Quality Traits of Conventional and Transgenic Lettuce (Lactuca sativa L.) at Harvesting by NMR Metabolic Profiling

Metabolism of genetically modified (GM) lettuce (Lactuca sativa L.) leaves was investigated by comparing NMR metabolic profiles of three lines (T3B12, T7B7, and T7B14) overexpressing the E. coli asparagine synthetase A gene with those of the wild type (WT) at 24, 56, and 64 days after sowing (DAS). Statistical analyses based on hydro-soluble compound profiles significantly and maximally discriminated the WT from GM-lines at optimal harvest time (56 DAS). The T7B14 metabolic variations were opposite to those of both T3B12/T7B7 lines, suggesting that unexpected effects of transgenesis had occurred. Compared to controls, the T3B12/T7B7 plants shared the leaf mass increase, higher amino acid (asparagine, glutamine, valine, and isoleucine) and protein levels, and lower nitrate contents, accompanied by a modest sink of organic acids (α-chetoglutarate, succinate, fumarate, and malate), sucrose, fructose, and inulins. Incongruously, the T7B14 butter heads were less leafy than the controls and showed lowered amino acid/protein contents and overstored inulin. To further investigate the metabolic discrepancies among the GM-lines, a set of key nitrogen and inulin genes was monitored. The T3B12/T7B7 lines shared comparable gene expression changes, including the induction of the endogenous ASPARAGINE SYNTHETASE1 and NITRATE REDUCTASE1 that supported the targeted enhancement of nitrogen status. Transgene product malfunctioning and T-DNA rearrangements throughout generations were proposed to explain the decreased asparagine content and the complex expression pattern of N genes in T7B14 leaves. In the latter, the inulin accumulation was associated with the upregulation of fructan biosynthesis genes and the intense repression of fructan hydrolases.