Tomato (Solarium lycopersicum) SIIPT4, encoding an isopentenyltransferase, is involved in leaf senescence and lycopene biosynthesis during fruit ripening

Background: Lycopene is an important carotenoid pigment in red fruits and vegetables, especially in tomato. Although lycopene biosynthesis and catabolism have been found to be regulated by multiple factors including phytohormones, little is known about their regulatory mechanism. Cytokinins are crucial to various aspects of plant growth. Isopentenyltransferases (IPTs) catalyze the initial rate-limiting step of cytokinins biosynthesis, however, their roles in fruit ripening remain unclear. Results: Here, the functions of SIIPT4, encoding an isopentenyltransferase, were characterized via RNAi-mediated gene silencing in tomato. As we expected, silencing of SIIPT4 expression resulted in accelerated leaf senescence. However, down expression of SIIPT4 generated never red orange fruits, corresponding with a dramatic reduction of lycopene. Among lycopene biosynthesis related genes, the fact of remarkable decrease of ZISO transcript and upregulation of other genes, revealed that SIIPT4 regulates positively lycopene biosynthesis via directly affecting ZISO expression, and also supported the existence of regulatory loops in lycopene biosynthesis pathway. Meanwhile, the accumulation of abscisic acid (ABA) was reduced and the transcripts PSY1 were increased in SIIPT4-RNAi fruits, supporting the feedback regulation between ABA and lycopene biosynthesis. Conclusion: The study revealed the crucial roles of SIIPT4 in leaf senescence and the regulatory network of lycopene biosynthesis in tomato, providing a new light on the lycopene biosynthesis and fruit ripening.