Resistence to leprosis and alternaria brown spot in citrus

Author(s)

Marines Bastianel

• Document type: Doctoral Thesis
• Press: Campinas, SP.
• Institution: Universidade Estadual de Campinas (UNICAMP). Instituto de Biologia
• Defense date: 2005-12-16

Examining board members

Marcos Antonio Machado; Dario Grattapaglia; Renato Beozzo Bessanezi; Anete Pereira de Souza; Maria Elisa Ayres Guidetti Zagatto Paterniani

• Advisor: Marcos Antonio Machado

Abstract

Leprosis and Alternaria brown spot are important citrus diseases. The knowledge of the genetic mechanisms involved in the resistance to these diseases, observed in some citrus species and hybrids, is of fundamental importance for the development of strategies for the genetic improvement based on varietal resistance. The main objective of this dissertation was to study the inheritance of citrus resistance to leprosis and Alternaria brown spot, based on genetic mapping and analysis of QTLs (quantitative trait loci) and to analyze genes differentially expressed in response to leprosis through suppression subtractive hybridization (SSH). A progeny of 143 individuals obtained from controlled crosses between 'Murcott¿ tangor [Citrus reticulata (L.) Blanco x C. sinensis (L.) Osbeck], resistant to leprosis and susceptible to brown spot, and 'Pêra¿ sweet orange (C. sinensis (L.) Osb.), susceptible to leprosis and resistant to brown spot, were established in the field and assessed for disease resistance during a three-year period. The plants were inoculated with citrus leprosis virus, cytoplasmic type (CiLV-C) through its viruliferous vector, the mite Brevipalpus phoenicis (Geijskes, 1939) (Acari: Tenuipalpidae), and naturally infected with the fungus Alternaria alternata pathotype tangerine, causal agent of the Alternaria brown spot. The plants were genotyped with AFLP (amplified fragment length polymorphism) and RAPD (random amplified polymorphic DNA) markers. Genetic diversity among hybrids was characterized by molecular markers using the software NTSYS v.2.0. Leprosis incidence and severity were assessed and exhibited high values of heritability at the third year of evaluation (0.88 to 0.96), and low values for parameters related to growth (plant height and trunk diameter) (0.64 and 0.68). The percentage of affected shoots by Alternaria brow spot was assessed for two years. Genetic maps for each parent and an integrated map were obtained, respectively, using Mapmaker v.2.0 and JoinMap 3.0. The analysis of QTLs was done using the software MAPQTL v.4.0 on the integrated map and showed a consistent QTL for leprosis incidence and severity, and another QTL for percentage of brown spot affected shoots. The analysis of sequences differentially expressed under CiLVC inoculation through SSH allowed the identification a large number of genes expressed in the parent resistant 'Murcott¿ tangor. Many of these genes seem to be involved in processes related to the resistance against pathogens, such as oxidative stress responses, signal perception and transduction, programmed cell death, selective degradation of proteins, and synthesis of pathogenesis-related proteins. Our results suggest a great genetic diversity amongst the progeny and that both leprosis and Alternaria brown spot diseases are possibly controlled by few genes. This is the first study on the genetic inheritance of leprosies resistance and of citrus genes expressed in response to CiLV-C (AU)