Figure 7.

Proposed Model for Bidirectional Secretions from Glandular Trichomes of Pyrethrum That Enable Immunization of Seedlings.

STLs are secreted in an apical direction from the white apical cells to the subcuticular space (yellow). In the blue subapical cells, the terpene substrate of pyrethrin biosynthesis is synthesized. This leads to the formation of chrysanthemyl diphosphate and COH. COH is oxidized into CA and then exported to the pericarp (green arrow). The enzyme GLIP, which forms pyrethrin esters by joining chrysanthmoyl-CoA to lipid alcohols, resides in the pericarp and is active in the apoplast according to Kikuta et al. (2012), but the question mark indicates that it is unclear where and how the esterification occurs precisely. Pyrethrins subsequently accumulate in the intercellular spaces of pericarp, surrounding the embryo. During seed maturation, they are absorbed into the embryo. Seeds germinate with high pyrethrin concentrations in the seedlings. Trichomes are a biseriate stack of five paired cells oriented along the groove of the seed. The inset shows the lateral and frontal views of the trichomes for the purpose of clarity. The figure is based on an original cryo-scanning electron microscopy image shown in Supplemental Figure 2 online.