Establishment of an Efficient Agrobacterium rhizogenes-Mediated Hairy Root Transformation System for Functional Analysis in Passion Fruit

Passion fruit (<i>Passiflora edulis</i> Sims), belonging to the Passifloraceae family, is an economically important plant in tropical and subtropical regions. The advances in functional genomics research of passion fruit have been significantly hindered by its recalcitrance to regeneration and stable transformation. This study establishes the first efficient <i>Agrobacterium rhizogenes</i>-mediated hairy root transformation system for passion fruit. Utilizing the <i>eGFP</i> marker gene, transformation efficiencies of 11.3% were initially achieved with strains K599, MSU440, and C58C1, with K599 proving most effective. Key transformation parameters were systematically optimized to achieve the following: OD₆₀₀ = 0.6, infection duration 30 min, acetosyringone concentration 100 μM, and a dark co-cultivation period of 2 days. The system's utility was further enhanced by incorporating the red visual marker RUBY, enabling direct, instrument-free identification of transgenic roots via betaxanthin accumulation. Finally, this system was applied for functional analysis using <i>PeMYB123</i>, which may be involved in proanthocyanidin accumulation. Overexpression of <i>PeMYB123</i> produced a higher content of proanthocyanidin in hairy roots. Additionally, the <i>PeANR</i> gene involved in the proanthocyanidin pathway was strongly activated in the transgenic hairy roots. This rapid and efficient visually simplified hairy root transformation system provides a powerful tool for functional gene studies in passion fruit.