Phytochelatin and proline-related genes expression in tomato exposed to heavy metals

Abstract

Heavy metals are one of the abiotic stress factor for plants, they effect the plant life cycles binding to sulfhydryl groups in amino acids and proteins. The presence of heavy metal in plant tissues changes the plant’s molecular, biochemical and physiological responses such as variations in gene expression, enzyme activities and amino acid content. This study aimed to determine the expression of P5CS and PCS genes to investigate their roles in the leaves of tomato plants subjected to increasing doses of heavy metals. The cultivars of tomato (Solanum lycopersicum cv.) were cultivated in plastic pots containing peat and garden soil (1:1) under greenhouse conditions. Three weeks after the plantings, the tomato plants were exposed to 10, 20 and 50 ppm doses of CdCI2, CuSO4 and Pb(NO3)2 salts. The leaves of tomato were harvested for sampling two weeks after the treatments and total RNA was extracted by using the Plant RNA Mini-Preps Kit. The first-strand cDNA was synthesized from the isolated RNA; the expression of the selected genes was detected by using RTqPCR with SYBR Green RT-PCR kit. Also, free proline content and metal-chelating ability was identified spectrophotometrically. The application of heavy metals significantly increased the expression of PCS1 gene in the leaves of tomato compared to control plants. The most evident expressions were observed in plants given the high doses of heavy metals. Transcript expression of P5CS1 changed depending on heavy metal doses; treatments of Cu (20 and 50 ppm), Cd and Pb (10 and 20 ppm) remarkably increased P5CS1 expression. The metal-chelating potency of the extract of tomato leaves exposed to Pb and Cd was higher than that of untreated plants. The proline content in the leaf extract significantly increased with the addition of 10 and 20 ppm of Pb and Cd, but significantly decreased at high doses (50 ppm) (p<0.05). The metal-chelating activity was higher in intervention plants cultivated in Pb and Cd-containing soils than in control plants (p<0.05). The results showed that the proline content had a positive correlation with P5CS1 transcript expression when the tomato plant is subjected to the three heavy metals. In addition, there was a positive relation between the PCS gene expression and metalchelating ability of Cd-stressed plants. In this context, the gene expressions of PCS1 and P5CS1 related with the PCs and proline content, respectively were correlated in the leaves of tomato under heavy metal stress. To further define and elucidate the responses of plants under stress conditions, the expression of genes and their corresponding enzymes associated with PCs and proline metabolisms, such as P5CR, P5CDH, PDH, POX, ProT, ?- ECS, and PC synthase, should be studied together to reveal the depth of responses in tomato subjected to various environmental conditions.