Fast-tracking kiwifruit breeding through mutagenesis

Incursion of a highly virulent strain of Pseudomonas syringae pv. actinidiae (Psa) into New Zealand in 2010 prompted the rapid development of methods to generate new genetic variation from which resistant kiwifruit cultivars might be developed. Traditional methods of mutation induction that treat bud material with chemical or physical mutagens typically result in chimeras that need to be removed by grafting over several cycles. This requires large greenhouse/field space, and is time-consuming. Our in vitro-based technique for the mass production of non-chimeric mutants uses either ethyl methanesulfonate (1% solution for 1 h) or gamma ray-treated (30 Gy from a ⁶⁰Co source) leaf callus. More than 10,000 putative mutant plants of Actinidia chinensis 'Hort16A' were regenerated from treated callus tissue, and 4138 plants have been planted out in orchards in the Te Puke area, where Psa has become epiphytotic since its incursion in 2010. Another 6000 plants have been screened using an in vitro assay. About 400 putative mutant plants have so far survived in the field, and their survival in the 2013/14 season is being monitored. We have also developed a cell culture of 'Hort16A' and have been investigating pathways to activate transposons for the generation of tagged mutagenic populations. The development of methodologies for somatic embryogenesis, cryopreservation of embryogenic tissue, and generation and screening (RNAseq) of a mutant population developed through transposon activation by exposure to a range of methylation, biotic and abiotic treatments are included in this programme.