The mitochondrial connection: Arginine degradation versus arginine conversion to nitric oxide.

Arg catabolism to cytoplasmic urea and glutamate is initiated by two mitochondrial enzymes, arginase and ornithine aminotransferase. Mutation of either enzyme leads to Arg sensitivity, and at least in the former, an arginine-induced seedling morphology similar to exogenous auxin treatment. We reported that single mutants lacking either of two arginase isozymes exhibited more NO accumulation and efflux, and increased responses to auxin (measured by DR5 reporter expression and auxin-induced lateral roots). We discuss evidence for stimulation of NO by arginine, either directly, or via polyamines derived from arginine. We favor the "direct" route because mitochondria are sites of NO 'hot spots,' and the location of arginine-degrading enzymes and the NO-associated protein1. The polyamine "branch" invokes more than one cell compartment, at least two intermediates (polyamines and H(2)O(2)) between Arg and NO, and is not consistent with enhanced lateral root formation in arginine decarboxylase mutants. Genetic tools are at our disposal to test the two possible routes of arginine-derived NO.