Role of Light and Plant Hormones in Stem Parasitic Plant (Cuscuta and Cassytha) Twining and Haustoria Induction.

Cuscuta and Cassytha are two distinct stem parasitic plant genera developing haustoria at their stem. The initial step to parasitization is twining onto the host plant. Although twining is the critical first step, less attention has been paid to this aspect in stem haustoria parasitic plant studies. As tendril coiling is also controlled by light and plant hormones, we investigated the role of light (blue, red and far-red) and hormones (auxin, brassinolide, cytokinin) in twining of stem parasitic plants (Cuscuta japonica and Cassytha filiformis). In general, both Cuscuta and Cassytha showed similar behavior to light cues. The data show that blue light is essential for twining, and a lower far-red/red light (FR/R) ratio is important for subsequent haustoria induction. Regarding plant hormones, seedlings with solely auxin or cytokinin (iP) under blue light showed not only twining but also haustoria induction, demonstrating that auxin and iP appear to be especially important for induction. Seedlings with solely brassinolide showed no positive influence, but brassinolide together with iP caused twining even under dark conditions. This points to the presence of cross-talk between brassinolide and cytokinin for twining.