The concept of the holobiont has been proposed for plants. The term ‘holobiont’ was first defined by Lynn Margulis in 1991, as a simple biological unit involving a host and a single inherited symbiont. Zilberg-Rosenberg and Rosenberg (2008) expanded this definition to the entire microbiota. These authors have further proposed the holobiont to be a selection unit, which underlies the hologenome based theory of evolution.

This Research Topic is one of two volumes on the Plant Holobiont:
Volume I: Microbiota as part of the holobiont; challenges for agriculture
Volume II: Impacts of the Rhizosphere on Plant Health

Plants live in close association with the microbes that inhabit the rhizosphere, and the presence of this complex plant-associated microbial community is important for plant health. In nature, plants are continuously exposed to a variety of environmental factors that influence both the above- and below-ground structures. In particular, the composition of the root microbiome may be altered by specific biotic and/or abiotic stressors and, subsequently, has consequences on the reconfiguration of the microbiome, which may aid the plant in reacting, and in some cases overcoming, the adverse constraints. Re-modulation of the rhizosphere microbiome composition is often accompanied by active root excretion of selective compounds that can be antagonistic/unfavorable and/or proliferative to microbes. It is of great interest to understand the signaling molecules produced by plants when faced by abiotic and biotic stressors, and to determine how these compounds can be responsible for the recruitment of “helper” microbes in the rhizosphere that aid the plant defense response.

At the same time, the interaction between the plant and its root microbiome can affect plant defense responses and subsequently influence crop productivity and quality. Some root symbiotic microbes may establish positive interactions with plants either by attracting other beneficial microbes or insects, directly or indirectly or by increasing plant tolerance to specific stresses.

This Research Topic aims to present investigations on plant-microbe interactions occurring in the rhizosphere when plants are subject to biotic or abiotic stress conditions. This will examine the molecular dialogue that occurs not only between the root microbiome and its host plant but also among the components/members of the related microbiota, with special interest on those associations that elicit plant beneficial effects.
This Research Topic will consider the submission of Reviews, Technology Report, Methods, Opinion, and Original Research Articles highlighting the following aspects:

· Recognition of basic signaling compounds, including nutrients, that are responsible for maintaining a healthy exo- and/or endophytic microbial community with the plant root system;
· Identification of “help signals” released in the rhizosphere by the stressed plants that aid in the recruitment or “activation” of beneficial microbes to the victim;
· Determine the effects of root colonization by microbes that can systemically influence plant defenses to above-ground stresses;
· Modulation of the plant rhizosphere microbiome in response to both biotic and abiotic factors.

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Frontiers in Plant Science

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