Unrelated Fungal Rust Candidate Effectors Act on Overlapping Plant Functions

Rust fungi cause epidemics that threaten the production of important plant species, such as wheat and soy. Melampsora larici-populina (Mlp) causes the poplar rust and encodes at least 1184 candidate effectors (CEs) whose functions are poorly known. In this study, we sequenced the transcriptome and used mass spectrometry to analyze the metabolome of Arabidopsis plants constitutively expressing 14 Mlp CEs and of a control line to discover alterations leading to plant susceptibility. We found 2299 deregulated genes across the experiment. Genes involved in pattern-triggered immunity, such as FRK1, PR1, RBOHD, and WRKY33, as well as AUX/IAA genes were down-regulated. We further observed that 680 metabolites were deregulated in at least one CE-expressing transgenic line, with highly unsaturated and phenolic compounds and peptides enriched among down- and up-regulated metabolites. Interestingly, transgenic lines expressing unrelated CEs had correlated patterns of gene and metabolite deregulation, while expression of CEs belonging to the same family deregulated different genes and metabolites. Thus, our results uncouple effector sequence similarity and function. This supports that effector functional investigation in the context of their virulence activity and effect on plant susceptibility requires the investigation of the individual effector and precludes generalization based on sequence similarity.

Automated summary

The study revealed that transgenic plants expressing candidate effectors of poplar rust fungus Mlp caused significant gene and metabolite deregulation. Genes involved in pattern-triggered immunity were down-regulated, and similar effector sequences led to correlated patterns of gene and metabolite deregulation. This highlights the importance of investigating individual effectors rather than generalizing based on sequence similarity when studying plant susceptibility.