The Moniliophthora roreri Effector MrB1 Affects Cellular Homeostasis and Induces Genotype-Dependent Necrosis in Theobroma cacao.

Frosty pod rot (FPR) is an invasive disease responsible for severe yield losses of cacao trees. In this study, we characterised the MrB1 protein (BASIDIN1 from Moniliophthora roreri), a homologue of the BASIDIN effector protein from M. perniciosa. Recombinant MrB1 (rMrB1) induced morphological changes typical of necrosis in Nicotiana benthamiana cell suspensions, including cell swelling, reduced GFP (green fluorescent protein) fluorescence, plasma membrane damage, and electrolyte leakage. Proteomic analysis of the cells indicated that rMrB1 treatment modulated proteins associated with cellular homeostasis, including reduced abundance of proteins related to energy metabolism and protein folding, such as BiP (Binding Immunoglobulin Protein), whose non-detection was validated by western blotting. In cacao leaves, spraying with rMrB1 (12.5 μM) resulted in necrotic lesions affecting 54% of the leaf area after 10 days, increasing to 77% in the presence of 0.5% urea. At a lower concentration (5 μM), the recombinant protein promoted hydrogen peroxide accumulation, supporting the induction of oxidative stress. The phenotypic response to rMrB1 varied among cacao genotypes with different levels of resistance to FPR and witches' broom disease, with the susceptible Catongo genotype being the most affected, while resistant genotypes, such as ICS-95 and Scavina-6, did not show visible symptoms. These results position MrB1 among the few functionally characterised effector proteins in the M. roreri-Theobroma cacao pathosystem and suggest that this effector may be further evaluated as a candidate marker of susceptibility in cacao breeding programmes.