Sugar loading of crop seeds – a partnership of phloem, plasmodesmal and membrane transport

Abstract

Summary Sugar loading of developing seeds comprises a cohort of transport events that contribute to reproductive success and seed yield. Understanding these events is most advanced for grain crops (Brassicaceae, Fabaceae and Gramineae) and Arabidopsis. For these species, 75–80% of their final seed biomass is derived from phloem‐imported sucrose. Sugar loading consecutively traverses three genomically distinct, and symplasmically isolated, seed domains: maternal pericarp/seed coat, filial endosperm and filial embryo. Sink status of each domain co‐ordinately transitions from growth to storage. The latter is dominated by embryos (Brassicaceae and Fabaceae) or endosperms (Gramineae). Intradomain sugar transport occurs symplasmically through plasmodesmata. Interdomain sugar transport relies on plasma‐membrane transporters operating in efflux (maternal and endosperm) or influx (endosperm and embryo) modes. Discussed is substantial progress made in identifying, and functionally evaluating, sugar symporters (STPs, SUTs or SUCs) and uniporters (SWEETs). These findings have underpinned a mechanistic understanding of seed loading. Less well researched are possible physical limitations imposed by hydraulic conductivities of differentiating protophloem and of subsequent plasmodesmal transport. The latter is coupled with sugar homeostasis within each domain mediated by sugar transporters. A similar conclusion is ascribed to fragmentary understanding of regulatory mechanisms integrating transport events with seed growth and storage.

Phaseolus vulgaris STP13.1 is an H+‐coupled monosaccharide transporter, present in source leaves and seed coats, with higher substrate affinity at depolarized potentials

“…During the pre-storage phase of Fava bean seed development, a high apoplasmic hexose/sucrose ratio is created by extracellular invertase activity (Weber et al, 1995). Unlike other STPs that have been shown to be expressed in the maternal and filial tissues (Pegler et al, 2023), PvSTP13.1 appears to have a seed coat-specific role in common bean. Specifically, PvSTP13.1 may play a significant role in maternal/filial competition for hexoses during the pre-storage phase to preferentially support coat development.…”

“…Of the sugar transporter families expressed in sinks, the functional role of STPs is least understood. A circumstance possibly arises from the common characteristic of high numbers of STP isoforms being co‐expressed in a sink conferring redundancy for hexose transport (Pegler et al, 2023 ; Wen et al, 2022 ). Unlike SWEET uniporters, STPs are high affinity (apparent K m 2 to 100 μm) hexose symporters that underpin a potential capacity to accumulate hexoses to high concentrations from apoplasmic spaces in post‐phloem unloading routes (Pegler et al, 2023 ; Pommerrenig et al, 2020 ; Wen et al, 2022 ).…”

“…A circumstance possibly arises from the common characteristic of high numbers of STP isoforms being co‐expressed in a sink conferring redundancy for hexose transport (Pegler et al, 2023 ; Wen et al, 2022 ). Unlike SWEET uniporters, STPs are high affinity (apparent K m 2 to 100 μm) hexose symporters that underpin a potential capacity to accumulate hexoses to high concentrations from apoplasmic spaces in post‐phloem unloading routes (Pegler et al, 2023 ; Pommerrenig et al, 2020 ; Wen et al, 2022 ). Thus far, this functional role has only been demonstrated for STPs expressed in storage cells of fleshy fruits that accumulate hexoses to high levels (Milne et al, 2018 ; Wen et al, 2022 ).…”

“…Although members of many STP sub-clades are expressed in developing seeds (Pegler et al, 2023), to date, STP13s have not been included in this cohort. In order to explore the role of STPs in seed filling, we investigated whether STP13 was expressed in source leaves and recipient-developing seeds (coat and embryo separately) across the pre-and storage phases of Phaseolus vulgaris L. (common bean).…”

Genome‐wide transcriptional responses to water deficit during seed development in Pisum sativum, focusing on sugar transport and metabolism

“…Throughout seed development, reproductive success and yield are controlled by a cohort of transport events (Pegler et al, 2023). When carbon supply is limited, for instance by environmental factors, competition for carbon can occur between sinks, like roots and seeds.…”