A suite of sucrose transporters expressed in coats of developing legume seeds includes novel pH‐independent facilitators

Abstract

SummaryA suite of newly discovered sucrose transporter genes, PsSUF1, PsSUF4, PvSUT1 and PvSUF1, were isolated from the coats of developing pea (Pisum sativum L.) and bean (Phaseolus vulgaris L.) seeds. Sequence analysis indicated that deduced proteins encoded by PsSUF1, PvSUT1 and PvSUF1 clustered in a separate sub-group under sucrose transporter Clade I, whereas the deduced protein encoded by PsSUF4 clustered in Clade II. When expressed in yeast, these genes were shown to encode sucrose transporters with apparent Michaelis Menten constant (K m ) values ranging from 8.9 to 99.8 mM. PvSUT1 exhibited functional characteristics of a sucrose/H þ symporter. In contrast, PsSUF1, PvSUF1 and PsSUF4 supported the pH-and energy independent transport of sucrose that was shown to be bi-directional. These transport properties, together with that of counter transport, indicated that PsSUF1, PvSUF1 and PsSUF4 function as carriers that support the facilitated diffusion of sucrose. Carrier function was unaffected by diethylpyrocarbonate and by maltose competition, suggesting that the sucrose binding sites of these transporters differed from those of known sucrose/H þ symporters. All sucrose transporters were expressed throughout the plant and, of greatest interest, were coexpressed in cells considered responsible for sucrose efflux from seed coats. The possible roles played by the novel facilitators in sucrose efflux from seed coats are discussed.

Review: Nutrient loading of developing seeds

“…Early models for sucrose unloading envisaged a pump leak system whereby passively released sucrose is taken up by sucrose symport that functions to regulate net rates of sucrose release. Such a model is consistent with observed expression of sucrose symporter genes in maternal tissues of developing seeds of both monocots (Bagnall et al 2000;Weschke et al 2000;Furbank et al 2001) and dicots (Weber et al 1997a;Tegeder et al 1999;Meyer et al 2004;Zhou et al 2007). However, there are several observations that do not support a pump/leak model of sucrose release.…”

“…Interestingly a low affinity and sulfhydryl modifier-independent facilitated transport of sucrose has been shown to function at high external concentrations (Ritchie et al 2003). This transport behaviour is consistent with transport properties of three sucrose facilitators (SUFs) recently cloned from coats of developing pea and French bean seeds (Zhou et al 2007; Fig. 1).…”

Genetic Control of Carbon Partitioning in Grasses: Roles of Sucrose Transporters and Tie-dyed Loci in Phloem Loading

“…The SUT homolog PsSUF4 from pea (Pisum sativum), showing 73% amino acid identity to LjSUT4, was characterized by heterologous expression in yeast and shown to be a Suc facilitator, rather than a symporter, that functions independently of a H + gradient (Zhou et al, 2007). The subcellular localization of PsSUF4 in plants was not reported, but if similarly localized to the tonoplast, it may be responsible for Suc entry into the vacuole.…”

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

“…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) .…”