Identification of Actively Filling Sucrose Sinks

Abstract

ABSTRACTpropose that they should be more correctly termed sucrolysis and sucroneogenesis. Before recent work it was customary to assume that sucrose synthase action resulted in the formation of UDP-glucose and then other nucleotide sugars leading into sugar polymer synthesis, such as plant cell walls. However, a substrate level pool of PPi was measured in plants (2,7,27), and we successfully tested the pyrophosphorolysis of UDPglucose feeding glucose 1-P directly into plant metabolism (32). These steps are an integral part of the recently proposed sucrose synthase pathway (1,15,30). Then, of course, from glycolysis carbon can be directed into essentially every metabolic activity of a cell. Therefore, the ability of a tissue or organ to metabolize sucrose must be one determinant of sink strength. Here we have tested the feasibility of biochemically measuring sucrose sink strength by assaying sucrose cleavage activities, i.e. sucrolysis via either the invertases or by the sucrose synthase pathway. The reasons we developed this UDP and PPi-dependent assay for sucrose synthase activity were (a) to measure activity in the sucrose breakdown direction whereas many other workers measured the opposite, namely sucrose synthesis (3,4,22,26), and (b) others who measured sucrose breakdown often assayed for UDP-glucose accumulation as a precursor of the synthesis of cell walls or other nucleotide sugars (6, 22). But in our assay we couple through the PPi-dependent UDP-glucopyrophosphorylase, which is very active in plants, to the formation of glucose 1-P which feeds carbon directly into glycolysis and possibly on to starch formation (31,32

Sucrose Synthase, Starch Accumulation, and Tomato Fruit Sink Strength

“…High sucrose synthase activity may lead to reduced concentrations of sucrose in pericarp cells and increased sucrose gradient from the source leaves to the fruit, resulting in greater sink strength (Walker et al, 1978). Severa1 authors (Claussen et al, 1986;Sung et al, 1989) have suggested that the activity of sucrose synthase could be used as a biochemical marker for sink strength. Consistent with this suggestion, we observed that, at 65 d after germination, the young fruit had the highest level of sucrose synthase activity (Table I).…”

Sucrose synthase expression pattern in the rhythmically growing shoot of common oak (Quercus robur L.)

“…At this date, SuSy activity was higher than in other shoot organs. Since SuSy is considered a good indicator of the sink strength of an organ [24], the shoot apex on the 2nd day of the flush appears therefore to be the strongest sink of the shoot. This fact is supported by the non-vascular localization of SuSy protein observed here.…”

“…A positive correlation between sucrose synthase activity, sink strength and sucrose import has been demonstrated in many sink organs, suggesting a role for this enzyme in carbohydrate partitioning between sink and source organs [24,28,30]. In addition, SuSy appears to play a key role in supplying energy for loading and unloading in phloem by providing substrate for respiration [16].…”

“…SuSy is ubiquitous in higher plants and assigns a variety of important roles. It is the predominant sucrose cleavage enzyme in cereal endosperm and in storage organs such as tubers, fruits, seed or immature leaves [24,28]. Sucrose synthase in these carbohydrate sink organs provides substrates for respiration [29], 586 R. Le Hir et al…”

What Is Phloem Unloading?

“…It is, therefore, significant that the enzyme sucrose synthase has recently been shown to play a major role in determining the sink strength of storage organs which accumulate starch as a major reserve (12).…”