Effect of NaCl Stress on Proline Metabolism in Two Varieties of Habanero Pepper

Although the role of proline (Pro) as an important osmolyte has been extensively studied, there are few comprehensive studies on their metabolism under salinity conditions. We investigated Pro metabolism in two habanero pepper varieties with contrasting salinity responses: Mayan Chan (tolerant) and Mayan Ba’alche (sensitive). First, a phylogenetic analysis of enzymes participating in their biosynthesis, P5CS and P5CR, and in its degradation, PDH, was performed. Additionally, the levels of their transcripts, the enzymatic activity, and Pro content were determined in plants subjected to 150 mM NaCl by short (0, 24, 48 and 72 h) and long (seven days) periods. Potassium flux in roots exposed to NaCl, in the absence or presence of Pro, was also measured. Phylogenetic analysis showed that the sequences were grouped according to their taxonomic family and not by salt tolerance of the species. Molecular and biochemical analyses showed significant differences between organs and varieties; the tolerant variety showed highest levels of transcripts, biosynthesis enzymes activities and accumulation of Pro. The results suggested that Pro metabolism in habanero pepper is a complex process, that is regulated at different levels and differentially between organs and varieties. Exogenous Pro only reduced potassium efflux in the sensitive variety exposed to NaCl, suggesting that a precise threshold of this amino acid is required to perform this function.