Seeking the interspecies crosswalk for filamentous microbe effectors

Exploring the potential role of four Rhizophagus irregularis nuclear effectors: opportunities and technical limitations

Arbuscular mycorrhizal fungi (AMF) are obligate symbionts that interact with the roots of most land plants. The genome of the AMF model species Rhizophagus irregularis contains hundreds of predicted small effector proteins that are secreted extracellularly but also into the plant cells to suppress plant immunity and modify plant physiology to establish a niche for growth. Here, we investigated the role of four nuclear-localized putative effectors, i.e., GLOIN707, GLOIN781, GLOIN261, and RiSP749, in mycorrhization and plant growth. We initially intended to execute the functional studies in Solanum lycopersicum, a host plant of economic interest not previously used for AMF effector biology, but extended our studies to the model host Medicago truncatula as well as the non-host Arabidopsis thaliana because of the technical advantages of working with these models. Furthermore, for three effectors, the implementation of reverse genetic tools, yeast two-hybrid screening and whole-genome transcriptome analysis revealed potential host plant nuclear targets and the downstream triggered transcriptional responses. We identified and validated a host protein interactors participating in mycorrhization in the host.S. lycopersicum and demonstrated by transcriptomics the effectors possible involvement in different molecular processes, i.e., the regulation of DNA replication, methylglyoxal detoxification, and RNA splicing. We conclude that R. irregularis nuclear-localized effector proteins may act on different pathways to modulate symbiosis and plant physiology and discuss the pros and cons of the tools used.

Effectors and environment modulating rice blast disease: from understanding to effective control

Rice blast is a highly destructive crop disease that requires the interplay of three essential factors: the virulent blast fungus, the susceptible rice plant, and favorable environmental conditions. Although previous studies have focused mainly on the pathogen and rice, recent research has shed light on the molecular mechanisms by which the blast fungus and environmental conditions regulate host resistance and contribute to blast disease outbreaks. This review summarizes significant achievements in understanding the sophisticated modulation of blast resistance by Magnaporthe oryzae effectors and the dual regulatory mechanisms by which environmental conditions influence rice resistance and virulence of the blast fungus. Furthermore, it emphasizes potential strategies for developing blast-resistant rice varieties to effectively control blast disease.

Filamentous pathogen effectors enter plant cells via endocytosis

Recent findings demonstrate that cytoplasmic effectors from fungal and oomycete pathogens enter plant cells via clathrin-mediated endocytosis (CME). This raises several questions: Does effector secretion pathway facilitate host uptake? How is CME triggered in host cells? How are the effectors released from endosomal compartments to reach diverse subcellular destinations?

Unconventional secretion of Magnaporthe oryzae effectors in rice cells is regulated by tRNA modification and codon usage control

Microbial pathogens deploy effector proteins to manipulate host cell innate immunity, often using poorly understood unconventional secretion routes. Transfer RNA (tRNA) anticodon modifications are universal, but few biological functions are known. Here, in the rice blast fungus Magnaporthe oryzae, we show how unconventional effector secretion depends on tRNA modification and codon usage. We characterized the M. oryzae Uba4-Urm1 sulfur relay system mediating tRNA anticodon wobble uridine 2-thiolation (s2U34), a conserved modification required for efficient decoding of AA-ending cognate codons. Loss of s2U34 abolished the translation of AA-ending codon-rich messenger RNAs encoding unconventionally secreted cytoplasmic effectors, but mRNAs encoding endoplasmic reticulum-Golgi-secreted apoplastic effectors were unaffected. Increasing near-cognate tRNA acceptance, or synonymous AA- to AG-ending codon changes in PWL2, remediated cytoplasmic effector production in Δuba4. In UBA4+, expressing recoded PWL2 caused Pwl2 super-secretion that destabilized the host-fungus interface. Thus, U34 thiolation and codon usage tune pathogen unconventional effector secretion in host rice cells.

Characterization of Arbuscular Mycorrhizal Effector Proteins

Plants are colonized by various fungi with both pathogenic and beneficial lifestyles. One type of colonization strategy is through the secretion of effector proteins that alter the plant's physiology to accommodate the fungus. The oldest plant symbionts, the arbuscular mycorrhizal fungi (AMF), may exploit effectors to their benefit. Genome analysis coupled with transcriptomic studies in different AMFs has intensified research on the effector function, evolution, and diversification of AMF. However, of the current 338 predicted effector proteins from the AM fungus Rhizophagus irregularis, only five have been characterized, of which merely two have been studied in detail to understand which plant proteins they associate with to affect the host physiology. Here, we review the most recent findings in AMF effector research and discuss the techniques used for the functional characterization of effector proteins, from their in silico prediction to their mode of action, with an emphasis on high-throughput approaches for the identification of plant targets of the effectors through which they manipulate their hosts.