University of North Carolina Chapel Hill, North Carolina
Body of Abstract: Arabidopsis thaliana REGULATOR OF G SIGNALING 1 (AtRGS1) G protein complex plays a role in sensing extracellular signals. Millimolar concentration of some sugars induce AtRGS1 endocytosis, therefore activate G protein signaling but this is not likely direct, rather indirect through a sugar metabolite. One metabolite at the plasma membrane is UDP-glucose produced by SUCROSE SYNTHASEs (SUS). UDP-glucose activates AtRGS1 with an EC50 of 17 µM. Our direct measurements indicate that the Kd that UDP-glucose binds AtRGS1 (3 mM) is insufficient to account for activation but AtRGS1 forms a complex with SUS1 and SUS4 that have Km values for UDP-glucose of 44 µM and 69 µM, respectively, and may be the mechanism for binding. The interaction between SUS and AtRGS1 requires AtRGS1 at a specific phosphocluster and AtRGS1 interaction with its substrate, AtGPA1, the G subunit of the heterotrimeric G protein complex. In the absence of UDP-glucose, both SUS1 and SUS4 stabilize this complex and repress both the growth and defense basal states. UDP-glucose induces rapid phosphorylation of AtRGS1 at its phosphocluster site leading to AtRGS1 activation. We show that downstream of UDP-glucose activation of AtRGS1 is a reprogramming of transcription, largely involving defense gene transcription consistent with UDP-glucose attenuation of pathogen ingress. These results provide the first direct evidence for UDP-glucose as a newly-recognized low-molecular weight signal controlling key aspects of plant physiology. This new UDP-glucose pathway shares no similarity to the well-characterized nucleotide-sugar signaling pathway mediated by P2Y receptors in animals.