Development of peltate glandular trichomes of peppermint

Cryofixation and conventional chemical fixation methods were employed to ex amine the ultrastructure of developing peltate glandular trichomes of peppe rmint (Mentha X piperita). Our results are discussed in relation to monoter pene production and the mechanism of essential oil secretion. Peltate gland s arise as epidermal protuberances (initials) that divide asymmetrically to produce a vacuolate basal cell, a stalk cell, and a cytoplasmically dense apical cell. Further divisions of the apical cell produce a peltate trichom e with one basal cell, one stalk cell, and eight glandular (secretory) disc cells. Presecretory,oland cells resemble meristematic cells because they c ontain proplastids, small vacuoles, and large nuclei. The secretory phase c oincides with the separation and filling of the sub-cuticular oil storage s pace, the maturation of glandular disc cell leucoplasts in which monoterpen e biosynthesis is known to be initiated, and the formation of extensive smo oth endoplasmic reticulum at which hydroxylation steps of the monoterpene b iosynthetic pathway occur. The smooth endoplasmic reticulum of the secretor y cells appears to form associations with both the leucoplasts and the plas ma membrane bordering the sub-cuticular oil storage cavity, often contains densely staining material, and may be involved with the transport of the mo noterpene-rich secretion product. Associated changes in the ultrastructure of the secretory stage stalk cell are also described, as is the ultrastruct ure of the fragile post-secretory gland for which cryofixation methods are particularly well suited for the preservation of organizational integrity.