Sucrose-Metabolizing Enzymes in Transport Tissues and Adjacent Sink Structures in Developing Citrus Fruit

Abstract

Juice tissues of citrus lack phloem; therefore, photosynthates enroute to juice sacs exit the vascular system on the surface of each segment. Areas of extensive phloem unloading and transport (vascular bundles + segment epidermis) can thus be separated from those of assimilate storage Uuice sacs) and adjacent tissues where both processes occur (peel). Sugar composition, dry weight accumulation, and activities of four sucrose-metabolizing enzymes (soluble and cell-wall-bound acid invertase, alkaline invertase, sucrose synthase, and sucrose phosphate synthase) were measured in these transport and sink tissues of grapefruit (Citrus paradisi Macf.) to determine more clearly whether a given enzyme appeared to be more directly associated with assimilate transport versus deposition or utilization. Results were compared at three developmental stages. Activity of sucrose (per gram fresh weight and per milligram protein) extracted from zones of extensive phloem unloading and transport was significantiy greater than from adjacent sink tissues during the stages (II and ll) when juice sacs grow most rapidly. In stage 11 fruit, activity of sucrose synthase also significantly surpassed that of all other sucrose-metabolizing enzymes in extracts from the transport tissues (vascular bundles + segment epidermis). In contrast, sucrose phosphate synthase and alkaline invertase at this stage of growth were the most active enzymes from adjacent,

Companion-Cell Specific Localization of Sucrose Synthase in Zones of Phloem Loading and Unloading

“…This observation is consistent with its detection in phloem exudates (Lehmann, 1973) and more recent evidence demonstrating phloem-specific expression of a maize Suc synthase gene in transgenic tobacco plants (Yang and Russell, 1990). A similar pattern of localization is suggested by work showing elevated levels of activity in the midrib of exporting leaves (Claussen et al, 1985) as well as the vascular bundles of developing citrus fruit (Lowell et al, 1989, Tomlinson et al, 1991. These data suggest that the presence of Suc synthase in or immediately adjacent to vascular strands could be a common attribute of these tissues.…”

“…This may indicate a similar function for the enzyme in companion cells of both source and sink tissues. Elevated activity of Sue synthase has been shown previously in the functioning transport tissues of this assimilate unloading zone during rapid import (Lowell et al, 1989). Although early work suggested that phloem unloading could take place through the activity of an ATPase driven by a Sue-proton antiport (Humphreys, 1978), little evidence has been obtained demonstrating that this model is directly involved in the unloading process.…”

“…A band 1 cm wide was excised approximately 20 cm from the maize leaf tip and from the middle of the citrus leaf. Five grapefruit, each in the expansion phase of growth (stage II, Lowell et al, 1989), were harvested randomly and the region of peel containing the major vascular bundles and adjacent segment epidermis was isolated from the fruit. Portions of both fruit and leaf tissues were cut into 3x3 mm pieces, randomized and fixed in formalin acetic acid (47.5% [v/v] ethanol, 0.87 M glacial acetic acid, 3.7% [v/v] formaldehyde) at 25°C for 24 h (Sass, 1940).…”

Postphloem, Nonvascular Transfer in Citrus

“…Second, even if sucrose were hydrolyzed and rapidly resynthesized during transport, percentages of "'C-hexoses indicate that the process occurs to approximately the same degree throughout the vascular, nonvascular, and sink tissues examined. In addition, activities of acid invertases in these tissues drop to barely detectable levels at this stage of development (23), and pH along the transport path is not likely to be low enough for rapid action of the nonenzymatic sucrose hydrolysis reported in acid limes (5). The suggestion that sucrose hydrolysis in this system is more closely related to storage than transport is also supported by the observation that instances of most pronounced formation of "'C-hexoses occurred in the final storage sites (juice sacs) rather than en route.…”

Physiology of citrus fruiting

“…Sink strength in citrus fruits has been associated with the presence of sucrose metabolizing enzymes, mainly sucrose synthase (Hockema and Etxeberria, 2001;Komatsu et al, 2002;Etxeberria et al, 2005), and in particular with a specific isoenzyme of this activity, CitSUSA, that is induced during citrus fruit ripening (Komatsu et al, 2002). Thus, while alkaline invertase and sucrose-phosphate synthase activities are mainly detected in the sink cells, sucrose synthase activity appears to be associated with the vascular bundles (Lowell et al, 1989;Tomlinson et al, 1991), specifically in the companion cells. In citrus fruit, juice sacs containing sink cells are physically separated from vascular bundles and hence the above observations indicate a role for sucrose synthase activity in phloem loading and unloading processes (Nolte and Koch, 1993).…”