MINOR VEIN DIFFERENTIATION AND THE DEVELOPMENT OF SPECIALIZED PLASMODESMATA BETWEEN COMPANION CELLS AND CONTIGUOUS CELLS IN EXPANDING LEAVES OF MORICANDIA-ARVENSIS (L) DC (BRASSICACEAE)

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-contiguous cell interfaces. A morphological classification of min or veins into different orders proved inadequate as an indicator of mi nor Vein anatomy, yet neither total vein cell number nor number of sie ve element-companion cell complexes (SE-CCCs) proved accurate as vein order predictors. Therefore, minor veins were divided into three class es (IV V, and VI) according to the total number of cells within the bu ndle sheath, class VI having the smallest number. A basipetal trend in maturation was observed for all classes, with larger minor veins matu ring before smaller ones. Most minor veins were not structurally matur e before the cessation of assimilate import; nevertheless, an associat ion was seen between phloem maturation and the transition from importi ng to nonimporting status, as all class IV and V minor veins had some mature SEs in nonimporting tissue. A cell-specific ultrastructural spe cialization of plasmodesmata was observed on the companion cell side o f the interface with contiguous cells. The timing of plasmodesmal deve lopment was the same for all minor vein classes and was associated wit h SE-CCC differentiation. Specialized plasmodesmata began to different iate when SE-CCCs were still undifferentiated. These plasmodesmata wer e structurally mature by the time companion cells appeared fully diffe rentiated, 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, establ ishing M. arvensis as a sucrose-translocating species. Development of specialized plasmodesmata is discussed in relation to the apoplastic p hloem-loading function of companion cells.