Visualisation of stromules in transgenic wheat expressing a plastid-targeted yellow fluorescent protein.

Stromules are stroma-filled tubules that extend from the plastids in all multicellular plants examined to date. To facilitate the visualisation of stromules on different plastid types in various tissues of bread wheat (Triticum aestivum L.), a chimeric gene construct encoding enhanced yellow fluorescent protein (EYFP) targeted to plastids with the transit peptide of wheat granule-bound starch synthase I was introduced by Agrobacterium-mediated transformation. The gene construct was under the control of the rice Actin1 promoter, and EYFP fluorescence was detected in plastids in all cell types throughout the transgenic plants. Stromules were observed on all plastid types, although the stromule length and abundance varied markedly in different tissues. The longest stromules (up to 40 μm) were observed in epidermal cells of leaves, whereas only short beak-like stromules were observed on chloroplasts in mesophyll cells. Epidermal cells in leaves and roots contained the highest proportion of plastids with stromules, and stromules were also abundant on amyloplasts in the endosperm tissue of developing seeds. The general features of stromule morphology and distribution were similar to those shown previously for tobacco (Nicotiana tabacum L.) and arabidopsis (Arabidopsis thaliana (L.) Heynh.).