Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:Although other metabolic engineering efforts in amino acid metabolism such as over expressing glutamine synthetase and glutamate synthase resulted in improved total free amino acids and growth phenotypes in transgenic plants (Ameziane et al., 2000; Migge et al., 2000; Chichkova et al., 2001; Fuentes et al., 2001; Oliveira et al., 2002; Fu et al., 2003), the bacterial L-aspartate-a-decarboxylase was not previously expressed in transgenic
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