Dead of Alive, A study of the molecular and physiological events that determine kiwifruit skin skin will stay alive or die during fruit development

Abstract

The formation of fruit skin has unresolved origins that are being uncovered through the application of ‘omics’ technologies. In this dissertation, I examine the formation of fruit skin (exocarp) in the context of kiwifruit vines grown in New Zealand. The fruit of kiwifruit (Actinidia sp) was found to segregate into two broad categories, those that retain epidermal exocarp throughout development and those that replace epidermis with peridermis during development. The control switch between normal development and lateral growth via cell division and differentiation is explored in this thesis using various Actinidia sp which vary based on the appearance of their fruit exocarp. A review of the literature found the kiwifruit periderm to be more alike potato tuber skin and tree bark than the temporal russet affecting other fruit such as apple, pear and tomato. A study regarding the genomic locations relevant to exocarp formation found a highly significant QTL for russeting on chromosome 19, and other significant QTLs for cuticle formation on chromosome 11, 24. Gene mining within QTL regions found candidate genes homologous to genes with conserved roles regarding cuticle formation, cork meristem induction and periderm formation. A time-course study of kiwifruit transcriptomics was combined with a microscopy study and hormone analysis. This multi-faceted approach identified key regulatory networks that regulate cell fate in kiwifruit including transcripts assumed to fold into functional proteins and others that act as signals or transcription factors. Patterns of hormone concentration throughout development confirmed significant correlations between various classes of hormones, which furthermore coincided with trends in transcription of key regulatory genes. This body of work advances the current knowledge base of cork meristem formation in plants and more specifically the regulation of exocarp formation in kiwifruit. The comprehensive study lacks metabolomics and proteomics data and yet finds highly significant candidate genes for the regulation of complex and important signalling pathways in plants. The summation of results suggests periderm formation in kiwifruit is a developmentally controlled organogenesis event involving from a previously uncharacterised secondary lateral meristem that is initiated in some Actinidia sp before significant cuticle damage of the exocarp occurs.