Overexpression of the Brassica rapa SRS7 gene in pot-type chrysanthemum [Chrysanthemum morifolium Ramat] reduces plant height

Abstract

Retardation of plant growth is important in horticultural plant production and is generally achieved through the application of various chemical plant growth regulators (PGRs). Chrysanthemum plants grown in pots (pot-mums), are typically treated with PGRs to produce a compact form; however, these compounds can be hazardous to the environment and human health. In an effort to develop a method to produce compact chrysanthemum plants without using PGRs, we introduced the BrSRS7 gene from Brassica rapa L. ssp. pekinensis into chrysanthemum with its expression controlled by the promoter of the chrysanthemum actin gene (CmActin). As a first step, we evaluated the regeneration capacities of seven pot-mum cultivars and identified the Peace Copper (PC) cultivar as having the best regeneration efficiency. After transformation of the BrSRS7 gene into PC followed by selection and 5 rounds of vegetative subcultivation, 11 transgenic lines were identified for analysis. Internode lengths were reduced in all lines compared to control plants, but plant heights of some lines were similar to those of control plants. Four transgenic lines were selected for a sixth round of subcultivation and for further study. We determined the relative expression levels of gibberellin- and auxin-related genes in the four transgenic lines. Of the auxin-related genes, ARF2 and YUCCA10 showed higher expression in the transgenic lines than in the control. The expression levels of GA-related genes decreased overall in P-33, which showed the greatest decrease in height. Subjecting the transgenic lines to a dwarfing treatment with the PGR B9 showed that the plants could be reduced to actual marketable size with only a single dwarfing treatment. Our results demonstrate the possibility of using the BrSRS7 gene to produce compact chrysanthemum plants with reduced or no use of PGRs.