Abstract
The ability to develop secondary (post-cytokinetic) plasmodesmata (PD) is an important evolutionary advantage that helps in creating symplastic domains within the plant body. Developmental regulation of secondary PD formation is not completely understood. In flowering plants, secondary PD occur exclusively between cells from different lineages, e.g., at the L1/L2 interface within shoot apices, or between leaf epidermis (L1-derivative), and mesophyll (L2-derivative). However, the highest numbers of secondary PD occur in the minor veins of leaf between bundle sheath cells and phloem companion cells in a group of plant species designated “symplastic” phloem loaders, as opposed to “apoplastic” loaders. This poses a question of whether secondary PD formation is upregulated in general in symplastic loaders. Distribution of PD in leaves and in shoot apices of two symplastic phloem loaders, Alonsoa meridionalis and Asarina barclaiana, was compared with that in two apoplastic loaders, Solanum tuberosum (potato) and Hordeum vulgare (barley), using immunolabeling of the PD-specific proteins and transmission electron microscopy (TEM), respectively. Single-cell sampling was performed to correlate sugar allocation between leaf epidermis and mesophyll to PD abundance. Although the distribution of PD in the leaf lamina (except within the vascular tissues) and in the meristem layers was similar in all species examined, far fewer PD were found at the epidermis/epidermis and mesophyll/epidermis boundaries in apoplastic loaders compared to symplastic loaders. In the latter, the leaf epidermis accumulated sugar, suggesting sugar import from the mesophyll via PD. Thus, leaf epidermis and mesophyll might represent a single symplastic domain in Alonsoa meridionalis and Asarina barclaiana.
Highlights
IntroductionCells can be connected by primary and/or secondary plasmodesmata (PD)
In land plants, cells can be connected by primary and/or secondary plasmodesmata (PD)
We considered PD between leaf epidermis and mesophyll as secondary because epidermis and mesophyll originate from different cell lineages corresponding to the L1 and L2 layers, respectively in the leaf primordium (Satina and Blakeslee, 1941; Kang and Dengler, 2018)
Summary
Cells can be connected by primary and/or secondary plasmodesmata (PD). Transfer of assimilates from the mesophyll into the phloem is assisted by highly developed PD at the boundary between bundle sheath cells and phloem companion cells. As these PD appear during the maturation of the leaf, i.e., after cell divisions have been completed, they represent secondary PD. The number of these PD can be very high in symplastic phloem loaders, especially in species that contain phloem companion cells of the “intermediary cell” and in “intermediary-cell-like” types (Gamalei, 1991; Batashev et al, 2013)
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