Defence responses of Norway spruce seedlings to elicitors of ectomycorrhizal fungus Pisolithus tinctorius and pathogen Heterobasidion annosum are affected by zeatin riboside

Abstract

Cytokinins are known to attenuate defence responses of plants after elicitor application or inoculation with fungi. To evaluate their role in the regulation of colonisation of Norway spruce (Picea abies) seedlings with mycorrhizal and pathogenic fungus, we analysed the effects of zeatin riboside (ZR) on: i) growth of ectomycorrhizal fungus Pisolithus tinctorius and pathogen Heterobasidion annosum in axenic cultures, ii) colonisation intensity of selected fungi on P. abies seedlings and iii) induction of defence reactions of spruce seedlings following elicitor treatment. Mycorrhizal fungus P. tinctorius showed increased growth at concentrations higherthan 10-2 µM ZR that was accompanied by increased ergosterol concentrations. In contrast, decreased growth of pathogen H. annosum was observed at the highest ZR (10 µM) concentration. ZR treatment also increased colonisation of spruce seedlingswith the mycorrhizal fungus. Application of cell wall preparations of both fungi increased peroxidase (POD) activity in the roots of treated spruce seedlings, whereas only elicitors of H. annosum increased also phenylalanine ammonia-lyase (PAL) activity,levels of soluble phenolics and salicylic acis (SA) concentrations. Application of ZR negated the increased activity of POD that was observed in elicitor treated seedlings, accompanied by increased levels of soluble phenolics in the roots of elicitated seedlings. In contrast, no effects of ZR treatment on PAL activity and SA accumulation could be observed. Our results suggest involvement of ZR in the regulation of cell wall modifications during the fungal colonisation with P. tinctorius and formation of ectomycorrhizae, by affecting the growth of fungal partner and non-specific defence reactions of the plant host.