Tracing 3D apoplasmic complexity in vascular tissue of live conifer needles by X-ray computed tomography

Abstract

Architecture and conducting area of vascular elements along conifer needles are fundamentally different from broad leaves. We hypothesised that the needles unique transfusion tissue offers different mechanisms for water allocation and used multimodal imaging to dissect the critical water exchange interfaces in this xerophytic leaf type. Our study examined intact conifer needles with X-ray computed tomography (XCT) and water-soluble tracers, allowing to render the functional 3D structure of the water-filled apoplast and the complementary symplasmic domain inside the bundle sheath. Segmentation of these data, together with fluorescence and electron microscopy of axial phloem and xylem elements along the needle, enabled quantification of the dimensions of the conducting tissue complex. The transfusion tracheid system between the endodermis-type bundle sheath and the axial venation formed a sponge-like apoplast domain. Transfusion parenchyma cell chains bridged this domain not directly but as tortuous symplasmic pathways between bundle sheath and axial phloem, which is nearly exclusively accessible at flanks. The transfusion tissue extends the plasma membrane surface for phloem loading and provides a large volume space. We discuss that this unique tissue plays an important role in the subtle interplay between water uptake/storage and sugar transport that has evolved to cope with desiccation stress.