Characterization of Arbuscular Mycorrhizal Effector Proteins

Abstract

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.

Keywords: arbuscular mycorrhizal fungi; effector proteins; functional validation; interactomics; secretome.

Figure 1

Mode-of-action of the characterized nuclear-localized effector protein SP7 and RiNLE1. (a) The R. irregularis effector protein SP7 is secreted from the fungal hyphae, translocated to the plant cell, and internalized into the host nucleus. Then, it binds to the ERF19 to suppress the expression of pathogenesis-related genes in M. truncatula. (b) RiNLE1 is secreted from the arbuscule and further compartmentalized to the plant nucleus where it binds the M. truncatula H2B, hindering its ubiquitination to reduce the expression of plant defense-related genes.

Figure 2

Representation of the main nucleus-localized effector-related processes and the principal wet lab techniques used for the identification of plant targets. AMF effectors are synthesized and processed for signal peptide-directed secretion to the apoplast where some of them translocate to the intracellular space and further to other compartments, such as the nucleus. Once in the plant cell, the effectors associate with RNA, DNA, or RNA to modulate their activity. Although PPI techniques, such as Y2H and IP have been proven useful for the identification of plant proteins targeted by AMF effectors, alternative methods that include protein proximity labeling can be exploited. No AMF effector with DNA or RNA manipulating function has been studied yet. Nevertheless, DNA-targeted sequences by other fungal effector proteins have been identified with ChIP and its derived variants, whereas RNA-bound regions can be studied by means of RIP and iCLIP.