Plant and mycorrhizal regulation of rhizodeposition

Abstract

Contents Introduction 460 What is rhizodeposition? 460 Regulation of rhizodeposition 460 How large is the root exudation C flux? 463 How responsive is the root exudation C flux? 463 How responsive is the microbial community to root exudation? 464 The role of root exudates in nutrient acquisition 464 Mycorrhizal fungi and rhizodeposition 471 Future thoughts 474 Acknowledgements 474 References 474 Summary The loss of carbon from roots (rhizodeposition) and the consequent proliferation of microorganisms in the surrounding soil, coupled with the physical presence of a root and processes associated with nutrient uptake, gives rise to a unique zone of soil called the rhizosphere. In this review, we bring together evidence to show that roots can directly regulate most aspects of rhizosphere C flow either by regulating the exudation process itself or by directly regulating the recapture of exudates from soil. Root exudates have been hypothesized to be involved in the enhanced mobilization and acquisition of many nutrients from soil or the external detoxification of metals. With few exceptions, there is little mechanistic evidence from soil‐based systems to support these propositions. We conclude that much more integrated work in realistic systems is required to quantify the functional significance of these processes in the field. We need to further unravel the complexities of the rhizosphere in order to fully engage with key scientific ideas such as the development of sustainable agricultural systems and the response of ecosystems to climate change.

Behavior of mercury in a soil–plant system as affected by inoculation with the arbuscular mycorrhizal fungus Glomus mosseae

“…It has been expected that topsoil with high organic matter and high microbial activity is the major source of Hg reduction and evaporation (Schlüter 2000). Mycorrhizal fungi can secrete organic materials and regulate root exudation into soil (Jones et al 2004), leading to a change in the quantity and composition of soil organic matter. Furthermore, mycorrhizal inoculation has been identified to result in an increase in soil microbial activity particularly in the rhizosphere soil (Wu et al 2008;Huang et al 2009), which can benefit the reduction of Hg(II) and Hg evaporation.…”

“…AM inoculation can influence soil properties and soil microbial community (Smith and Read 1997;Li and Christie 2001). Qualitative changes in root exudation and rhizodeposition following AM colonization have also been reported (Jones et al 2004). All of these are speculated to change the speciation, mobility, and bioavailability of Hg in soil, therefore influencing its behavior in soil-plant system.…”

Identification of Bacterial Groups Preferentially Associated with Mycorrhizal Roots of Medicago truncatula

“…The lower fertility of the Mas d'Imbert soil together with the higher mycorrhization rate of the Myc ϩ genotypes (Table 3) have led to a more significantly beneficial effect of AM symbiosis on plant growth and development in the Mas d'Imbert than in the Châteaurenard soil (Table 2). Since root exudates are influenced quantitatively and qualitatively by the plant growth and development (15,16,17,20), the more significant plant growth promotion by AM in the Mas d'Imbert soil than in the Châteaurenard soil is expected to have impacted more root exudation of the wild type compared to the nonmycorrhizal mutants in the Mas d'Imbert soil than in the Châteaurenard soil. Root exudates are known to influence the genetic structure and diversity of microbial communities in the rhizosphere (18,40), and differences between the bacterial communities associated with mycorrhizal and nonmycorrhizal genotypes in the two soils could then to be related to root exudate variations resulting from the greater promoting effect of AM in the Mas d'Imbert than in the Châteaurenard soil (Table 2).…”

Redox potential (Eh) and pH as drivers of soil/plant/microorganism systems: a transdisciplinary overview pointing to integrative opportunities for agronomy

“…In addition, this new framework for analysis of soil/ plant/microorganism systems could help in bridging the gap between chemistry-physics and biology and may be a useful tool for the integration between disciplines, which is needed for the development of new perspectives in agronomy (Jones et al 2004). Sparks (2001), for instance, regards the studies of microbedriven redox reactions and redox transformations of C, N, P, and S according to various redox limits as new frontiers of research at the interface between chemistry and biology.…”