Establishment of tissue culture plants for the evaluation of defence-related genes in Persea americana Mill

Abstract

The avocado (Persea americana Mill.) is a popular fruit crop eaten throughout the world. It has a number of health benefits and uses and is a good source of fibre, potassium, Vitamin C, K and B6, folate, and antioxidants. Numerous breeding programs for avocado exist worldwide and some of the characteristics which are selected for include tolerance to white root rot caused by Rossellinia necatrix, rootstocks with increased salt tolerance and scion breeding objectives which include the improvement of fruit quality and tolerance to post-harvest diseases. One of the most important breeding objectives however, is the selection of rootstocks which show increased tolerance to Phytophthora cinnamomi Rands. The development of new cultivars is very slow, since the avocado has a high level of heterozygosity which makes it difficult to predict the genotype of the progeny. Furthermore, the avocado also has a very long juvenile phase and it takes extended periods of time for new cultivars to be evaluated in the field. Also, very little is known about the molecular mechanisms involved in plant-pathogen interactions. Plant tissue culture techniques may be a useful tool to help accelerate plant breeding strategies.

The aim of this MSc was therefore to evaluate different tissue culture techniques and media in order to produce plant material which can be used to aid in the development of new cultivars as well as molecular studies. Defence-related genes in avocado in response to P. cinnamomi were also investigated in five different rootstocks to gain a better understanding of the plant-pathogen interaction mechanism and the tolerance/susceptible response in avocado.

Chapter 1 provides a comprehensive overview of the developmental and reproductive biology of the avocado as well as its interaction with the oomycete, P. cinnamomi. In addition, plant tissue culture and the progress made in the in vitro culture and transformation of avocado is also discussed. Chapter 2 describes the evaluation of different tissue culture techniques in order to find the most efficient and reliable method for the germination, micropropagation, rooting, acclimatization and callus initiation of avocado zygotic embryos. The use of in vitro generated plant material in pathogenicity trials and for gene expression studies is also evaluated. In Chapter 3, the expression of four defence-related genes is studied in five rootstock cultivars over seven different time points after inoculation with P. cinnamomi. Chapter 4 provides a general discussion in which the results obtained and conclusions drawn from the two experimental chapters and the Literature review are discussed along with some of the future applications of the results.