Plant Genetic Engineering Laboratory

Plants need to perceive and accordingly respond to an incredible number of signals from their surrounding environment in order to survive. Aside from the sheer number of them, the nature of these signals can be extremely variable; chemical nutrients in the rhizosphere, day length (or more correctly, night), pathogenic microbes, water shortage, heat, cold, herbivores, gravity, etc. In order to recognize such a varied array of signals they have evolved a large number of receptors with very different structural characteristics. However, perception of a stimulus is only the first step in a long and complicated process culminating in the production of correct responses that can be as diverse as the original stimuli. Coupling each specific stimulus to the appropriate response is essential and can be the difference between life and death.  Aside from specificity, controlling the magnitude of the response is also important and needs to be proportionate to the perceived signal. Finally, while some responses such as flowering in response to photoperiod can be produced over a relatively wide window of time, in other cases speed is of the essence and stomatas need to be quickly closed in a hot mid-day before dehydration results in irreversible damage to the plant. In order to control the specificity, speed and magnitude of the response, plants have evolved sophisticated signal transduction mechanisms that in most cases are far from linear events. A single stimulus can trigger one or more receptors and each activated receptor can initiate several signalling cascades. To complicate it all, signalling to different stimuli can have extensive cross-talk as is the case in the response to hormones.

We are studying a family of signal transduction molecules called heterotrimeric G-proteins (G proteins) consisting of α, β and γ subunits that mediate many important signalling pathways in eukaryotes. They are involved in morphological development cell proliferation, ion-channel regulation, stomatal control, light perception, early seedling development, abiotic stresses and, most importantly, yield in very important crops such as rice.

Classically, ligand bound 7-transmembrane (7TM) spanning G protein-coupled receptors (GPCRs) catalyse the exchange of bound GDP for GTP on Gα, resulting in activation of the heterotrimer and dissociation of the two functional elements, the Gα subunit and the Gβγ dimer. Gα and Gβγ independently interact with multiple downstream effectors mediating specific signal transduction pathways. Eventually, the intrinsic GTPase activity of Gα hydrolyses GTP to GDP increasing its affinity for the Gβγ dimer, leading to the re-association of the inactive heterotrimer at the receptor.

Our group has made important contributions to the available knowledge of plant G proteins discovered several characteristics that make plant G proteins very different from their animal counterparts. For example we reported the first plant Gγ subunit (Mason and Botella 2001) and recently discovered a new class of Gγ proteins never before described in plant or animal systems, proving their involvement in development and guard cell physiology (Chakravorty et al. 2011). We also reported that two of the most important QTLs for rice yield are members of the new Gγ subunit family. We then provided a mechanistic model to explain the apparently contradictory results obtained by a number of groups working on these QTLs (Botella 2012). In our last published contribution, we have identified a set of completely new subunits to the heterotrimer. We have proven that, contrary to the current belief, XLG proteins can indeed be part of the G protein heterotrimer and replace the Gα subunit to form functional associations with the Gβγ dimer (Maruta et al. 2015).

Our research is now focused on establishing the molecular signalling relay used by G proteins to control all the above mentioned processes.