Reinvigoration of diploid strawberry (Fragaria vesca) during adventitious shoot regeneration.

Diploid strawberry (Fragaria vesca 'Baiguo') is a model plant for studying functional genomics in Rosaceae. Adventitious shoot regeneration is essential for functional genomics by Agrobacterium tumefaciens-mediated transformation. An efficient shoot regeneration method using diploid strawberry leaf explants was conducted on 1/2MS + 1/2B5 medium that contained 2.0 mg L-1 TDZ over 14 days of dark culture; this induced the maximum percentage of shoot regeneration (96.44 ± 1.60%) and the highest number of shoots per explant (23.46 ± 2.14) after 11 weeks of culture. The explants considerably enlarged after 12 days; then, turned greenish brown after 30 days, yellowish brown after 36 days, and completely brown and necrotic after 48 days. Large numbers of adventitious shoots were produced from 48 to 66 days, and the shoots elongated from 66 to 78 days; this represents a critical period of reinvigoration, which included 30 days for leaf explant chlorosis, 36 days for adventitious shoot appearance, and 48 days for generation of numerous shoots. During the reinvigoration process, higher expressions of the hormone synthesis-related genes Ciszog1, CKX2, CKX3, CKX7, YUC2, YUC6, YUC10, YUC9, and GA2ox were detected from 30 to 48 days. Our results indicate that these genes may regulate reinvigoration of shoot regeneration.