QS is important to plant-pathogen interactions, and their study has also contributed to the QS field more generally. The first X-ray crystallography results for some of the key proteins were those of "Pantoea stewartii" subsp. "stewartii" in maize/corn and "Agrobacterium tumefaciens", a crop pathogen with wider range of hosts. These interactions are facilitated by quorum-sensing molecules and play a major role in maintaining the pathogenicity of bacteria towards other hosts, such as humans. This mechanism can be understood by looking at the effects of N-Acyl homoserine lactone (AHL), one of the quorum sensing-signaling molecules in gram-negative bacteria, on plants. The model organism used is "Arabidopsis thaliana".