Multiple gene copies: carrying out biochemical and molecular studies in kiwifruit

Biochemical and molecular studies on kiwifruit have often been fraught with difficulty. The reasons are several. Biochemical studies have needed to combat fruit acidity, mucilaginous extracts and various other tissue-based problems associated with perennial woody plants. Molecular studies have required development of new RNA and DNA extraction techniques appropriate for kiwifruit whilst functional analysis of genes using transgenics has been slow due to large plant size and long life cycle. Both biochemical and molecular studies have needed to deal with the multiplicity of genes coding for a particular function. Early studies focused on ‘Hayward’ kiwifruit, the hexaploid commercial kiwifruit (Actinidia deliciosa). More recent studies have utilised diploid A. chinensis genotypes in an attempt to simplify approaches and interpretation. Several genes (sucrose-phosphate synthase (SPS), polygalacturonase (PG), actinidin) have been shown to belong to gene families and at least one (SPS) has three or more closely related homologues within a diploid A. chinensis genotype. With SPS, only one of the genes is variably expressed, whilst the others are present at low constitutive levels. At the enzyme level there is also evidence of multiple isoforms (actinidin, PG), indicating that caution should be used in interpreting results from physiological studies. The literature on kiwifruit reveals a growing number of enzyme purification studies, enzyme activity measurements, gene expression studies, analyses of families of genes, and reports in which the role of particular genes have been studied in transgenic plants. This existing base of knowledge and the development of new technology have allowed the application of genomic approaches to gene and protein discovery in kiwifruit. HortResearch has developed a database of >60,000 ESTs and at least 13,400 individual genes, from sequencing of over 22 libraries. This wealth of information provides a platform for new approaches to be taken to understand kiwifruit behaviour and the differences between species and genotypes.