Development of trichomes in the Abutilon nectary gland

Abstract

Nectary trichomes of Abutilon striatum var. thompsonii arise by sequential periclinal divisions of outpushings from epidermal cells so producing trichomes that, when mature, are about 12 cells long. All epidermal cells within the nectary undergo this transformation. Later, anticlinal divisions lead to a multiseriate lower part of the trichome. The original epidermal cell becomes the basal cell which increases substantially in volume during development, thus leading to lateral separation of the trichomes. Above the basal cell is the stalk cell which develops an apoplastic barrier in its anticlinal (outer) wall. Secretion ultimately takes place from a capitate tip cell. An initially very thin cuticular layer, which overlies the whole trichome, eventually becomes as thick as the cell wall itself (approx. 0.4 μm). The pre‐secretory hairs contain numerous small, condensed mitochondria; poorly differentiated plastids; dictyosomes with coated vesicles; small vacuoles; and a large amount of smooth endoplasmic reticulum (“secretory reticulum”) which contrasts with the rough endoplasmic reticulum seen during earlier developmental stages. As secretion proceeds, vacuolation becomes more extensive. Plasmodesmata are present between all the cells of the trichome and diminish in frequency from about 12.0 μm‐2 in the stalk cell to about 4.0 μm‐2 in the apical cells. This variation in plasmodesmatal frequency along the trichome is seen at all stages of development. The ultrastructural evidence would be consistent with the hypothesis that the pre‐nectar flows through the plasmodesmata from cell to cell, is loaded into a “secretory compartment”, and is then unloaded into the apoplast from all cells of the trichome distal to the stalk cell.