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The P34 Syringolide Elicitor Receptor Interacts with a Soybean Photorespiration Enzyme, NADH-Dependent Hydroxypyruvate Reductase

¹Department of Plant Pathology and Center for Plant Cell Biology, University of California, Riverside 92521 U.S.A.; ²Plant Genetics Unit, USDA-ARS Donald Danforth Plant Science Center, 975 N. Warson Rd., St. Louis, MO 63132 U.S.A.; ³Dupont Experimental Station, Rt. 141 and Henry Clay Road, Wilmington, DE 19880 U.S.A.

The syringolide receptor P34 mediates avrD-Rpg4 gene-for-gene complementarity in soybean. However, the mechanism underlying P34 signal transmission after syringolide binding is unknown. In an effort to identify a second messenger for P34, soybean leaf proteins were run though a P34-affinity column. A 42-kDa protein which specifically bound to the column was identified as a putative plant NADH-dependent hydroxypyruvate reductase (HPR) by N-terminal peptide sequencing. HPR is an important enzyme involved in the plant photorespiration system. Screening of a soybean cDNA library yielded two distinct HPR clones that encoded proteins with 97% identity (P42-1 and P42-2). Surprisingly, only P42-2 displayed good binding with P34 in a yeast two-hybrid assay, indicating that P42-2, but not P42-1, is a potential second messenger for P34. Glycerate and its analogs, which are utilized in the photorespiration system, were tested for their inhibitory effect on syringolide-induced hypersensitive response (HR) to evaluate the biological significance of P42-2. Interestingly, the downstream products of HPR (glycerate and 3-phosphoglycerate) inhibited HR but the upstream compounds (hydroxypyruvate or serine) did not have a significant effect on HR. These results suggest that P42-2 is a primary target for a P34/syringolide complex and that P42-2 binding with the complex probably induces HR by inhibiting one or more HPR functions in soybean.