Co-evolution within the plant holobiont drives host performance.

Plants interact with a diversity of microorganisms that influence their growth and resilience, and they can therefore be considered as ecological entities, namely "plant holobionts," rather than as singular organisms. In a plant holobiont, the assembly of above- and belowground microbiota is ruled by host, microbial, and environmental factors. Upon microorganism perception, plants activate immune signaling resulting in the secretion of factors that modulate microbiota composition. Additionally, metabolic interdependencies and antagonism between microbes are driving forces for community assemblies. We argue that complex plant-microbe and intermicrobial interactions have been selected for during evolution and may promote the survival and fitness of plants and their associated microorganisms as holobionts. As part of this process, plants evolved metabolite-mediated strategies to selectively recruit beneficial microorganisms in their microbiota. Some of these microbiota members show host-adaptation, from which mutualism may rapidly arise. In the holobiont, microbiota members also co-evolved antagonistic activities that restrict proliferation of microbes with high pathogenic potential and can therefore prevent disease development. Co-evolution within holobionts thus ultimately drives plant performance.

Transfer and engineering of immune receptors to improve recognition capacities in crops.

Immune receptors are pivotal elements of the plant immune system that act as sentinels for microbial invasion. Knowingly or unknowingly, breeding for resistance has largely relied on the transfer of immune receptor recognition specificities between plant genotypes. For decades such transfers were limited to crossable species. However, advents in transgene technologies have allowed overcoming species barriers. Novel strategies for mining of recognition specificities, combined with our recently increased understanding of immune receptor functioning, allows to increase and alter recognition specificities, which should ultimately increase the spectrum of recognition specificities that are available to control plant diseases in crops.

Interspecific hybridization impacts host range and pathogenicity of filamentous microbes.

Interspecific hybridization is widely observed within diverse eukaryotic taxa, and is considered an important driver for genome evolution. As hybridization fuels genomic and transcriptional alterations, hybrids are adept to respond to environmental changes or to invade novel niches. This may be particularly relevant for organisms that establish symbiotic relationships with host organisms, such as mutualistic symbionts, endophytes and pathogens. The latter group is especially well-known for engaging in everlasting arms races with their hosts. Illustrated by the increased identification of hybrid pathogens with altered virulence or host ranges when compared with their parental lineages, it appears that hybridization is a strong driver for pathogen evolution, and may thus significantly impact agriculture and natural ecosystems.

Gene silencing to investigate the roles of receptor-like proteins in Arabidopsis.

Receptor-like proteins (RLPs) are cell surface receptors that play important roles in various processes. In several plant species RLPs have been found to play a role in disease resistance, including the tomato Cf and Ve proteins and the apple HcrVf proteins that mediate resistance against the fungal pathogens Cladosporium fulvum, Verticillium spp., and Venturia inaequalis, respectively. The Arabidopsis genome contains 57 AtRLP genes. Two of these, CLV2 (AtRLP10) and TMM (AtRLP17), have well-characterized functions in meristem and stomatal development, respectively, while AtRLP52 is required for defense against powdery mildew. We recently reported the assembly of a genome-wide collection of T-DNA insertion lines for the Arabidopsis AtRLP genes. This collection was functionally analyzed with respect to plant growth, development and sensitivity to various stress responses including pathogen susceptibility. Only few new phenotypes were discovered; while AtRLP41 was found to mediate abscisic acid sensitivity, AtRLP30 (and possibly AtRLP18) was found to be required for full non-host resistance to a bacterial pathogen. Possibly, identification of novel phenotypes is obscured by functional redundancy. Therefore, RNA interference (RNAi) to target the expression of multiple AtRLP genes simultaneously was employed followed by functional analysis of the RNAi lines.