SUCROSE SYNTHASE IN WILD TOMATO, LYCOPERSICON-CHMIELEWSKII, AND TOMATO FRUIT SINK STRENGTH

Here it is reported that sucrose synthase can be readily measured in g rowing wild tomato fruits (Lycopersicon chmielewskii) when suitable me thods are adopted during fruit extraction. The enzyme also was present in fruit pericarp tissues, in seeds, and in flowers. To check for nov el characteristics, the wild tomato fruit sucrose synthase was purifie d, by (NH4)2SO4 fraction and chromatography with DE-32, Sephadex G-200 , and PBA-60, to one major band on sodium dodecyl sulfate-polyacrylami de gel electrophoresis. The following characteristics were obtained: n ative protein relative molecular weight 380,000; subunit relative mole cular weight 89,000; K(m) values with: sucrose 53 millimolar, UDP 18.9 micromolar, UDP-glucose 88 micromolar, fructose 8.4 millimolar; pH op tima between 6.2 to 7.3 for sucrose breakdown and 7 to 9 for synthesis ; and temperature optima near 50-degrees-C. The enzyme exhibited a hig h affinity and a preference for uridylates. The enzyme showed more sen sitivity to divalent cations in the synthesis of sucrose than in its b reakdown. Sink strength in tomato fruits also was investigated in rega rd to sucrose breakdown enzyme activities versus fruit weight gain. Su crose synthase activity was consistently related to increases in fruit weight (sink strength) in both wild and commercial tomatoes. Acid and neutral invertases were not, because the published invertase activity values were too variable for quantitative analyses regarding the role s of invertases in tomato fruit development. In rapidly growing fruits of both wild and commercially developed tomato plants, the activity o f sucrose synthase per growing fruit, i.e. sucrose synthase peak activ ity X fruit size, was linearly related to final fruit size; and the ac tivity exceeded fruit growth and carbon import rates by at least 10-fo ld. In mature, nongrowing fruits, sucrose synthase activities approach ed nil values. Therefore, sucrose synthase can serve as an indicator o f sink strength in growing tomato fruits.