Caracterização da interação “mandioca × podridões radiculares” por meio de mapeamento associativo, transcriptômica e caracterização de espécies patogênicas em suporte à obtenção de plantas resistentes

Abstract

Cassava (Manihot esculenta Crantz) is a species that has food, economic and social importance in Africa, Asia and Latin America. However, several diseases can affect its productivity, among them root rot, caused by a complex of pathogens, highlighting the fungi of the genus Fusarium, the family Botryosphareacea and oomycetes of the genus Phytophthora. The losses in production might achieve 100%, depending on the intensity of the incidence, severity of symptoms and susceptibility of cassava varieties. Planting resistant varieties is the most economical and efficient approach to control root rot. Advances in plant physiology, biochemistry and genomics have contributed to the understanding of plant-pathogen interaction behavior and the responses of defense mechanisms. However, studies on the interaction between M. esculenta and the fungi causing root rot are incipient in Brazil. Therefore, the objective of this work was to identify the resistance mechanisms related to root rot in cassava by evaluating the enzymatic activity of pathogens and molecular characterization of the infection process, aiming to suggest a model of resistance to root rot in cassava. For this, three strategies were used: 1) The study of the in vitro growth, aggressiveness and enzymatic activity of fungal isolates associated to root rot, where 71 fungi were isolated, classified as belonging to six different genera (Gongronella sp., Myrothecium sp., Colletotrichum sp., Dipodascus sp., Fusarium sp. and Lasiodiplodia sp.) composing a small part of the soil microbiota of a cassava cultivation area. These isolates differed in their growth and were grouped into four classes of mycelial growth velocity (high, moderate, low and extremely low), being the fungi of Lasiodiplodia sp. those with higher growth velocity. In relation to severity, the isolated Lasiodiplodia sp. were also the most aggressive, causing greater areas of damage to cassava roots. Regarding the extracellular enzymatic activity, the isolates associated to dry rot produced the seven extracellular enzymes evaluated (pectinase, pectin lyase, amylase, protease, urease, cellulase andlaccase), while the isolates associated to black rot did not produce amylase and urease. 2) The study of genetic resistance to root rot in field conditions, where 148 cassava genotypes belonging to the Cassava Breeding Program of Embrapa Cassava and Fruticulture, were planted in an area with high incidence of rot. Based on the values of phenotypic analysis of resistance (survival and disease index) and agronomic traits of interest (plant height, above-ground yield and fresh root yield) these genotypes were grouped into five clusters, and classified as "Extremely Susceptible", "Susceptible", "Moderately Resistant" and "Resistant". The 10 best performing genotypes in the field (TOP-10) were selected as parental potentials for the development of segregation progeny. A genome-wide association study analysis was also performed. It was detected five significant SNPs for root productivity in rotten-infested soil, whose gene annotation indicates functions related to defense mechanisms against biotic and abiotic stresses. 3) Transcriptoma analysis, using the RNASeq technique; Two contrasting cassava genotypes with regard the response to root rot infection, were challenged by a mix of black rot and dry rot pathogens where 18 candidate genes that play important roles in resistance against the disease were identified and can be used in molecular marker-assisted selection for identification and development of resistant genotypes. Our results represent important advances related to the understanding of epidemiology and disease dynamics in the field, correlated with knowledge about the defense mechanisms involved in root rot resistance.