Submitted to: Amino Acids
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/30/2011
Publication Date: 8/4/2011
Citation: Kausch, K.D., Sobolev, A.P., Goyal, R.K., Fatima, T., Laila-Beevi, R., Saftner, R.A., Handa, A.K., Mattoo, A.K. 2011. Methyl jasmonate deficiency alters cellular metabolome including the aminome of tomato (Solanum lycopersicum L.) fruit. Amino Acids. 42:843-856. Interpretive Summary: Ripening of fruits is a genetically regulated terminal developmental program of a plant, which involves a myriad of gene-expression and metabolic changes that together transform an inedible fruit to an edible, consumer-acceptable product. One of the genes implicated in flavor, taste and nutritional quality of fruits is called lipoxygenase (LOX). Hydroperoxides produced through the action of LOX are subsequently metabolized to a C6 volatile molecule called (Z)-hex-3-enal, which is one of the most valuable aroma components in tomato fruit. To study the role of LOX in tomato metabolism, we genetically silenced the expression of ripening-related LOXs in tomato fruit. The LOX-silenced fruits were found deficient in the production of a stress hormone called methyl jasmonate (MJA). Thirty one fruit metabolites were profiled including different classes of amino acids, organic acids, carbohydrates and other signaling molecules. The levels of a majority of these metabolites were negatively impacted in LOX-silenced transgenic fruit as compared to the wild type line, particularly at the beginning of the ripening process. These data suggest that MJA regulates shift in important molecules during fruit ripening and therefore LOX together with MJA can contribute towards the organoleptic characteristics of a fruit. These data are important to scientists and agriculturists interested in developing novel strategies to further the aroma and taste of fruits.
Technical Abstract: Lipoxygenase (LOX) catalyzes oxidation of C-13 atom of C:18 polyunsaturated fatty acids and produces jasmonic acid and other oxylipins that have important biological relevance. However, the role of these important molecules in cellular metabolism is barely understood. We have used a transgenic approach to discern the role of LOX (and indirectly its product jasmonate) in influencing the nutrient content of tomato (Solanum lycopersicum L.) fruit. Thus, homology-dependent gene silencing of a ripening-associated LOX was achieved by expressing a truncated fruit-specific LOX gene under the control of the constitutive promoter, CaMV35S. The fruits from tomato plants homozygous to the introduced transgene were deficient in LOX transcripts, LOX protein and methyl jasmonate content at all stages of ripening. However, the transgenic fruits were equivalent to the wild type in lycopene level and chlorophyll degradation rates but had slightly reduced levels of ethylene. These transgenic fruit were used to evaluate the effects of LOX and methyl jasmonate deficiency on cellular metabolites in ripening tomato fruits using NMR spectroscopy. During on-shelf ripening, transgenic fruits were significantly reduced in the content of 19 out of 30 metabolites examined, including Ile, Val, Ala, Thr, Asn Tyr, Glu, Gln, His, Phe, Trp, GABA, citrate, succinate, myo-inositol, unidentified compound B, nucleic acid compound Nucl1, choline, and trigonelline as compared to the wild type azygous counterparts. A significant increase in ß-glucose levels in transgenic fruits was observed at the pink stage. Taken together, these results show that silencing of a developmentally regulated LOX significantly affects overall primary metabolism, especially aminome (amino acids) of ripening fruits. The role of jasmonates as regulators of some of the ripening events independent of ethylene is hypothesized.