Nitric oxide production shifts metabolic pathways toward lignification to alleviate Pb stress in Prosopis farcta

The mechanisms through which nitric oxide (NO) can influence plant tolerance to Pb are unknown, but may be based on the accumulation of secondary metabolites such as phenolic compounds and polyamines (PAs). This hypothesis was tested by exposing hydroponically-cultured Prosopis farcta to Pb alone and in combination with sodium nitroprusside (SNP; NO donor), L-NAME (NO synthase inhibitor) and tungstate (TUN; nitrate reductase inhibitor) for a time course of 72 h. The inhibitors strongly repressed NO generation, verifying that Pb and Pb + SNP-induced NO is probably mediated by nitrite and arginine-dependent pathways. The results show that the Pb-induced inhibition of growth was alleviated through application of NO donor, which decreased Pb accumulation in the plant samples, but deteriorated with the decrease in NO, which indicates a higher level of Pb absorbed. The elevated contents of phenolic acids under Pb and Pb + SNP treatments was reduced by the NOS and NR inhibitors and was significantly associated with phenylalanine ammonia-lyase (PAL) gene expression patterns. The arginine decarboxylase (ADC) gene in the PM biosynthesis pathway remained unchanged. Considerable alteration was observed in the conjugated PM in response to the Pb and Pb + SNP applications when compared with the NR and NOS inhibitors that exhibited an increase in free PM. The results suggest that Pb-evoked NO promotes homeostasis of metabolic pathways to phenolic acids and conjugated PM that enhance lignification to strengthen P. farcta against stress.