MADS about flowers: Identification of kiwifruit (Actinidia chinensis) AGAMOUS- and SEPALLATA-like genes

Abstract

Flowering plants produce a wide variety of fruiting structures, each regulated by the differential expression of a conserved group of genes. The patterning of tissues leading to such diverse fruiting bodies is set early in development by the genes required for carpel development. In the model plant Arabidopsis thaliana, both AGAMOUS and SEPALLATA are required to establish normal carpel identity; however, the fruit produced are dry and dehiscent. This is in marked contrast to the edible fruit of the fleshy model plant tomato (Solanum sp.). While kiwifruit (Actinidia sp.) fruits are similar to tomato in that they are also classified as a berry, the kiwifruit ovary is formed from the fusion of a much higher number of carpels. The aims of this research were to identify carpel identity genes from the kiwifruit genome and investigate the expression patterns of these genes and protein products in the context of flesh patterning. Arabidopsis protein sequences were used identify homologous protein sequences in kiwifruit. Candidate genes identified from the protein sequences were amplified from kiwifruit floral bud tissue to generate RNA probes. Kiwifruit floral bud structure was investigated using a range of microscopy techniques, mRNA transcripts of selected candidate genes were detected using in situ hybridisation, while the protein product of kiwifruit AGAMOUS (Acc20728.1- AG1) was detected using immunolocalisation. This research has shown that kiwifruit have a larger zone of AGAMOUS (Acc20728.1-AG1) expression compared to model plants. During development, transcripts were expressed at the highest level over the distal portion of the pistil and became isolated within the epidermal and subepidermal tissue layers of the ovary, as well as the locules and stigma. Transcripts were not associated with the developing vasculature network. Kiwifruit AGAMOUS (Acc20728.1-AG1) proteins were located throughout the ovary in both the outer pericarp and core tissue, specifically within the nucleolus. Overall, this research has shown that the pattern of kiwifruit AGAMOUS transcript and protein expression is similar to the pattern of the model plants Arabidopsis and tomato. The location of identified transcripts provides further evidence that flesh patterning may occur in the absence of AGAMOUS once carpel identity has been established.