DYNAMICS OF ARBUSCULE DEVELOPMENT AND DEGENERATION IN A ZEA MAYS MYCORRHIZA

Abstract

A quantitative light and electron microscope study of developing and degenerating mycorrhizal arbuscules of Glomus fasciculatum in Zea mays was carried out in order to estimate three parameters during the colonization cycle. These were: 1) Vv(f,c), the fraction of the host cell volume occupied by a volume of fungus; 2) Vv(cy,c), the fraction of the host cell volume occupied by host cytoplasm; 3) Sv(pr,c), the surface‐area‐to‐volume ratio of the host protoplast to the whole host cell. Uninfected cortical cells had an Sv(pr,c) of 0.13 μm2/μm3. As the fungus penetrates the cell wall, the protoplast invaginates, causing a decrease in protoplast volume and an increase in protoplast Sv. The Sv(pr,c) of a cell containing a mature arbuscule is 1.275 μm2/μm3. Because of the shrinkage of the protoplast, the Sv of the protoplast to its own volume rather than the original cell volume is 2.55 μm2/μm3, or almost a 20‐fold increase. Total cell size is unaffected. When the arbuscule is mature, the fungus occupies 42% of the cell, with 24% as 1‐μm‐diam branches, and 18% as trunk. Arbuscular branch formation progresses at a linear rate and is the most important factor in causing the increased host Sv. The correlation coefficient for Vv(br,c) the volume fraction for arbuscular branches, vs. Sv(pr,c) is r = 0.932 (P < 0.001). Degeneration of the arbuscule is marked by a rapid decrease in branches, host Sv, and host cytoplasm. The trunk develops and degenerates at a slower rate than the branches.