Impact of root exudates and plant defense signaling on bacterial communities in the rhizosphere. A review

Despite significant advances in crop protection, plant diseases cause a 20% yield loss in food and cash crops worldwide. Therefore, interactions between plants and pathogens have been studied in great detail. In contrast, the interplay between plants and non-pathogenic micro- organisms has received scant attention, and differential responses of plants to pathogenic and non-pathogenic microorganisms are as yet not well understood. Plants affect their rhizosphere microbial communities that can contain beneficial, neutral and pathogenic elements. Inter- actions between the different elements of these communi- ties have been studied in relation to biological control of plant pathogens. One of the mechanisms of disease control is induced systemic resistance (ISR). Studies on biological control of plant diseases have focused on ISR the last decade, because ISR is effective against a wide range of pathogens and thus offers serious potential for practical applications in crop protection. Such applications may however affect microbial communities associated with plant roots and interfere with the functioning of the root micro- biota. Here, we review the possible impact of plant defense signaling on bacterial communities in the rhizosphere. To better assess implications of shifts in the rhizosphere microflora we first review effects of root exudates on soil microbial communities. Current knowledge on inducible defense signaling in plants is discussed in the context of recognition and systemic responses to pathogenic and beneficial microorganisms. Finally, the as yet limited knowledge on effects of plant defense on rhizosphere microbial communities is reviewed and we discuss future directions of research that will contribute to unravel the molecular interplay of plants and their beneficial microflora.