Transcriptomic and phytohormonal profiling of the adventitious shoot regeneration of Camellia reticulata and functional characterization of the CrSAUR20 gene.

In establishing in vitro regeneration of Camellia reticulata, we found that the callus from the wild C. reticulata plant could differentiate into the adventitious shoot (AS). In contrast, the callus from C. reticulata cultivar 'Purple Gown' has not yet differentiated into the AS, and the mechanism of AS regeneration was still unclear. The molecular mechanisms underlying AS regeneration were investigated using transcriptomic profiling and phytohormone quantification. Results showed that the levels of cytokinin, auxin, and other hormones in the callus of the wild C. reticulata plant were significantly changed compared with those of 'Purple Gown'. Transcriptomic profiling identified differentially expressed genes (DEGs) involved in phytohormone signaling and hormone biosynthesis pathways. Among these, the expression of response regulators (ARRs), auxin response factors (ARFs), indole-3-pyruvate monooxygenase (YUCCAs), small auxin up-regulated RNAs (SAURs), ethylene-responsive transcription factor 1/2 (ERF1/2), and others significantly differed between the wild C. reticulata plant and 'Purple Gown', affecting the development of AS in the callus of the C. reticulata. To further validate the gene function related to hormones, CrSAUR20 was cloned as an example and its overexpression vector was constructed. The overexpression of CrSAUR20 promoted an increased ratio of auxin-to-cytokinin in transgenic tobacco plants and affected the differentiation of AS and root from the leaf disc of transgenic tobacco plants, suggesting a positive regulatory role of CrSAUR20 in auxin signaling transduction. Genotype, phytohormone ratio, and phytohormone crosstalk were critical factors in enhancing AS regeneration capacity in the callus of C. reticulata.