New developments in understanding the role of salicylic acid in plant defence

Salicylic acid (SA) is currently considered as a key molecule in activating plant defence against various insects and pathogenic invaders. Its role in determining the so-called systemic acquired resistance (SAR), that confers long-lasting protection against a broad spectrum of pathogens, has definitely been established. The activation of an SA-dependent signalling pathway is also associated with more specific plant defences, such as those determined by gene-for-gene recognition of the pathogen by plants carrying resistance genes (R-genes). A recent further impulse in the investigation of the role of SA, both in local defences and SAR, has come either from genetic screens of Arabidopsis or from SA-degrading transgenic plants that express the bacterial salicylate hydroxylase gene (NahG). Genetic screens of Arabidopsis and the recently developed microarray technology allowed us to identify several genes involved in SA synthesis and signalling. However, the biochemical mechanisms by which SA can exert its many functions are not clear yet and the scope of this review is to report on such mechanisms. SA signalling is indicated here as a complex network operating through gene expressions and biochemical pathways which are finely regulated by SA concentration. According to its level in plants, SA may operate as a beneficial anti-inflammatory compound or as a cell killer. New developments in understanding the role of SA in plant defence are reported from studies on root response to soil pests, such as root-knot nematodes. The role of SA in diseases of roots has been reviewed here for the first time, as most of our knowledge on this matter is based on studies carried out on leaves.