The effect of gamma aminobutyric acid (GABA) foliar application on some biochemical characteristics and expression pattern of PAL and CHS genes in Qızıl Uzum grape (Vitis vinifera L.)

Grape is one of the most important fruit products globally and has a high nutritional value with strong antioxidant and anti-cancer activity. Today, the use of healthy natural compounds, including organic nitrogen compounds, has become very important to improve the qualitative performance of fruits. The use of amino acids as one of the natural compounds of nitrogen can increase the nutritional values of fruit crops. In the present study, gamma aminobutyric acid was used as a non-protein amino acid to improve the quality characteristics of Qizil-Uzum grape fruit.

The experiment was performed in two separate orchards located in two regions of Urmia city with different microclimatic conditions in a completely randomized design with GABA foliar application at 4 concentrations (0, 5, 10, 25 mM) in two stages (veraison stage and one week later) with 3 replications on 13-year-old cv. Qızıl Uzum grapevines. Some fruit quality characteristics include titratable acids (TA), total soluble solids (TSS), total antioxidant content, total phenol, total flavonoid, total anthocyanin, the activity of phenylalanine ammonialyase (PAL), catalase enzymes, phenolic compounds of fruit including flavonols, flavan-3-ols and phenolic acids, and also a relative expression of PAL and CHS genes were evaluated.

Based on the results, GABA at a concentration of 10 mM, caused the highest content of titratable acids, total soluble solids, total phenol, total flavonoids, total antioxidant and total anthocyanin of fruit. The highest activity of PAL enzyme was also observed at this concentration. Catalase enzyme had the maximum activity at 25 mM. The phenolic compounds that were measured by HPLC in this study included the flavonol compounds: myricetin, quercetin, kaempferol, syringetin; the flavan-3-ols compounds: catechin; and the non-flavonoid compounds: gallic acid, caffeic acid, p-coumaric acid and resveratrol, most of which had the highest level at 10 mM, followed by 5 mM GABA. Also, PAL and CHS genes had the highest expression at both sampling times (48 and 72 hours after foliar application) at the concentration of 10 mM GABA and their lowest expression was at the concentration of 25 mM GABA.

This study showed that GABA at the concentration of 10 mM at the veraison stage and one week later had an effect on increasing fruit quality indicators, including total soluble sugars, as a basic substrate for the biosynthetic pathway of effective fruit quality compounds, and with effect on antioxidant content improvement, as well as enhancing the expression of related genes for PAL enzyme activity, as a key enzyme of the biosynthesis of the phenolic compound, can improve fruit quality and marketability of grape fruit of Qızıl Uzum cultivar.