A Cationic Amino Acid Transporter NtCAT1 Promotes Leaf Senescence by the Accumulation of ABA in Nicotiana tabacum

Abstract

Leaf senescence is a comprehensive process performed by integrating various internal and external signals, followed by nutrient reallocation, especially via organic nitrogen (e.g., amino acids) mobilization. Amino acid (AA) transporters play an important role in crop growth and development by participating in the process of organic nitrogen remobilization. However, the biological functions and downstream effectors of amino acid transporters involved in leaf senescence are still poorly understood. In the present study, we cloned and characterized a cationic amino acid transporter gene, NtCAT1, from tobacco (K326). We found that NtCAT1 transcript levels were induced by age and abscisic acid (ABA). The NtCAT1 protein was highly localized in the plasma membrane. The overexpressing NtCAT1 line (OECAT1) showed early leaf senescence, accompanied with increased reactive oxygen species (ROS) and ABA content. By contrast, the NtCAT1 mutant (ntcat1-36) generated by the CRSPR/Cas9 system, showed a delayed-senescence phenotype with a decreased accumulation of ROS and ABA. Moreover, we discovered that the overexpression of NtCAT1 could downregulate the expression of the target of rapamycin (TOR) kinase gene and upregulate the transcript levels of ABA-related genes during leaf senescence compared with wild type (WT), while the expression of these genes in ntcat1-36 plants exhibited inverse trends. Furthermore, an analysis of the amino acid concentration demonstrated that NtCAT1 transgenic plants displayed dramatic changes in the amino acid profile during leaf senescence. In summary, our results suggest that NtCAT1 could promote leaf senescence via the increased biosynthesis of ABA, and our study provides new insights into the molecular mechanism of leaf senescence.