LOCALIZATION OF THE YELLOW PIGMENT FORMED IN ROOTS OF GRAMINEOUS PLANTS COLONIZED BY ARBUSCULAR FUNGI

Many plants form yellow coloured roots when colonized by arbuscular my corrhizal (AM) fungi. In maize, a yellow pigment is first visible as s mall droplets in parenchyma cells of roots in the vicinity of arbuscul es, 3-4 weeks after mycorrhizal colonization. During the course of the development of the plants, the yellow pigment spreads all over the ce lls of the cortex (with the exception of the exodermis) and of the end odermis, whereas the other stelar elements remain uncoloured. Other gr amineous plants (wheat, barley, millet) show the same pattern of pigme nt formation. In contrast, the deposition of this pigment is not detec ted in roots of Tagetes, garden bean, onion, or leek. Weak yellow fluo rescence is also seen in the fungal structures, particularly in the ar buscules of the investigated probes. This is, however, clearly differe nt from the intense yellow colour of the pigment formed in root cells of grasses. The yellow pigment is even detected in such cells which ar e never colonized by fungal structures (e.g., endodermal cells). A maj or constituent of the yellow pigment of AM-colonized root cells has be en 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 specifical ly by arbuscular fungi.