Abstract
BACKGROUND: The phenylalanine ammonia-lyase (PAL) gene, a well-studied plant defense gene, is crucial for growth, development, and stress resistance. The PAL gene family has been studied in many plants. Citrus is among the most vital cash crops worldwide. However, the PAL gene family has not been comprehensively studied in most Citrus species, and the biological functions and specific underlying mechanisms are unclear. RESULTS: We identified 41 PAL genes from nine Citrus species and revealed different patterns of evolution among the PAL genes in different Citrus species. Gene duplication was found to be a vital mechanism for the expansion of the PAL gene family in citrus. In addition, there was a strong correlation between the ability of PAL genes to respond to stress and their evolutionary duration in citrus. PAL genes with shorter evolutionary times were involved in more multiple stress responses, and these PAL genes with broad-spectrum resistance were all single-copy genes. By further integrating the lignin and flavonoid synthesis pathways in citrus, we observed that PAL genes contribute to the synthesis of lignin and flavonoids, which enhance the physical defense and ROS scavenging ability of citrus plants, thereby helping them withstand stress. CONCLUSIONS: This study provides a comprehensive framework of the PAL gene family in citrus, and we propose a hypothetical model for the stress resistance mechanism in citrus. This study provides a foundation for further investigations into the biological functions of PAL genes in the growth, development, and response to various stresses in citrus.