A novel approach to enhance resistance to vascular disease by expressing cell-death-inducing fungal elicitors in the xylem tissue.

Constitutive expression of hypersensitive cell-death-inducing effectors secreted by pathogens has proved to be difficult in plants. Herein, xylem-specific promoters (P(VND7) or P(VND2)) were utilized to express three cell-death-inducing proteins (FocSIX1A, FocSIX6 or TvSm1) of fungal origin in banana plants. The regenerated plants were not only healthy but also showed improved resistance to fungal (Foc vcg0124 and Foc vcg0125) infection. These banana plants developed systemic resistance-like symptoms, including elevated jasmonic acid content, callose depositions, increased xylem wall thickness and expression of several defence-associated genes. Interestingly, expression of NAC105, a homologue of VND (vascular-related NAC domain) transcription factor, was elevated in all these transgenic lines. Employing a novel positive-feedback loop using VND genes, banana lines with thicker xylem walls devoid of any ectopic lignification were developed. Enhanced disease resistance of these lines against Foc strains, increased expression of defence genes, elevated jasmonic acid content and quantitative elevation of phenolic compounds provide evidence that elevation in xylem wall thickness does indeed contribute to systemic resistance by playing a crucial role in regulating immune responses. The current study demonstrates that (a) plants that express cell-death-inducing proteins in xylem can be regenerated, (b) these plants are healthy and show enhanced disease resistance and (c) a novel feedback loop involving vascular proteins, which specifically elevates xylem wall thickness without negatively affecting growth, exists in plants.