Stomatal myrosin cells in Caricaceae. Taxonomic implications for a glucosinolate-containing family

Stomatal myrosin cells are shown to be present in Carica goudotiana, C, papaya, C. quercifolia and Jarilla caudata of the Caricaceae. They are found in the epidermis of all green parts, viz. leaves, stems, and immature fruits, and also of cotyledons from germinating seeds. They are structurally the same as the stomatal myrosin cells in other glucosinolate- and myrosinase-containing families, Resedaceae, Tovariaceae and Bataceae, and comparable to the idioblastic myrosin cells from e.g. Brassicaceae and Capparaceae. The stomatal myrosin cells have vacuoles filled with proteinaceous material and cytoplasm rich in rough endoplasmic reticulum. During embryogenesis single adaxial epidermal cells of the cotyledons can be distinguished as myrosin cells, since their protein bodies are homogeneous and without globoids and become filled with protein earlier than the protein bodies with globoids present in the other epidermal cells or the mesophyll cells of the cotyledons. This is analogous to myrosin cells in embryos from Brassicaceae. In germinating seeds the single epidermal myrosin cells divide to form precursors of guard cells, thus turning into stomatal myrosin cells in the green cotyledons. The presence of myrosin cells supports a taxonomic treatment of Caricaceae together with the majority of the other glucosinolate-containing families in the major glucosinolate clade.