Sucrose Synthase in Wild Tomato,

Here it is reported that sucrose synthase can be readily measured in growing wild tomato fruits (Lycopersicon chmielewskii) when suitable methods are adopted during fruit extraction. The enzyme also was present in fruit pericarp tissues, in seeds, and in flowers. To check for novel characteristics, the wild tomato fruit sucrose synthase was purified, by (NH4)2SO4 fraction and chromatography with DE-32, Sephadex G-200, and PBA-60, to one major band on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The following characteristics were obtained: native protein relative molecular weight 380,000; subunit relative molecular weight 89,000; Km values with: sucrose 53 millimolar, UDP 18.9 micromolar, UDP-glucose 88 micromolar, fructose 8.4 millimolar; pH optima between 6.2 to 7.3 for sucrose breakdown and 7 to 9 for synthesis; and temperature optima near 500C. The enzyme exhibited a high affinity and a preference for uridylates. The enzyme showed more sensitivity to divalent cations in the synthesis of sucrose than in its breakdown. Sink strength in tomato fruits also was investigated in regard to sucrose breakdown enzyme activities versus fruit weight gain. Sucrose 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 roles of invertases in tomato fruit development. In rapidly growing fruits of both wild and commercially developed tomato plants, the activity of sucrose synthase per growing fruit, i.e. sucrose synthase peak activity X fruit size, was lineariy related to final fruit size; and the activity exceeded fruit growth and carbon import rates by at least 10-fold. In mature, nongrowing fruits, sucrose synthase activities approached nil values. Therefore, sucrose synthase can serve as an indicator of sink strength in growing tomato fruits. In working on sucrose breakdown in various types of plant tissues, we have sought to identify metabolic activities that act as control points and that may be useful as biochemical indicators of plant sink strength. The enzyme activities identified include sucrose synthase or acid invertase in specific plant sink tissues along with the PPi-dependent phosphofructokinase and fructokinase in some sink tissues (20-22, 28). ' Recipient of a study stipend from Jiangsu Agricultural College, Yangzhou, The People's Republic of China. For example, from work with various plant sinks, it was concluded that sucrose synthase activity was a biochemical indicator of sink strength in growing Solanum tuberosum tubers, in developing lima bean seeds, and seasonally in the vascular cambial tissue of tree roots (20, 21, 28). In contrast, recent work on carbohydrate assimilation with tomato fruits (29, 30) reported that sucrose synthase activity was not detectable at any time during fruit development with the wild tomato (Lycopersicon chmielewskii), and acid invertase activity was low throughout fruit development. In the same studies, fruits of commercial tomato varieties (Lycopersicon esculentum), expressed both an active sucrose synthase and an acid invertase. With commercial tomatoes, the specific activity of sucrose synthase also changed markedly during fruit development in correlation with ADP-glucose pyrophosphorylase; and both of these activities correlated with a transient starch accumulation pattern found in developing fruits (17). Other work also suggests that ADP-glucose pyrophosphorylase activities may correlate with sink development, particularly in starch-accumulating sinks (15). The apparent absence of sucrose synthase in a low-yielding wild tomato plant fruits and its presence at good activity levels in high-yielding commercial tomato varieties appeared to be related to yield and sink strength in tomatoes (17, 29, 30). However, a recent report (10) with another wild tomato (Lycopersicon hirsutum) demonstrated a low-activity sucrose synthase throughout fruit development. In earlier sink strength work with tomato fruits, other workers concluded that "sink activity" was a more primary determinant of sink strength than sink size (1, 26). Most enzyme work on tomato fruit sink activity suggested that invertase activity may be one determinant. A few reports have appeared on sucrose synthase in tomato fruits (8, 10, 17, 29, 30), but none concluded that its activity was an indicator of sink strength or that it is strongly related to fruit growth. Indeed, the possible absence of sucrose synthase in wild tomato fruit could strengthen the theory that invertase activity correlates with tomato fruit sink growth (8, 9, 27). Because of our studies on identifying metabolic components of sink strength to use in genetic and molecular modifications of plant sinks, we investigated the three initial enzymes of sucrose breakdown in developing tomato fruits. In addition, the inability to detect sucrose synthase in wild tomato (30) concerned us because we had found an active enzyme in growing tubers ofthe wild potato (Solanum acule),