Figure 1.

Events associated with plant protection by Streptomyces GB 4-2. The stronger capacity to activate defence responses at the sites of secondary infection results in a decrease in disease. (A) Upon 3-wk treatment of Norway spruce roots by the phytopathogen Heterobasidion abietinum, root cortex is colonised by the fungus. (B) Prior treatment with GB 4-2 causes the formation of cell wall thickenings (arrowheads) that block further plant colonisation by the fungus. Whereas the fungus has lysed most of the cells 4 wks after inoculation (C), pre-treatment with GB 4-2 leads to the formation of thick-walled, small diameter tracheids (D). Plant's resistance to secondary infections is also increased in the shoot by GB 4-2. The formation of brassica dark leaf spot (E) in leaves of Arabidopsis thaliana (arrowheads) is suppressed by pre-inoculation of plant roots by GB 4-2 (F). Bars 0.5 mm.