TY - JOUR

T1 - Tracing the opposing assimilate and nutrient flows in live conifer needles

AU - Gao, Chen

AU - Marker, Sean J V

AU - Gundlach, Carsten

AU - Poulsen, Henning F

AU - Bohr, Tomas

AU - Schulz, Alexander

N1 - © The Author(s) 2023. Published by Oxford University Press on behalf of the Society for Experimental Biology. All rights reserved. For permissions, please email: journals.permissions@oup.com.

PY - 2023

Y1 - 2023

N2 - The vasculature along conifer needles is fundamentally different from that in angiosperm leaves containing a unique transfusion tissue inside the bundle sheath. To identify the pathway of photoassimilates from mesophyll to phloem, and the opposing pathway of nutrients from xylem to mesophyll, we used specific tracers. For symplasmic transport we applied esculin to the tip of attached pine needles and followed tracer movement down the phloem. For apoplasmic transport we let detached needles take up a membrane-impermeable contrast agent and used micro-X-ray computed tomography to map critical water exchange interfaces and domain borders. Microscopy and segmentation of X-ray data enabled us to render and quantify the functional 3D structure of the water-filled apoplasm and the complementary symplasmic domain. The transfusion tracheid system formed a sponge-like apoplasmic domain that was blocked at the bundle sheath. Transfusion parenchyma cell chains bridged this domain as tortuous symplasmic pathways with strong local anisotropy which, as the accumulation of esculin, pointed to the phloem flanks as preferred phloem loading path. Simple estimates support the pivotal role of the bundle sheath, showing that a bidirectional movement of nutrient ions and assimilates is feasible and emphasising the bundle sheath's role in nutrient and assimilate exchange.

AB - The vasculature along conifer needles is fundamentally different from that in angiosperm leaves containing a unique transfusion tissue inside the bundle sheath. To identify the pathway of photoassimilates from mesophyll to phloem, and the opposing pathway of nutrients from xylem to mesophyll, we used specific tracers. For symplasmic transport we applied esculin to the tip of attached pine needles and followed tracer movement down the phloem. For apoplasmic transport we let detached needles take up a membrane-impermeable contrast agent and used micro-X-ray computed tomography to map critical water exchange interfaces and domain borders. Microscopy and segmentation of X-ray data enabled us to render and quantify the functional 3D structure of the water-filled apoplasm and the complementary symplasmic domain. The transfusion tracheid system formed a sponge-like apoplasmic domain that was blocked at the bundle sheath. Transfusion parenchyma cell chains bridged this domain as tortuous symplasmic pathways with strong local anisotropy which, as the accumulation of esculin, pointed to the phloem flanks as preferred phloem loading path. Simple estimates support the pivotal role of the bundle sheath, showing that a bidirectional movement of nutrient ions and assimilates is feasible and emphasising the bundle sheath's role in nutrient and assimilate exchange.

U2 - 10.1093/jxb/erad334

DO - 10.1093/jxb/erad334

M3 - Journal article

C2 - 37668473

VL - 74

SP - 6677

EP - 6691

JO - Journal of Experimental Botany

JF - Journal of Experimental Botany

SN - 0022-0957

IS - 21

ER -