Hyphal penetration of Brassica (Cruciferae) roots by a vesicular-arbuscular mycorrhizal fungus

In situ observations of a vesicular-arbuscular mycorrhizal fungus Glomus mosseae (Nicol. & Gerd.) Gerdemann & Trappe with roots of Brassica cultivars containing low and normal levels of glucosinolates (7 to 524 mg g-1fresh weight) revealed differences between fungal penetrations of all Brassica roots compared to compatible hosts. Hyphae penetrated older, thicker Brassica roots, failed to form arbuscules, and exhibited no post-penetration stimulation of hyphal branching either internal or external to the roots. Some internal hyphae formed vesicles. Ultrastructural examination revealed that fungal hyphae penetrated Brassica roots intercellularly between dead cortical cells. Internal hyphae found near the healthy stele of the root lacked cytoplasm and had not penetrated the stele. Hyphae did not penetrate Brassica roots with healthy cortical cells; hyphae which contacted the surface of these cells became vacuolated and retracted their cytoplasm within a day. These observations indicate that functional mycorrhizas were not formed with Brassica roots, and that the level of glucosinolates in roots was not correlated with failure to establish symbiosis.