Symposium V: The Plasma Membrane: A Happening Place

Abs # 50002: Heterotrimeric G protein signaling in Arabidopsis guard cells

Presenter:	Assmann, Sarah M, sma3@psu.edu
Authors	Assmann, Sarah M (A)   Fan, Liu-Min  (A)   Coursol, Sylvie  (A)   Wang, Xi-Qing  (A)   Le Stunff, Herve  (B)   Ullah, Hemayet  (C)   Jones, Alan  (C)   Gilroy, Simon  (A)   Spiegel, Sarah  (B)
Affiliations:	(A): Department of Biology, Penn State University, University Park, PA, 16802-5301, USA (B): Department of Biochemistry, Medical College of Virginia, Virginia Commonwealth University, Richmond, VA 2398-0614, USA (C): Department of Biology, University of North Carolina Chapel Hill, NC 27599-3280, USA

In humans, heterotrimeric G proteins composed of a, b, and g subunits couple stimulus perception by G protein-coupled receptors (GPCRs) with numerous downstream effectors, including those mediating vision, olfaction, and aspects of gustation. In contrast, despite plants’ equal or greater complexity in signal perception, the Arabidopsis genome encodes only single canonical Ga and Gb subunit genes (GPA1 and AGB1, respectively), and two Gg subunits (AGG1 and AGG2). In animal systems, G proteins regulate both Ca²⁺ and K⁺ ion channels. We are investigating the role of G proteins in ion channel regulation in stomatal guard cells from Arabidopsis. In guard cells, inhibition of inwardly rectifying K⁺ channels and activation of slow anion channels by the plant hormone abscisic acid (ABA) plays a crucial role in inhibition of stomatal opening and promotion of stomatal closure in response to drought. Null mutations in GPA1 and AGB1 result in guard cells that are insensitive to ABA-inhibition of stomatal opening, while retaining a wild-type response to ABA promotion of stomatal closure. Consistent with these data, whole cell patch clamp analyses indicate that ABA inhibition of inward K⁺ channels is eliminated in these mutant lines, while ABA-activation of slow anion channels is retained through a backup or parallel mechanism involving ABA signaling via cytosolic pH changes. Nevertheless, rates of transpirational water loss are greater in gpa1 and agb1 mutants than in wild-type plants. Recent results indicate that the sphingolipid metabolite, sphingosine-1-phosphate (S1P), is an important intermediate in the ABA response. ABA increases the activity of sphingosine kinase, the enzyme responsible for S1P formation, in guard cells. S1P application to wild type epidermal peels and protoplasts mimics ABA regulation of stomatal apertures, K⁺ channels, and slow anion channels. These S1P responses are not observed in gpa1 null lines, indicating that S1P signals via a heterotrimeric G protein in guard cells. This result is consistent with a known mammalian paradigm, in which S1P interacts with GPCRs to initiate downstream signaling events. G protein research in SMA’s laboratory is supported by grants from the National Science Foundation and U.S. Department of Agriculture.