Abstract
BackgroundLycopene 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.ResultsHere, the functions of SlIPT4, encoding an isopentenyltransferase, were characterized via RNAi-mediated gene silencing in tomato. As we expected, silencing of SlIPT4 expression resulted in accelerated leaf senescence. However, down-expression of SlIPT4 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 SlIPT4 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 SlIPT4-RNAi fruits, supporting the feedback regulation between ABA and lycopene biosynthesis.ConclusionThe study revealed the crucial roles of SlIPT4 in leaf senescence and the regulatory network of lycopene biosynthesis in tomato, providing a new light on the lycopene biosynthesis and fruit ripening.
Highlights
Lycopene is an important carotenoid pigment in red fruits and vegetables, especially in tomato
Carotenoids are more than just pigments, they act as membrane stabilizers and the precursors of important plant hormones, such as abscisic acid (ABA) and strigolactone, to play important roles in photosynthesis and a variety of physiological processes including plant growth, fruit development and response to abiotic stress [4,5,6]
Lycopene is produced from carotenoid biosynthesis pathway, and has been proposed to proceed through a poly-cis pathway: geranylgeranyl diphosphate (GGPP) → 15-cis-phytoene → 9,15,9′-tri-cis-ζ-carotene → 9,9′-di-cis-ζ-carotene → prolycopene → all-trans-lycopene, catalyzed by phytoene synthase (PSY), phytoene desaturase (PDS), ζ-carotene isomerase (ZISO), ζ-carotene desaturase (ZDS), and carotene isomerase (CrtISO), respectively [2, 3, 10,11,12]
Summary
Lycopene is an important carotenoid pigment in red fruits and vegetables, especially in tomato. Carotenoids are a group of terpenoid pigments, naturally synthesized in plants, fungi, algae and photosynthetic bacteria [1], and usually give bright colors to fruit, flower and seed in plants [2, 3]. Carotenoids are more than just pigments, they act as membrane stabilizers and the precursors of important plant hormones, such as abscisic acid (ABA) and strigolactone, to play important roles in photosynthesis and a variety of physiological processes including plant growth, fruit development and response to abiotic stress [4,5,6]. Lycopene is a bright red linear carotenoid and widely exists in red fruits and vegetables.
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