Molecular plant-pathogen interactions with special reference to Eucalyptus Grandis polygalacturonase-inhibiting proteins and fungal polygalacturonases

Abstract

Phytopathogenic fungi parasitise plants for survival. Knowledge of the interactions between plants and these fungi is of importance in designing control strategies. In this thesis the interactions of Eucalyptus polygalacturonase-inhibiting proteins (PGIPs) and fungal polygalacturonases (PGs) were characterised. About thirty years of research on the interaction between PGIPs and PGs was reviewed in Chapter 1. The gene sequences of the mature PGIP peptides from selected Eucalyptus species were determined in Chapter 2. The PGIP genes have very high similarities among themselves, signifying conservation of the function of PGIPs in this genus. Molecular characterisation of the endoPG gene of C. cubensis was presented in Chapter 3. This endoPG gene occurs more than two times in the genome of C. cubensis. The gene sequences of the other endoPGs remain to be determined. EndoPG production by C. cubensis, a Eucalyptus canker pathogen, was presented in Chapter 4. It was found that polygalactw:onases probably have a minor role in determining the levels of pathogenicity in different C. cubensis isolates. However, the hypovirus CHVl-173 has a role in reducing polygalacturonase production in infected hypovirulent C. cubensis isolates. Chapter 5 dealt with the interaction between Eucalyptus PGIPs and endoPGs from different fungal pathogens. Coniothyrium zuluense and Botryosphaeria dothidea produced more PGs than C. cubensis and Phytophthora cinnamomi in liquid culture. Disease tolerant Eucalyptus TAG5 clones produced PGIPs that are more effective in inhibiting endoPGs from C. zuluense than the susceptible Eucalyptus ZG14 clones. In Chapter 6 the gene sequence of the endoPG of Fusarium circinatum was detennined. This :fungus causes the . pitch canker disease in pine trees. This endoPG gene occurs as a single copy and is related to those of other Fusarium spp. The results presented in this thesis add to the current scientific knowledge pertaining to the role of PGIPs and PGs in pathogenic interactions, especially, between Eucalyptus PGIPs and fungal endoPGs. Interaction of Eucalyptus PGIPs and fungal endoPGs was demonstrated; therefore it is possible to genetically engineer Eucalyptus for more disease tolerant Eucalyptus plants using PGIP genes, especially, against Coniothyrium zuluense. This means that more research is needed to identity genes that will give a more global protection against many fungal pathogens.