The enzyme guanylate cyclase (GC) forms cGMP which acts as a signalling molecule in many physiological responses. We have recently identified and partially characterised the first molecules from higher plants with GC activity. Several of these molecules appear to have multiple functions. Our aims are to determine if the GC functional domain is active and the biological importance of the GC domain in plant growth and development. The outcomes will provide new insight into the biological role and catalytic mechanisms of novel classes of multi-task enzymes with key functions in higher plants.