Genetic engineering of chrysanthemum (Chrysanthemum morifolium): Current progress and perspectives

We give an overview of the advances of an Agrobacterium-mediated transformation system, clarifying its problems and their solutions, and then show the latest version of our transformation system and examples of the introduction of agronomically important traits into chrysanthemums. Typical problems with the Agrobacterium-mediated transformation in chrysanthemum include low transformation efficiency, high chimerism and cultivar specificity. Using a co-cultivation medium containing acetosyringone and casamino acids for high transformation efficiency and an antibiotic-selection step for transgenic calli before plant regeneration to eliminate the chimerism, we established an efficient and stable transformation system for chrysanthemum. In addition, this system was used to successfully introduce useful agronomical traits, such as insect resistance and new flower color, into chrysanthemums. These traits have been stably and highly expressed to confer the expected characteristics upon the transgenic chrysanthemums. Before applying a field trial of the genetically modified (GM) chrysanthemums, male and female sterility were introduced into the transformants to exclude the transgene flow from the GM plants to their wild relatives. So far, using RNAi technology, some of the transgenic chrysanthemums have displayed complete male sterility with very weak female fertility.