Metabolism of endogenous arginine in tomato fruit harvested at different ripening stages

• The expression of LeARG1 and LeARG2 decreased with fruit ripening.
• Polyamines production mainly depended on the expression of ADC but not ODC.
• NO content and NOS activity in mature green fruit was higher than that in other stages.
• OAT expression increased at later stages and accompanied by proline accumulation.
• The roles of arginine in fruit ripening depend on the coordination of its metabolic pathways.

Tomato (Solanum lycopersicum cv. Messina) fruit, at five ripening stages (mature green, breaker, pink, light red and full red), have been analyzed for the expression of genes involved in arginine metabolism, polyamines and arginine-related amino acids content, as well as the nitric oxide synthase (NOS) activity and nitric oxide (NO) content. During ripening, the expression of LeARG1 and LeARG2, two genes encoding arginase, and NOS activity, as well as NO content decreased and the highest levels of them were found in mature green fruit. The expression of gene encoding arginine decarboxylase (ADC) increased with fruit ripening and reached the highest value at pink stage, which possibly contributed to the increased polyamines concentration. While the ornithine decarboxylase (ODC) might play a minor role in polyamines biosynthesis during tomato fruit ripening, the gene expression pattern of which differed with that of polyamines accumulation. The expression of gene encoding ornithine aminotransferase (OAT) increased during the later ripening stages of tomato, which was accompanied by proline accumulation. From all the amino acids tested, glutamate content was the most abundant and showed a marked increase during the course of fruit ripening. In contrast, arginine and ornithine contents remain relatively uniform throughout fruit ripening. These results implicate that the unique physiological roles of arginine in fruit ripening may depend on the coordination of different pathways of arginine metabolism.