Accumulation of secondary compounds in barley and wheat roots in response to inoculation with an arbuscular mycorrhizal fungus and co-inoculation with rhizosphere bacteria

Colonization of Hordeum vulgare L. cv. Salome (barley) and Triticum aestivu m L. cv. Caprimus (wheat) roots by the arbuscular mycorrhizal fungus Glomus intraradices Schenck & Smith leads to de novo synthesis of isoprenoid cycl ohexenone derivatives with blumenin [9-O-(2'-O-beta-glucuronosyl)-beta-gluc opyranoside of 6-(3-hydroxybutyl)-1,1,5-trimethyl-4-cyclohexen-3-one] as th e major constituent and to transient accumulation of hydroxycinnamate amide s (4-coumaroylagmatine and -putrescine). Accumulation of these compounds in mycorrhizal wheat roots started 2 weeks after sowing together with the ons et of arbuscule formation and proceeded with mycorrhizal progression. Highe st levels were reached in 3- to 4-week-old secondary roots (foot branches o f first and higher order) characterized by the formation of vesicles. In th e final developmental stages, the fungus produced massive amounts of spores , enclosing the stele of older root parts (older than 5 weeks) characterize d by cortical death. In these root parts, the secondary compounds were dete cted in trace amounts only, indicating that they were located in the cortic al tissues. Some rhizosphere bacteria tested, i.e. Agrobacterium rhizogenes , Pseudomonas fluorescens, and Rhizobium leguminosarum, markedly stimulated both fungal root colonization and blumenin accumulation, thus, acting as m ycorrhiza-helper bacteria (MHB). Application of blumenin itself strongly in hibited fungal colonization and arbuscule formation at early stages of myco rrhiza development. This was associated with a markedly reduced accumulatio n of the hydroxycinnamate amides 4-coumaroylputrescine and -agmatine. The r esults suggest that both the isoprenoid and the phenylpropanoid metabolism are closely linked to the developmental stage and the extent of fungal colo nization. Their possible involvement in the regulation of mycorrhiza develo pment is discussed.