Suppressing the rhamnogalacturonan lyase gene FaRGLyase1 preserves RGI pectin degradation and enhances strawberry fruit firmness

Plant rhamnogalacturonan lyases (RGLyases) cleave the backbone of rhamnogalacturonan I (RGI), the “hairy” pectin and polymer of the disaccharide rhamnose (Rha)-galacturonic acid (GalA) with arabinan, galactan or arabinogalactan side chains. It has been suggested that RGLyases could participate in remodeling cell walls during fruit softening, but clear evidence has not been reported. To investigate the role of RGLyases in strawberry softening, a genome-wide analysis of RGLyase genes in the genus Fragaria was performed. Seventeen genes encoding RGLyases with functional domains were identified in Fragaria × ananassa. FaRGLyase1 was the most expressed in the ripe receptacle of cv. Chandler. Transgenic strawberry plants expressing an RNAi sequence of FaRGLyase1 were obtained. Three transgenic lines yielded ripe fruits firmer than controls without other fruit quality parameters being significantly affected. The highest increase in firmness achieved was close to 32%. Cell walls were isolated from ripe fruits of two selected lines. The amount of water-soluble and chelated pectins was higher in transgenic lines than in the control. A carbohydrate microarray study showed a higher abundance of RGI epitopes in pectin fractions and in the cellulose-enriched fraction obtained from transgenic lines. Sixty-seven genes were differentially expressed in transgenic ripe fruits when compared with controls. These genes were involved in various physiological processes, including cell wall remodeling, ion homeostasis, lipid metabolism, protein degradation, stress response, and defense. The transcriptomic changes observed in FaRGLyase1 plants suggest that senescence was delayed in transgenic fruits.