This study investigated arginine metabolism in Spiroplasma citri, a leafhopper-transmitted pathogen of citrus plants. Although previous reports claimed S. citri could not hydrolyze arginine, pH fluctuations in arginine-supplemented cultures suggested metabolic activity. The researcher grew S. citri in basal medium with various combinations of arginine, glucose, and fructose, measuring pH changes and ammonia production daily. Electrophoresis with radioactively labeled arginine tracked amino acid transformations. Results showed that S. citri metabolizes arginine via the arginine dihydrolase pathway, producing up to 30 mM ammonia when sugars were present. The pathway converts arginine to ornithine and citrulline, but citrulline accumulated incompletely, even under optimal growth conditions. Complete arginine utilization occurred only with glucose or fructose supplementation. The author concluded that previous negative results arose because arginine alone provided insufficient energy for growth and ammonia production to exceed detection limits using standard phenol red indicators.

Key findings

• Spiroplasma citri metabolizes arginine via the arginine dihydrolase pathway, contradicting earlier reports of no arginine hydrolysis activity
• Arginine metabolism produces substantial ammonia (up to 30 mM) only when glucose or fructose are present as additional energy sources
• Citrulline accumulates as a degradation intermediate and is not completely converted to ornithine, limiting ATP generation from this pathway
• Arginine alone supports minimal ammonia production and poor growth, explaining why standard arginine hydrolysis tests failed to detect activity in this organism