Functional study in tomato on the role of the hormone auxin in the induction of symptoms resulting from infection by Moniliophtora perniciosa, causal agent of witches' broom in cocoa

Abstract

"Witches' broom", caused by the basidiomycete fungus Moniliophthora perniciosa, is an important disease of cacao (Theobroma cacao). In contrast to other hemibiotrophic pathogens, the biotrophic stage of infection in cacao is symptomatic and can last for several weeks. The main symptoms of the disease are the increase in infected branches and the accentuated induction of lateral shoots (brooms), which lead to a reduction in the plant's fruit production. The interaction between M. perniciosa and cacao is considered complex due to the biological characteristics of the host and pathogen. The S-biotype of M. perniciosa is capable of naturally infecting some members of the Solanaceae family, and by artificial inoculation, it is also capable of infecting tomato (Solanum lycopersicum). The infected tomato plant presents the main symptoms present in the biotrophic phase of the disease in cacao. The 'Micro-Tom' (MT) cultivar has a short life cycle, small size and ease of genetic transformation and genomic editing, and proved to be an adequate genetic model for the study of this interaction. A strategy common to pathogens is the manipulation of host hormonal signaling to favor the development of pathogenesis. In this context, it was demonstrated that M. perniciosa produces the hormone auxin during interaction with tomato and cacao. An analysis of RNAseq data treated the induction of auxin-responsive genes in infected cacao tissues. Inoculation of mutant MT plants with greater sensitivity to auxin (integer) by M. perniciosa (biotype-S) evolved in more pronounced symptoms when compared to inoculated MT plants. Such evidence suggests a possible contribution of auxin imbalance in favoring the infection. However, there are not enough data that stated how auxin acts in the process of infection, colonization and symptom induction. This project aims at the symptomatic evaluation of inoculated MT plants with contrasting stability for auxin signaling (whole and diageatropic mutants) compared to controls. The level of pathogen colonization in infected tissues will be quantified by qPCR, as well as the productivity of all genotypes in response to infection.