Large lipid droplets in leaf epidermis of angiosperms

• Large lipid droplets are present constantly in the leaf epidermis of angiosperms.
• Earlier they were considered typical only of mesophyll and bundle-sheath cells.
• Now they are found in stomata, in particular, in their subsidiary cells.
• We observed that breakdown of large lipid droplets takes place in dying leaves.
• We suggest that lipids from lipid droplets are used to maintain functionality of stomata.

Methods of light, scanning and transmission electron microscopy were used to study the lower epidermis of the leaves in evergreen Acokanthera oblongifolia (Apocynaceae), Exbucklandia populnea (Hamamelidaceae), Ternstroemia gymnanthera (Theaceae), and Viburnum suspensum (Caprifoliaceae). All these species show similar leaf anatomy. Our results together with those of earlier studies show that large lipid droplets (LDs) can regularly be found in leaf epidermis of some flowering plants. Their diameter varies in studied species from 3 to 11 μm. Large lipid droplets are typical for cells of stomatal complexes and can have irregular shape. They also occur in some ordinary cells. The formation of large LDs is accompanied by the presence of thick cuticle covering ordinary and subsidiary cells, prominent cuticular outer ledges and rims on stomata, waxy plugs in the outer stomatal cavities (in A. oblongifolia), and abundant plastoglobules in plastids. All these indicate that intensive lipid synthesis occurs in leaf epidermis. The stomatal complexes are the place with the highest occurrence of the largest LDs. They are formed in subsidiary cells, which lie beneath the stomata. In these cells plastids and mitochondria are always present. The former contain lipid inclusions. We demonstrated that breakdown of LDs takes place in dying leaves of V. suspensum. This might indicate mobilization of lipids from the globules into the LDs. Based on these new data we suggest that the LDs found in cells of stomatal complexes play important role in maintaining normal function of stomata.