Abstract
Abscisic acid (ABA) plays an important role in fruit development and ripening. Here, three NCED genes encoding 9-cis-epoxycarotenoid dioxygenase (NCED, a key enzyme in the ABA biosynthetic pathway) and three CYP707A genes encoding ABA 8'-hydroxylase (a key enzyme in the oxidative catabolism of ABA) were identified in tomato fruit by tobacco rattle virus-induced gene silencing (VIGS). Quantitative real-time PCR showed that VIGS-treated tomato fruits had significant reductions in target gene transcripts. In SlNCED1-RNAi-treated fruits, ripening slowed down, and the entire fruit turned to orange instead of red as in the control. In comparison, the downregulation of SlCYP707A2 expression in SlCYP707A2-silenced fruit could promote ripening; for example, colouring was quicker than in the control. Silencing SlNCED2/3 or SlCYP707A1/3 made no significant difference to fruit ripening comparing RNAi-treated fruits with control fruits. ABA accumulation and SlNCED1transcript levels in the SlNCED1-RNAi-treated fruit were downregulated to 21% and 19% of those in control fruit, respectively, but upregulated in SlCYP707A2-RNAi-treated fruit. Silencing SlNCED1 or SlCYP707A2 by VIGS significantly altered the transcripts of a set of both ABA-responsive and ripening-related genes, including ABA-signalling genes (PYL1, PP2C1, and SnRK2.2), lycopene-synthesis genes (SlBcyc, SlPSY1 and SlPDS), and cell wall-degrading genes (SlPG1, SlEXP, and SlXET) during ripening. These data indicate that SlNCED1 and SlCYP707A2 are key genes in the regulation of ABA synthesis and catabolism, and are involved in fruit ripening as positive and negative regulators, respectively.
Highlights
Abscisic acid (ABA) plays an important role in plant growth, stomatal movement, seed dormancy, and germination (Melcher et al, 2009; Nishimura et al, 2009)
SlNCED1 decreases from 10 days after full bloom (DAFB) to the mature green (MG) stage, it increased sharply and peaked at the turning stage; after that it declined to a low level at the over ripe (OR) stage (Fig. 1A)
For SlCYP707A2, expression was downregulated in the fruits under both ABA treatment and dehydration at 1 days after treatment (DAT) (Fig. 1D, F), and it significantly increased at 2 DAT (Fig. 1D, F)
Summary
Abscisic acid (ABA) plays an important role in plant growth, stomatal movement, seed dormancy, and germination (Melcher et al, 2009; Nishimura et al, 2009). Major progress has been made in research into the role of ABA in the regulation of fleshy fruit ripening (Rodrigo et al, 2006; Zhang et al, 2009a, b; Giribaldi et al, 2010). These physiological processes controlled by ABA are primarily regulated by the bioactive ABA pool size, which is thought to be maintained by its biosynthesis, and by its catabolism (Sawada et al, 2008). The carotenoid-biosynthetic pathway begins with the formation of phytoene from two molecules of geranylgeranyl diphosphate (GGPP) in the central isoprenoid pathway.
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