Sugar Retrieval by Coats of Developing Seeds of Phaseolus vulgaris L. and Vicia faba L.

Abstract

;Influxes of glucose, fructose and sucrose were characterised for coat cells of developing seeds of Phaseolus vulgaris L. and Vicia faba L. by monitoring uptake of [14 C]sugars into excised seed-coat halves and two different protoplast populations derived from seed coats. Sugar influxes by the two populations of protoplasts were similar for each sugar species [sucrose > (fructose » glucose)] and hexoses competed with sucrose. Concentration-dependent influxes of all three sugars by excised seed coats could be described by a simple directly proportional relationship between concentration ([S]) and uptake rate (v) in the physiological range of sugar concentrations (v » A.[S]). Alternatively, with the exception of fructose influx by Vicia, all could be fitted to a Michaelis-Menten relationship, as could sucrose uptake by Vicia protoplasts. Apparent K m values were high (» 100-500 mM) compared with those reported for other systems. Sucrose transport was distinct from glucose and fructose transport in both species. Sugar influx was decreased by p-chloromercuribenzenesulfonic acid, carbonylcyanide m-chlorophenylhydrazone and erythrosin B. These responses are consistent with sugar/H + symport acting to retrieve photoassimilates leaked to the apoplasm during post-sieve element transport within seed coats.

A Cascade of Sequentially Expressed Sucrose Transporters in the Seed Coat and Endosperm Provides Nutrition for the Arabidopsis Embryo

“…In legume seeds, results from inhibitor studies led to the proposal that nonselective passive pores are responsible for sucrose efflux (van Dongen et al, 2001;Ritchie et al, 2003). Neither the predicted antiporters nor the nonselective pores have been identified at the molecular level to date.…”

“…How sucrose is released from maternal tissues (seed coat) to support filial tissues (embryo) also remains unclear, except for the contribution of a subset of transporters of the SUT sucrose/H + cotransporter family. Evidence from studies of pea (Pisum sativum) and bean (Phaseolus vulgaris) seed coats implicated SUF transporters, which appear to have lost proton coupling to act as uniporters, in sucrose efflux from seed coat (Ritchie et al, 2003;Zhou et al, 2007). These uncoupled SUFs appear to have evolved from recent gene duplications and likely represent a specific adaptation in legumes to sustain the large seeds in some of these legumes, yet they have not been found in other plants, such as Arabidopsis or maize (Zea mays) .…”

Review: Nutrient loading of developing seeds

“…First, estimates of sucrose release by simple diffusion at best only account for 25% of the observed sucrose flux (Table 1). Second, sucrose symport activity is not detectable in maternal tissues of developing wheat grains Bagnall et al 2000) and only marginally so in seed coats of pea (de Jong et al 1996;de Jong and Borstlap 2000) and of two bean species at low (<10 mM) external sucrose concentrations (Ritchie et al 2003).…”

“…van . In this context, cells located along the postphloem symplasmic pathway may function in retrieval of sucrose (Ritchie et al 2003;VfSUT1, Weber et al 1997a;PsSUT1, Tegeder et al 1999) and amino acids (PsAAP1, Tegeder et al 2000a;VfAAP3, Miranda et al 2001) leaked to the seed apoplasm while in transit to the principal site(s) of release.…”

“…Therefore, under these conditions, sucrose symporters are likely to function in sucrose retrieval modes in non-vascular cells of seed coats (cf. Ritchie et al 2003). However, assuming a phloem sucrose concentration of 500 mM, sucrose symporters could function as effluxers from sieve elements/vascular parenchyma for apoplasmic concentrations up to 10 mM (predicted from Eqn 4).…”