Genome-wide identification, bioinformatics characterization and functional analysis of pectin methylesterase inhibitors related to low temperature-induced juice sac granulation in navel orange (Citrus sinensis Osbeck)

Pectin methylesterase inhibitors (PMEI) plays crucial roles in cell wall modification by inhibiting the pectin methylesterase (PME) activity in plant growth and development. Although PMEI has been well characterized in model plants, the knowledge of the molecular evolution and biological functions of PMEI gene family in Citrus remains limited. PMEI proteins play key roles in regulating pectin content, and the accumulation of pectin is a typical symptom of low temperature-induced juice sac granulation in navel orange. To reveal the roles of PMEI gene family in juice sac granulation processes in Citrus, we performed a genome-wide characterization of the PMEI family including identification of PMEI genes, chromosomal localization, phylogenetic relationships, expression patterns, subcellular localization, and functional characterization. A total of 45 PMEI genes were identified from the Citrus sinensis genome, and these 45 PMEI genes were further divided into three clades based on their phylogenetic relationship. The expression patterns analyses of CsPMEI genes during fruit ripening, juice sac granulation, and low-temperature treatment revealed that CsPMEI genes were involved in the low temperature-induced juice sac granulation in navel orange fruits. Moreover, subcellular localization analysis suggested that CsPMEI19 was localized in the cytoplasm and CsPMEI32 was localized on the plasma membrane. Furthermore, stable transformation in navel orange calli showed that over-expression of CsPMEI19 significantly increased the pectin content. Our comprehensive analyses of evolution, expression pattern, and subcellular localization of the PMEI gene family in citrus, and provides a novel insight into the biological functions of CsPMEI genes in juice sac granulation of navel orange.