Characterization of Glutamate-1-Semialdehyde Aminotransferase and Its Activity Throughout Tomato Fruit Development and Ripening

Abstract

Glutamate-1-semialdehyde aminotransferase (GSAT) is a key enzyme in the regulation of chlorophyll biosynthesis and a plausible site to regulate chlorophyll loss during fruit development via a decrease in its activity. The primary objective of this research was to characterize GSAT from tomato fruit by varying substrate concentration and pH and to measure GSAT in various organs of the tomato plant. A secondary objective was to measure GSAT activity and chlorophyll concentration at 5-day intervals from 10 d to 60 d postanthesis during tomato fruit development and ripening. An optimum pH of 7.0 was found, and a maximum velocity (Vmax) of 2.3 nmol·g–1 of tissue and a Km of 119 mmol·L–1 were calculated for GSAT. Activity among organs was greatest at 3.84 nmol·g–1 tissue in leaves of 4-week-old seedlings, 1.67 nmol·g–1 tissue in fruits 15 days postanthesis, and below 0.70 nmol·g–1 tissue in roots, stems, and overripe fruits (65 d postanthesis). During fruit development, GSAT activity was greatest at 10 d postanthesis and declined to day 25. Activity increased from day 25 to a peak at day 40 and then decreased again during ripening to 60 d postanthesis. Total chlorophyll and chlorophyll a concentrations in tomato fruit declined throughout development and ripening with the greatest decrease between 10 d and 25 d postanthesis. Thus, GSAT activity may contribute to the control of chlorophyll concentration in tomato fruit from 10 to 25 d postanthesis when chlorophyll loss mirrors the decline in GSAT activity.