Transport and Metabolism of a Sucrose Analog (1′-Fluorosucrose) into Zea mays L. Endosperm without Invertase Hydrolysis

Abstract

ABSTRACr l'-Fluorosucrose (FS), a sucrose analog resistant to hydrolysis by invertase, was transported from husk leaves into maize (Zea mays L., Pioneer Hybrid 3320) kernels with the same magnitude and kinetics as sucrose. "C-Label from I"CqFS and l"lqsucrose in separate experiments was distributed similarly between the pedicel, endosperm, and embryo with time. FS passed through maternal tissue and was absorbed intact into the endosperm where it was metabolized and used in synthesis of sucrose and methanol-chloroform-water insolubles. Accumulation of 114q sucrose from supplied I'Cjglucosyl-FS indicated that the glucose moiety from the breakdown of sucrose (here FS), which normally occurs in the process of starch synthesis in maize endosperm, was available to the pool of substrates for resynthesis of sucrose. Uptake of FS into maize endosperm without hydrolysis suggests that despite the presence of invertase in maternal tissues and the hydrolysis of a large percentage of sucrose unloaded from the phloem, hexoses are not specifically needed for uptake into maize endosperm.The lack of direct vascular or cellular connections between maternal and embryonic tissues in developing seeds and caryopses, necessitates passage of phloem-imported assimilates through the apoplast (for review see Thorne [14]). It has been suggested that in maize and sorghum the apoplast ofthe placenta and chalazal regions, which contain many dead cells, functions as a temporary storage site for sugars before absorption into the endosperm (2, 6). Evidence for this is as follows: (a) high hexose concentrations are found in the placental sac, for sorghum 300 to 400 mm (6), and for maize 470 to 800 mm of glucose equivalents (12); (b) there is a buildup of radioactivity in the pedicel of maize after a pulse of "4CO2 to the leaf ear (2).In maize (7,10) Here we have used an analog of sucrose, FS,3 which is a poor substrate for invertase (4,8), to elucidate the role of invertase hydrolysis in uptake of phloem-supplied sucrose into maize endosperm. FS behaves similarly to sucrose in phloem-loading in leaf tissue (4) and in translocation in soybean and sugar beet plants (8). The KD for FS uptake by soybean cotyledon protoplasts is 0.9 mm compared to 2.0 mM for sucrose (4). FS is resistant to hydrolysis by invertase; the ratio of hydrolysis of sucrose to FS via yeast invertase is 4200, and hydrolysis of FS by crude extracts from developing soybean leaves or wheat germ is below measurement (8). In contrast, the ratio of breakdown of sucrose to FS by sucrose synthase from developing soybean leaves and wheat germ is 3.6 (8). Therefore, if invertase hydrolysis is necessary to provide hexoses for uptake into maize endosperm, FS should not enter the endosperm. If FS does enter the endosperm, the inversion to hexoses must not be a prerequisite for sugar entry, and once in the endosperm FS could be metabolized via sucrose synthase. MATERIALS AND METHODSSupply of "4C-Sugars. Zea mays L. (Pioneer Hybrid 3320) was grown in a greenhouse with supplemental lighting. C...

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

“…Similarities between results obtained in maize and sugarcane led to the general view that the described sequence of events proceeded the same way in many tissues. More recent histochemical studies of invertase in maize tissues have confirmed earlier localizations (8), but sucrose hydrolysis no longer appears essential for all sucrose import into kernels (29) or many other sink structures (15). Likewise, the extent of sucrose breakdown and activity of associated enzymes during sucrose import is now believed to vary considerably.…”

“…Earlier studies have clearly demonstrated the importance of acid invertase during assimilate transfer into immature sugarcane stems (13,14) and developing corn kernels (30). However, sucrose hydrolysis later was found not to occur during photosynthate entry into a variety of other sink tissues ( 16 and references cited therein, 29). In grapefruit, the capacity of both soluble and insoluble acid invertases drops to very low levels in tissues still rapidly accumulating dry matter.…”

Postphloem, Nonvascular Transfer in Citrus

“…Sink systems vary in the extent to which they hydrolyze imported sucrose, but data from maize (29) and other species (4,22) now indicate that although cleavage and resynthesis may occur, it does not appear essential for transport. Citrus results reported here are consistent with this hypothesis.…”

Sugar Uptake by Maize Endosperm Suspension Cultures

“…Short-term experiments with excised kernels showed passive sugar uptake (10), but neither sustained incorporation of external sugars into starch nor kernel growth has been shown with that experimental system. Although sucrose is hydrolyzed before or during uptake, this hydrolysis does not appear to be essential for sucrose movement into the endosperm (20). Thus, the contribution of invertase to the transport process is uncertain, and the mechanism of sugar transport into intact endosperm and its possible regulation is unclear.…”