During colonization of their hosts, pathogens secrete effector proteins to promote disease development through various mechanisms. Increasing evidence shows that the host microbiome plays a crucial role in health, and that hosts actively shape their microbiomes to suppress disease.We hypothesized that pathogens evolved to manipulate host microbiomes to their advantage in turn. Here, we show that the fungal plant pathogen Verticillium dahliae utilizes effector proteins for niche colonization through selective manipulation of host microbiomes by suppressing microbes with antagonistic activities. Moreover, we show that effector proteins are similarly exploited for microbiome manipulation in the soil environment, where the fungus resides in absence of a host. In conclusion, we demonstrate that pathogens utilize effector proteins to modulate microbiome compositions and propose that their effector catalogs represent an untapped resource for novel antibiotics.

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Background Scab, caused by the biotrophic fungus Venturia inaequalis, is the most economically important disease of apples worldwide. During infection, V. inaequalis occupies the subcuticular environment, where it secretes virulence factors, termed effectors, to promote host colonization. Consistent with other plant-pathogenic fungi, many of these effectors are expected to be non-enzymatic proteins, some of which can be recognized by corresponding host resistance proteins to activate plant defences, thus acting as avirulence determinants. To develop durable control strategies against scab, a better understanding of the roles that these effector proteins play in promoting subcuticular growth by V. inaequalis, as well as in activating, suppressing, or circumventing resistance protein-mediated defences in apple, is required. Results We generated the first comprehensive RNA-seq transcriptome of V. inaequalis during colonization of apple. Analysis of this transcriptome revealed five temporal waves of gene expression that peaked during early, mid, or mid-late infection. While the number of genes encoding secreted, non-enzymatic proteinaceous effector candidates (ECs) varied in each wave, most belonged to waves that peaked in expression during mid-late infection. Spectral clustering based on sequence similarity determined that the majority of ECs belonged to expanded protein families. To gain insights into function, the tertiary structures of ECs were predicted using AlphaFold2. Strikingly, despite an absence of sequence similarity, many ECs were predicted to have structural similarity to avirulence proteins from other plant-pathogenic fungi, including members of the MAX, LARS, ToxA and FOLD effector families. In addition, several other ECs, including an EC family with sequence similarity to the AvrLm6 avirulence effector from Leptosphaeria maculans, were predicted to adopt a KP6-like fold. Thus, proteins with a KP6-like fold represent another structural family of effectors shared among plant-pathogenic fungi. Conclusions Our study reveals the transcriptomic profile underpinning subcuticular growth by V. inaequalis and provides an enriched list of ECs that can be investigated for roles in virulence and avirulence. Furthermore, our study supports the idea that numerous sequence-unrelated effectors across plant-pathogenic fungi share common structural folds. In doing so, our study gives weight to the hypothesis that many fungal effectors evolved from ancestral genes through duplication, followed by sequence diversification, to produce sequence-unrelated but structurally similar proteins.

BackgroundScab, caused by the biotrophic fungus Venturia inaequalis, is the most economically important disease of apples worldwide. During infection, V. inaequalis occupies the subcuticular environment, where it secretes virulence factors, termed effectors, to promote host colonization. However, in the presence of corresponding host resistance proteins, these effectors are recognized as avirulence determinants to activate plant defences. To develop durable control strategies against scab, a better understanding of the molecular mechanisms underpinning subcuticular growth by V. inaequalis is required. Likewise, more information is needed on the role that effectors play in activating, suppressing or circumventing resistance protein-mediated defences.ResultsWe generated the first comprehensive RNA-seq transcriptome of V. inaequalis during colonization of apple. Analysis of this transcriptome revealed five in planta gene expression clusters or waves corresponding to three specific infection stages: early, mid and mid-late infection. Early infection was characterized by genes encoding plant cell wall-degrading enzymes (PCWDEs) and proteins associated with oxidative stress responses. Mid infection was characterized by genes encoding transporter proteins. Finally, mid-late infection was characterized by genes encoding PCWDEs and effector candidates (ECs), with most ECs belonging to expanded protein families. To gain insights into function, AlphaFold2 was used to predict the tertiary structures of proteinaceous ECs. Strikingly, many ECs were predicted to have structural similarity to avirulence proteins from other plant-pathogenic fungi, including members of the MAX, LARS, ToxA and FOLD structural effector families. In addition, several other ECs, including an EC family with amino acid similarity to the AvrLm6 effector from Leptosphaeria maculans, were predicted to adopt a KP6/ferredoxin-like fold. Thus, proteins with a KP6/ferredoxin-like fold may represent yet another structural family of effectors shared among plant-pathogenic fungi.ConclusionsOur study reveals the transcriptomic profile underpinning subcuticular growth by V. inaequalis, and reinforces the idea that fungal effectors share a limited number of structural folds. Importantly, our study also provides an enriched list of V. inaequalis ECs that can be investigated for roles in virulence and avirulence, and raises the possibility that apple resistance proteins can be engineered to recognize EC family folds or host components targeted by multiple EC family members.