Mining key genes related to root morphogenesis through genome-wide identification and expression analysis of gene family in citrus.

Morphogenesis of root is a vital factor to determine the root system architecture. Cytokinin response regulators (RRs) are the key transcription factors in cytokinin signaling, which play important roles in regulating the root morphogenesis. In this study, 29 RR proteins, including 21 RRs and 8 pseudo RRs, were identified from the genome of citrus, and termed as CcRR1-21 and CcPRR1-8, respectively. Phylogenetic analysis revealed that the 29 CcRRs could be classified into four types according to their representative domains. Analysis of -elements of indicated that they were possibly involved in the regulation of growth and abiotic stress resistance in citrus. Within the type A and type B , , , and highly expressed in roots and leaves, and dramatically responded to the treatments of hormones and abiotic stresses. , , and also highly expressed in roots under different treatments. Characteristic analysis revealed that the above 8 significantly and differentially expressed in the three zones of root, suggesting their functional differences in regulating root growth and development. Further investigation of the 3 highly and differentially expressed , , and , in 9 citrus rootstocks showed that the expression of , and was significantly correlated to the length of primary root, the number of lateral roots, and both primary root and the number of lateral roots, respectively. Results of this study indicated that were involved in regulating the growth and development of the root in citrus with different functions among the members.