Genetic transformation of the astaxanthin biosynthetic genes bkt and crtR-B into apple tree to increase photooxidation resistance

Apples may suffer sunburn during growing season because of photooxidation damage. Astaxanthin have a stronger antioxidant activity against photooxidation compared to its precursors zeaxanthin and β-carotene. Here, we transferred β-carotene ketolase (bkt) and β-carotene hydroxylase (crtR-B), two genes encoding key enzymes for astaxanthin biosynthesis, into “Brookfield Gala” apple plants using pCAMBIA1302-bkt-crtR-B via transformation mediated by Agrobacterium tumefaciens EHA105. We aimed to confer a stronger antioxidant ability against sunburn to apple plants by regulating metabolic pathways of carotenoids. RT-PCR and qPCR analysis confirmed that bkt and crtR-B genes were expressed simultaneously in transgenic plants. Southern blotting analysis confirmed that the transgene was stably integrated and determined transgene copy number. Transgenic plants synthesized novel ketocarotenoids, resulting in an accumulation of astaxanthin up to 12.06 μg/g and of canthaxanthin up to 6.38 μg/g per leaf fresh weight. The light antioxidant ability of transgenic plantlets was stronger than that of non-transgenic ones in high light conditions. We speculate that the transgenic apples accumulating astaxanthin are resistant to photooxidation and can therefore prevent sunburn.