Rekha Seshadri in Nature:
Close your eyes and imagine stalks of barley and corn waving gently against a butterscotch-coloured sky under the light of Phobos and Deimos — on the ruddy plains of a terraformed Mars! Although this science fiction fantasy has been re-imagined many times in cinema and print, the following recent studies bring this make-believe scenario infinitesimally closer to the realm of credibility. But more urgently, these studies begin to address potential solutions for alleviating pressures arising from agricultural burdens and climate change on our own planet.
Geddes, Paramasivan, Joffrin et al.1 successfully engineered Medicago trunculata (barrelclover) and Hordeum vulgare (barley, a cereal crop) with a synthetic pathway for bacterial-derived rhizopine, which was exuded into the plant root milieu and functioned as a signal to recruit a rhizopine biosensor-carrying Rhizobium leguminosarum. Engineering crops, cereals in particular, to gain important nutrients such as N2 without costly and environmentally detrimental fertilizer input is the ‘Holy Grail’ of agronomists, and the study by Geddes et al.1 proves this goal is achievable. N2 fixing bacteria or other growth-promoting bacteria could be specifically recruited and induced by engineering plant signalling and control. Also evidenced in this study, a lack of comprehensive functional knowledge of genes that participate in rhizopine metabolism proved an impediment, and considerable effort was expended in elucidating the native bacterial pathway, with partial success. Ultimately, success was achieved with an alternate rationally designed pathway (comprising genes from two different bacterial sources) that was transferred to the plant host with the desired effects.