Localization of the yellow pigment formed in roots of gramineous plants colonized by arbuscular fungi

Summary

Many plants form yellow coloured roots when colonized by arbuscular mycorrhizal (AM) fungi. In maize, a yellow pigment is first visible as small droplets in parenchyma cells of roots in the vicinity of arbuscules, 3–4 weeks after mycorrhizal colonization. During the course of the development of the plants, the yellow pigment spreads all over the cells of the cortex (with the exception of the exodermis) and of the endodermis, whereas the other stelar elements remain uncoloured. Other gramineous plants (wheat, barley, millet) show the same pattern of pigment formation. In contrast, the deposition of this pigment is not detected in roots ofTagetes, garden bean, onion, or leek. Weak yellow fluorescence is also seen in the fungal structures, particularly in the arbuscules of the investigated probes. This is, however, clearly different from the intense yellow colour of the pigment formed in root cells of grasses. The yellow pigment is even detected in such cells which are never colonized by fungal structures (e.g., endodermal cells). A major constituent of the yellow pigment of AM-colonized root cells has been identified as a carotenoid with 14 carbon atoms and two carboxylic groups and termed mycorradicin. This carotenoid is likely deposited in the vacuoles of root cells as a result of the colonization specifically by arbuscular fungi.