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:This phenotype is reminiscent of the phenotype observed in Arabidopsis plants expressing Hahb-4, a drought-inducible HD-Zip family I gene from sunflower that was implicated in ethylene-mediated control of senescence (Manavella et al. 2006).
Category: