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:However, when Hahb-4 was expressed from its own drought-inducible promoter, transgenic Arabidopsis plants did not show developmental effects but still were drought-resistant (Manavella et al. 2006).
Category: