Regulation Of Carotenoids Accumulation In Tomato Fruit By Jasmonic Acid And Brassinosteroid

Tomato(Solanum lycopersicum) fruit is of biological and dietary significance. One of the main characteristics of tomato fruit ripening is a massive accumulation of carotenoids (mainly lycopene), which may contribute to the nutrient quality of tomato fruit and its role in chemoprevention. Significant progress has been achieved in recent years in the understanding of carotenoid biosynthesis in tomato using biochemical and genetics approaches. The carotenoid metabolic network is temporally and spatially controlled, and plants have evolved strategic tactics to regulate carotenoid metabolism in response to various developmental and environmental factors. The mechanisms controlling carotenoid metabolism during fruit ripening are systematic and sophisticated. The present study investigated the role of new class phytohormone jasmonic acid (JA) and brassinosteroid (BR) in enhancing carotenoid accumulation of tomato fruit. The tomato mutants and transgenic plants related to the signal transduction and biosynthesis of phytohormones, as well as iTRAQ proteomics analysis approach were used to clarify the mechanism underlying the regulation of carotenoid accumulation during tomato fruit ripening. Following are the main results.1. The role of JA in controlling lycopene accumulation in tomato fruits was analysed by measuring fruit lycopene content and the expression levels of lycopene biosynthetic genes in JA-deficient mutants (spr2and defl) and a35S::prosystemin transgenic line (35S::prosys) with increased JA levels and constitutive JA signaling. The lycopene content was significantly decreased in the fruits of spr2and defl, but was enhanced in35S::prosys fruits. Simultaneously, the expression of lycopene biosynthetic genes followed a similar trend. Lycopene synthesis in methyl jasmonate (MeJA) vapour-treated fruits showed an inverted U-shaped dose response, which significantly enhanced the fruit lycopene content and restored lycopene accumulation in spr2and def1at a concentration of0.5μM. The results indicated that JA plays a positive role in lycopene biosynthesis. Previous studies have shown that ethylene plays a central role in promoting fruit ripening. In addition, the role of ethylene in JA-induced lycopene accumulation was also examined. Ethylene production in tomato fruits was depressed in spr2and defl while it increased in35S::prosys. However, the exogenous application of MeJA to Never ripe (Nr), the ethylene-insensitive mutant, significantly promoted lycopene accumulation, as well as the expression of lycopene biosynthetic genes. Based on these results, it is proposed that JA might function independently of NR-mediated ethylene to promote lycopene biosynthesis in tomato fruits.2. The BR response transcription factor Brassinazole resistant1(BZR1)-mediated BR signaling regulates many specific developmental processes including fruit ripening. The effect of2,4-epibrassinolide (EBR) and BZR1-1D over-expression on carotenoid accumulation and quality attributes of tomato fruit was explored. EBR-treated pericarp discs of Nr, accumulated significantly more carotenoid than those of the control. The results suggest that BR seems to be ethylene-independently involved in modulating pigments accumulation. When three independent transgenic lines over-expressing the AtBZR1-1D were used to evaluate the role of activated BZR1in regulating tomato fruit carotenoid accumulation and quality attributes, fruits of all three transgenic lines exhibited enhanced carotenoid accumulation and increased soluble solid, soluble sugar and ascorbic acid contents during fruit ripening. In addition, the fruits of two transgenic lines (BZR1-1D46and BZR1-1D#23) showed dark green shoulder at mature green stage, in accordance with the up-regulated expression level of SIGLK2, which is involved in chloroplast development. Transmission electron microscopy (TEM) revealed that BZR1-1D expression increased the number of MG fruit chloroplasts and plastoglobules per chloroplast profile, and promoted grana thylakoids development. Thus, BZR1-1D appears to be involved in regulating pigments accumulation in tomato fruits. The results indicate the importance of BZR1-centred BR signaling in regulating carotenoid accumulation and quality attributes of tomato fruit and the potential application of the BZR1-like(s) for improvement of nutritional quality and flavor of tomato through genetic engineering.3. To better understand the regulatory mechanisms underlying BZR1regulated fruit ripening, the comprehensive analysis of the proteome in tomato fruits at IM, MG, B and R stage of WT and two transgenic lines (BZR1-1D#6and BZR1-1D#23) were performed using high-throughput isobaric Tags for Relative and Absolute Quantification (iTRAQ) and high-resolution mass spectrometry.2338proteins representing2338individual genes were identified. Among them,389proteins were differently expressed. The proteins whose abundances change between BZR1-1D#6/BZR1-1D#23and WT at IM, MG, B and R stage were shown to be involved in various cellular processes, as well as the proteins differently expressed between BZR1-1D#6/BZR1-1D#23and WT during ripening stage transition from IM to MG and from B to R. The potential influence-targets of BZR1-1D are involved in several aspects including nutrient reservoir, cell defense, protein synthesis and processing, cell-wall metabolism and light harvest in photosynthesis.