Minor Vein Differentiation and the Development of Specialized Plasmodesmata between Companion Cells and Contiguous Cells in Expanding Leaves of Moricandia arvensis (L.) DC. (Brassicaceae)

Abstract

Expanding leaves of Moricandia arvensis undergoing the sink-to-source transition were sampled for transmission electron microscopy to follow minor vein differentiation and plasmodesmal development at companion cell-contignous cell interfaces. A morphological classification of minor veins into different orders proved inadequate as an indicator of minor vein anatomy, yet neither total vein cell number nor number of sieve element-companion cell complexes (SE-CCCs) proved accurate as vein order predictors. Therefore, minor veins were divided into three classes (IV, V, and VI) according to the total number of cells within the bundle sheath, class VI having the smallest number A basipetal trend in maturation was observed for all classes, with larger minor veins maturing before smaller ones. Most minor veins were not structurally mature before the cessation of assimilate import; nevertheless, an association was seen between phloem maturation and the transition from importing to nonimporting status, as all class IV and V minor veins had some mature SEs in nonimporting tissue. A cell-specific ultrastructural specialization of plasmodesmata was observed on the companion cell side of the interface with contiguous cells. The timing of plasmodesmal development was the same for all minor vein classes and was associated with SE-CCC differentiation. Specialized plasmodesmata began to differentiate when SE-CCCs were still undifferentiated. These plasmodesmata were structurally mature by the time companion cells appeared fully differentiated, although their associated SEs were still immature Sucrose, fructose, and glucose were detected by HPLC analysis in mature leaves and stems, while neither raffinose nor stachyose was detected, establishing M. arvensis as a sucrose-translocating species. Development of specialized plasmodesmata is discussed in relation to the apoplastic phloem-loading function of companion cells.