Abstract
Gibberellins (GAs) are phytohormones that regulate a wide range of developmental processes in plants. Levels of active GAs are regulated by biosynthetic and catabolic enzymes like the GA 2-oxidases (GA2oxs). In tomato (Solanum lycopersicum L.) C19 GA2oxs are encoded by a small multigenic family of five members with some degree of redundancy. In order to investigate their roles in tomato, the silencing of all five genes in transgenic plants was induced. A significant increase in active GA4 content was found in the ovaries of transgenic plants. In addition, the transgenic unfertilized ovaries were much bigger than wild-type ovaries (about 30 times) and a certain proportion (5-37%) were able to develop parthenocarpically. Among the GA2ox family, genes GA2ox1 and -2 seem to be the most relevant for this phenotype since their expression was induced in unfertilized ovaries and repressed in developing fruits, inversely correlating with ovary growth. Interestingly, transgenic lines exhibited a significant inhibition of branching and a higher content of active GA4 in axillary buds. This phenotype was reverted, in transgenic plants, by the application of paclobutrazol, a GA biosynthesis inhibitor, suggesting a role for GAs as repressors of branching. In summary, this work demonstrates that GA 2-oxidases regulate gibberellin levels in ovaries and axillary buds of tomato plants and their silencing is responsible for parthenocarpic fruit growth and branching inhibition.
Highlights
Gibberellins (GAs) are phytohormones that regulate a wide range of developmental processes in plants such as germination, stem elongation, leaf expansion, flowering, fruitset, and fruit growth (Yamaguchi, 2008; Mutasa-Göttgens and Hedden, 2009; Seymour et al, 2013)
This work demonstrates that GA 2-oxidases regulate gibberellin levels in ovaries and axillary buds of tomato plants and their silencing is responsible for parthenocarpic fruit growth and branching inhibition
The enzymes that catalyse the last steps of the biosynthesis pathways are Abbreviations: GA, gibberellin; GA2ox, gibberellin 2-oxidase; GA 3-oxidases (GA3ox), gibberellin 3-oxidase; GA 13-oxidase (GA13ox), gibberellin 13-oxidase; GA 20-oxidases (GA20ox), gibberellin 20-oxidase; PCB, paclobutrazol; RT–qPCR, reverse transcription-quantitative PCR; SE, standard error; shRNA, short-hairpin RNA; WT, wild type
Summary
Gibberellins (GAs) are phytohormones that regulate a wide range of developmental processes in plants such as germination, stem elongation, leaf expansion, flowering, fruitset, and fruit growth (Yamaguchi, 2008; Mutasa-Göttgens and Hedden, 2009; Seymour et al, 2013). The ent-kaurene is converted to GA12, the first GA of the pathway, by the action of two membrane-associated P450 enzymes (cytochrome P450 monooxygenases): ent-kaurene oxidase (KO) and ent-kaurenoic acid oxidase (KAO). From this step, there are two parallel pathways: the non-13-hydroxylation (leading to active GA4) and the early 13-hydroxylation (leading to active GA1), which depend on the action of the cytochrome P450 enzyme, GA 13-oxidase (GA13ox) (Magome et al, 2013). 5898 | Martínez-Bello et al. GA 20-oxidases (GA20ox) and GA 3-oxidases (GA3ox) which are 2-oxoglutarate dependent dioxygenases (2ODDs) (Yamaguchi, 2008; Hedden and Thomas, 2012)
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