Abstract
Leaves are asymmetric, with different functions for adaxial and abaxial tissue. The bundle sheath (BS) of C3 barley (Hordeum vulgare) is dorsoventrally differentiated into three types of cells: adaxial structural, lateral S-type, and abaxial L-type BS cells. Based on plasmodesmatal connections between S-type cells and mestome sheath (parenchymatous cell layer below bundle sheath), S-type cells likely transfer assimilates toward the phloem. Here, we used single-cell RNA sequencing to investigate BS differentiation in C4 maize (Zea mays L.) plants. Abaxial BS (abBS) cells of rank-2 intermediate veins specifically expressed three SWEET sucrose uniporters (SWEET13a, b, and c) and UmamiT amino acid efflux transporters. SWEET13a, b, c mRNAs were also detected in the phloem parenchyma (PP). We show that maize has acquired a mechanism for phloem loading in which abBS cells provide the main route for apoplasmic sucrose transfer toward the phloem. This putative route predominates in veins responsible for phloem loading (rank-2 intermediate), whereas rank-1 intermediate and major veins export sucrose from the PP adjacent to the sieve element companion cell complex, as in Arabidopsis thaliana. We surmise that abBS identity is subject to dorsoventral patterning and has components of PP identity. These observations provide insights into the unique transport-specific properties of abBS cells and support a modification to the canonical phloem loading pathway in maize.
Highlights
Leaves are typically asymmetric: there are often differences in the relative stomatal and trichome densities and cuticle properties between the abaxial and adaxial leaf surfaces
The BS cluster was further divided into two subclusters, the “upper” and “lower” subclusters, which later were assigned as abaxial and adaxial bundle sheath cells (Fig. 1d, Fig. 2)
Since barley Bundle sheath cells (BSCs) seem clearly differentiated, with S-type cells suspected to represent a preferential site of sucrose transfer into the phloem (Williams et al, 1989), we tested whether SWEET13a, b, and c mRNA would be present in adaxial and lateral BSCs
Summary
Leaves are typically asymmetric: there are often differences in the relative stomatal and trichome densities and cuticle properties between the abaxial and adaxial leaf surfaces. Abaxial BS (abBS) cells of rank-2 intermediate veins expressed three SWEET sucrose uniporters (SWEET13a, b, and c) and UmamiT amino acid efflux transporters.
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