesium chelatase subunit H (Cte-chlH) genes. Successful VIGS was demonstrated by gene silencing observed across various growth stages of C. tenuiflora when pTRV₂-CtePDS and pTRV₂-CtechlH were used, indicating that systemic viral infection was achieved. The photobleaching phenotype was observed 32 days after agro-infection and more effective with pTRV₂-CtechlH (80% photobleaching) than with pTRV₂-CtePDS (31% photobleaching). The phenotype of the silenced plants was significantly correlated with the downregulation of endogenous Cte-chlH and Cte-PDS (P ≤ 0.01). These results indicate that the use of Cc-AII-C58C1 and TRV-based VIGS with pTRV₂-CtechlH provides a tool to facilitate research on the functions of genes of interest involved in biotechnological processes in C. tenuiflora, yielding more significant information about gene silencing and genetic transformation in a non-model hemiparasitic medicinal plant.