2019 ASHS Annual Conference

Cold is a major limiting, even highly lethal, factor of citrus production worldwide. Poncirus trifoliata, a close relative of Citrus species, is extremely cold-hardy and has contributed its cold tolerance to many rootstocks widely used in the citrus industry in the world. Currently, the genes and genetic mechanisms for cold tolerance in Poncirus remain to be fully understood. We performed a comprehensive search for cold signaling-related genes in P. trifoliata, six Citrus species, and a primitive Citrus relative Atlantia buxifolia, using their orthologs that have been identified and characterized in Arabidopsis. The gene family analysis revealed a total of 46 gene families containing P. trifoliata genes and cold signaling pathway genes characterized in A. thaliana, such as CBF2, ICE1, MPK, and CAMTA. Additionally, there are 50 P. trifoliata gene families orthologous to either the first-wave transcription factor genes responsive to cold stress or CBF regulon genes in A. thaliana. For most of these gene families, P. trifoliata, Citrus species and A. buxifolia have equal or similar numbers of genes across the species. However, several gene families are present only in cold-hardy P. trifoliata and/or Citrus ichangensis but absent in cold-sensitive C. maxima and C. medica, and several other gene families have different numbers of genes between cold-hardy and cold-sensitive species. Among the genes specific to P. trifoliata, six were previously identified as being responsive to cold stress. Previous studies indicated that the hybrid proline-rich protein gene (PRP) positively regulates cold tolerance in P. trifoliata. Our analysis showed that P. trifoliata has two PRP genes (9 kb away from each other) while cold-sensitive Citrus species have only one PRP gene. The discovered gene families may play an important role in conferring cold tolerance in Poncirus. These findings may help understand the molecular mechanisms of cold tolerance in P. trifoliata and advance the development of cold-tolerant Citrus varieties.