Interaction of Moniliophthora perniciosa biotypes with Micro‐Tom tomato: a model system to investigate the witches' broom disease of Theobroma cacao

Abstract

The miniature tomato (Solanum lycopersicum) cultivar Micro‐Tom (MT) has become an important platform to investigate plant–pathogen interactions. In the case of the witches' broom disease of Theobroma cacao (cacao), the existence of Moniliophthora perniciosa isolates pathogenic to Solanaceae (S‐biotype) may enable the use of MT to circumvent limitations of the cacao host, whereas the availability of a non‐infective cacao C‐biotype allows the evaluation of contrasting responses of MT. Infection of MT by the S‐biotype led to stem swelling and axillary shoot growth to form broom‐like symptoms similar to the biotrophic phase in cacao, but the infected tissues did not progress to necrosis. Conversely, inoculation with the C‐biotype did not cause typical symptoms, but reduced plant height, appearing as a non‐host interaction. Histopathological characterization of the S‐biotype infection of MT by light and electron microscopy indicated limited germ tube penetration, preferentially through wounds at the base of trichomes or actively through the epidermis. No intracellular mycelium was observed, corroborating the lack of the necrotrophic stage of the pathogen. The analysis of gene expression during the interaction between the S‐ or C‐biotype with MT indicated that expression of plant defence‐associated genes differs for kinetics and intensity between a compatible or incompatible M. perniciosa–MT interaction. The pattern of spore germination and low rate of mycelia penetration suggests that the S‐biotype is not a fully adapted tomato pathogen, but possibly a case of broken non‐host resistance, and evidence suggests the occurrence of a non‐host MT response against the C‐biotype.