Comparison of Barley Succession and Take-All Disease as Environmental Factors Shaping the Rhizobacterial Community during Take-All Decline

The root disease take-all, caused by Gaeumannomyces graminis var. tritici, can be managed by monoculture-induced take-all decline (TAD). This natural biocontrol mechanism typically occurs after a take-all outbreak and is believed to arise from an enrichment of antagonistic populations in the rhizosphere. However, it is not known whether these changes are induced by the monoculture or by ecological rhizosphere conditions due to a disease outbreak and subsequent attenuation. This question was addressed by comparing the rhizosphere microflora of barley, either inoculated with the pathogen or noninoculated, in a microcosm experiment in five consecutive vegetation cycles. TAD occurred in soil inoculated with the pathogen but not in noninoculated soil. Bacterial community analysis using terminal restriction fragment length polymorphism of 16S rRNA showed pronounced population shifts in the successive vegetation cycles, but pathogen inoculation had little effect. To elucidate rhizobacterial dynamics during TAD development, a 16S rRNA-based taxonomic microarray was used. Actinobacteria were the prevailing indicators in the first vegetation cycle, whereas the third cycle—affected most severely by take-all—was characterized by Proteobacteria, Bacteroidetes, Chloroflexi, Planctomycetes, and Acidobacteria. Indicator taxa for the last cycle (TAD) belonged exclusively to Proteobacteria, including several genera with known biocontrol traits. Our results suggest that TAD involves monoculture-induced enrichment of plant-beneficial taxa.The root disease take-all, caused by the soilborne ascomycete fungus Gaeumannomyces graminis var. tritici, has a significant impact on the global production of wheat and barley (23). Usually, take-all is managed by crop rotation, but low disease levels may also be achieved by monoculture-induced disease suppressiveness (13, 50, 61). Indeed, a spontaneous reduction of disease severity can be observed during wheat or barley monoculture after at least one severe outbreak of the disease, a phenomenon called take-all decline (TAD). Disease symptoms then remain at a low level as long as monoculture continues. TAD is related to the enrichment of certain rhizosphere populations antagonistic to the pathogen (13) and parallel changes in G. graminis var. tritici populations (28).Though TAD is the best-studied example of induced disease suppressiveness (61), knowledge of the mechanisms involved in TAD is fragmentary. Take-all research so far has mainly focused on the role of fluorescent pseudomonads, as they are easy to cultivate and known to be antagonistic to various phytopathogens (21, 35, 62, 63). Several studies have indicated the involvement of other microorganisms in TAD, e.g., Bacillus species and various actinomycetes (3, 26, 45), but none of them has considered the whole bacterial community. Yet, it has been shown that community-based approaches are important in identifying microbial populations potentially involved in suppressiveness (10, 27).The first community-based assessment was carried out based on molecular fingerprinting and sequencing, and it showed that take-all disease of wheat induced changes in several rhizosphere bacterial populations (39). However, disease-suppressive stages were not included. More recently, the taxonomic microarray-based study of Sanguin et al. (48) revealed multiple changes in the rhizobacterial community of wheat when comparing take-all disease and suppressive stages, which paralleled changes in Pseudomonas populations (46). These differences were attributed to TAD, but it is not known whether they occurred as a result of repeated wheat cropping (leading to enrichment of particular bacterial taxa), a disease outbreak and subsequent attenuation (modifying nutrient conditions on roots), or both.Therefore, the objective of this study was to compare the significance of plant succession and take-all disease as environmental factors shaping the rhizobacterial community during TAD. Repeated barley cropping was carried out in microcosms inoculated with G. graminis var. tritici or not inoculated, and the physiologically active bacterial rhizosphere community was monitored using terminal restriction fragment length polymorphism (t-RFLP) (32), as well as microarray analysis of 16S rRNA.