Plasmodesmata and their role in the regulation of phloem unloading during fruit development

Abstract

Fruit consumption is fundamental to a balanced diet. The contemporary challenge of maintaining a steady food supply to meet the demands of a growing population is driving the development of strategies to improve the production and nutritional quality of fruit. Plasmodesmata, the structures that mediate symplasmic transport between plant cells, play an important role in phloem unloading and distribution of sugars and signalling molecules into developing organs. Targeted modifications to the structures and functioning of plasmodesmata have the potential to improve fruit development; however, knowledge on the mechanisms underpinning plasmodesmata regulation in this context is scarce. In this review, we have compiled current knowledge on plasmodesmata and their structural characterisation during the development of fruit organs. We discuss key questions on phloem unloading, including the pathway shift from symplasmic to apoplastic that takes place during the onset of ripening as potential targets for improving fruit quality.

Activation of apoplastic sugar at the transition stage may be essential for axillary bud outgrowth in the grasses

“…The activation of the apoplastic pathway at the transition stage during bud outgrowth in sorghum, sugarcane, and maize suggests axillary buds are initially formed using sucrose delivered through a non-apoplastic pathway. A transition from symplastic to apoplastic sugar is a common phenomenon during the development of other sink tissues such as seeds and fruits ( Paniagua et al., 2021 ). For example, a symplastic discontinuity at the placenta tissue that function as a conduit for assimilate transport from fruit to seed during seed development coincides with the expression of the tomato CWIN gene LIN5 ( Palmer et al., 2015 ; Ruan, 2022 ).…”

“…Results that indicate activation of apoplastic sugar during bud outgrowth have been documented in several species in the grasses ( Boussiengui-Boussiengui et al., 2016 ; Kebrom & Mullet, 2016 ; Dong et al., 2019 ). However, the mode of sugar supply during bud formation is still unclear, although in the analogous situation of the growth and development of sink plant tissues or organs a transition from symplastic to apoplastic sugar is common ( Paniagua et al., 2021 ). In this review, we posit that the bud transition stage may involve a shift in the axillary bud from relying on symplastic sugar during bud formation to apoplastic sugar for bud outgrowth ( Figure 1 ).…”

Callose metabolism and the regulation of cell walls and plasmodesmata during plant mutualistic and pathogenic interactions

“…Plasmodesmata are intercellular pores traversing cell walls that mediate the local and systemic transport of molecular signals to coordinate developmental and environmental responses (Figure 2 ). Plasmodesmata are often visualized as concentric pores formed by an outer specialized membranous domain that is a continuation of the plasma membrane, and an inner desmotubule which connects the endoplasmic reticulum (ER) of neighboring cells (Cheval & Faulkner, 2018 ; Paniagua et al, 2021 ). Molecular transport occurs through the cytoplasmic sleeve, the space between the plasma membrane and the desmotubule.…”

“…Other mechanisms, such as changes in plasmodesmata frequency or ultrastructure, are described to modify symplasmic transport allegedly independent of callose regulation. Various proteins which contribute directly or indirectly to callose metabolism and intercellular trafficking are found localized at plasmodesmata; thus, callose turnover is a dynamic process suited for rapid plasmodesmata regulation (Paniagua et al, 2021 ).…”

Sphingolipids at Plasmodesmata: Structural Components and Functional Modulators

“…The plasmodesmata (PD) channels facilitate the symplastic exchange of small molecules including photoassimilates, proteins, RNAs, hormones, and small peptides [ 1 , 2 , 3 ]. These substances flow either selectively or freely between cells satisfying the nutrient and signaling requirements during plant development and defenses [ 3 , 4 , 5 , 6 , 7 , 8 , 9 ]. The distribution and architecture of PD are adjusted according to diverse internal and external factors.…”