Poster: Plant-pathogen interactions

Abs # 549: Adi3, a protein kinase of the AGC family interacts cooperatively with the tomato resistance protein Pto and the Pseudomonas syringae effector protein AvrPto and negatively regulates host cell death

Presenter:	Devarenne, Timothy P., tpd8@cornell.edu
Authors	Devarenne, Timothy P. (A)   Ekengren, Sophia K. (A)   Martin, Greg B. (A) (B)
Affiliations:	(A): Boyce Thompson Institute for Plant Research (B): Department of Plant Pathology, Cornell University

Bacterial speck disease of tomato is caused by Pseudomonas syringae pv. tomato (Pst). Resistance to speck disease is conferred by the host protein kinase Pto which, upon interacting with the Pst protein AvrPto, activates various defense responses including the hypersensitive response (HR), a form of programmed cell death (PCD). A yeast 3-hybrid screen for host proteins that require both Pto and AvrPto for their interaction identified Adi3 (an AvrPto Pto-dependent interacting protein; Bogdanove and Martin, 2000). Adi3 belongs to the AGC family of protein kinases which are commonly regulated by the upstream kinase 3-phosphoinositide-dependent protein kinase-1 (PDK1). In mammals, PDK1 phosphorylates the AGC kinase PKB/Akt which then acts to suppress PCD. In vitro kinase assays demonstrate that a tomato PDK1 interacts with and phosphorylates Adi3. Adi3 kinase activity is not required for its interaction with Pto/AvrPto in the yeast 3-hybrid system and kinase assays demonstrate that Pto phosphorylates Adi3, but Adi3 does not phosphorylate Pto. Additionally, interaction of Pto and AvrPto with Adi3 blocks the ability of PDK1 to phosphorylate Adi3. Phosphorylation activity of Adi3 is also inhibited by Ca⁺². Silencing of Adi3 using virus-induced gene silencing (VIGS) causes localized PCD in leaves suggesting a role for Adi3 in plants similar to PKB/Akt. We hypothesize that, upon phosphorylation by PDK1, Adi3 normally acts as a negative regulator of host PCD. Interaction of Adi3 with Pto/AvrPto, in combination with defense-related increases in cellular Ca⁺² levels, then inhibits Adi3 activity. This inhibition of Adi3 may lead to the HR that is associated with resistance to Pst. Bogdanove and Martin (2000) Proc. Natl. Acad. Sci. USA 97:8836.