Pathogenesis-related proteins in roots: a role in plant resistance or an epiphenomenon?

Francés. Although Pathogenesis-Related (PR) proteins have been identified in numerous monocotyledons and dicotyledons across different genera and in relation to very different physiological situations, relatively few studies have been conducted on roots which are the site of multimicrobial interactions. PR proteins are considered to be inducible proteins elicited by many environmental and developmental stimuli, but an increasing body of data points to their occurrence as constitutive components in different plant organs (Edreva 2005), including roots. This is the case, for example, of a Nicotiana glutinosa x Nicotiana debney hybrid which over-expresses PR protein genes and shows constitutive resistance to root fungal pathogens, but susceptibility to root colonisation by symbiotic fungi. However, successful root colonisation by beneficial microbes must imply a permanent control over microbial development. This could result from defence mechanisms being modulated in their intensity by the beneficial microbe (Gianinazzi 1981). In mycorrhizal interactions this phenomenon is associated with the weak and localized expression of several PR protein genes, and it could be responsible for priming the localized and systemic resistance against subsequent attack by microbial pathogens that is induced in mycorrhizal root systems. The induction of resistance by mycorrhiza formation requires a yet undefined systemic signal in roots, the activity of which in the above-ground part of the plant is not clear. Moreover, PRprotein synthesis induced by fungal pathogens in roots is associated with the development of root disease which raises the question of the role of PR proteins in root control over microbial invasion (Dumas-Gaudot et al. 2000). Edreva A, 2005, Gen.Plant Physiology, 31,105-124 Gianinazzi S, 1981, Agric.Ecosys.Environ.35, 105-112 Dumas-Gaudot E et al., 2000, Arbuscular mycorrhizas: physiology and function, 173-200.