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Review
. 2013 May;6(3):230-40.
doi: 10.1111/1751-7915.12042. Epub 2013 Feb 25.

Phytopathogen type III effectors as probes of biological systems

Amy Huei-Yi Lee  1 , Maggie A Middleton, David S Guttman, Darrell Desveaux
Affiliations
Review

Phytopathogen type III effectors as probes of biological systems

Amy Huei-Yi Lee et al. Microb Biotechnol. 2013 May.

Abstract

Bacterial phytopathogens utilize a myriad of virulence factors to modulate their plant hosts in order to promote successful pathogenesis. One potent virulence strategy is to inject these virulence proteins into plant cells via the type III secretion system. Characterizing the host targets and the molecular mechanisms of type III secreted proteins, known as effectors, has illuminated our understanding of eukaryotic cell biology. As a result, these effectors can serve as molecular probes to aid in our understanding of plant cellular processes, such as immune signalling, vesicle trafficking, cytoskeleton stability and transcriptional regulation. Furthermore, given that effectors directly and specifically interact with their targets within plant cells, these virulence proteins have enormous biotechnological potential for manipulating eukaryotic systems.

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Figures

Figure 1
Figure 1
Eukaryotic cellular systems targeted by phytopathogenic type III secreted effectors (T3SEs). Bacterial phytopathogens use T3SEs (yellow boxes) to inhibit upstream signalling components such as targeting FLS2 for proteasome degradation (AvrPtoB), prevent the phosphorylation of BAK1 (AvrAC and AvrPto), or degrade BIK1 (HopAR1). In addition, phytopathogen T3SEs can inhibit the phosphorylation of downstream signalling components such as the MAPKKs (HopF2) and MAPKs (HopAI1). Two nuclear-localized T3SEs, AvrBs3 and XopD, bind to DNA and alter transcription in plant cells. Specifically, AvrBS3 is a transcription activator that binds to the UPA box of its target genes. XopD represses the activities of a eukaryotic transcription factor, MYB30, which consequently suppresses the transcription of plant immune response genes. HopU1 targets RNA-binding proteins such as AtGRP7, which alters RNA processing in the plant host. HopM1 targets a plant ARF–GEF (AtMIN7) for degradation by the proteasome and consequently inhibit the secretory pathway. HopZ1a is the first bacterial phytopathogen T3SE shown to bind to plant tubulin and causes microtubule destruction. In addition, HopZ1a inhibits the plant secretory pathway. Lastly, the HopAR1-elicited plant immune response requires an actin regulator, ADF4. However, the link between HopAR1 and ADF4 or HopAR1 and actin is currently unclear.
Figure 2
Figure 2
Phytopathogen T3SEs disrupt host-signalling pathways to suppress immune responses. A. Pattern recognition receptors such as FLS2 and EFR allow the plant host to recognize microbe-associated molecular patterns (MAMPs), such as bacterial flagellin and elongation factor Tu (EF-Tu) respectively. This MAMP recognition leads to the activation of signalling cascades via a series of phosphorylation events, which subsequently activates plant immunity. B. Bacterial phytopathogens use T3SEs (yellow boxes) to inhibit upstream signalling components. This includes the targeting of FLS2 for proteasome degradation (HopAB2), preventing the phosphorylation of BAK1 (AvrAC and AvrPto), or directly degrading BIK1 (HopAR1). In addition, phytopathogen T3SEs can inhibit the phosphorylation of downstream signalling components such as the MAPKKs (HopF2) and MAPKs (HopAI1).
Figure 3
Figure 3
DNA binding specificity of TAL effectors. TAL effectors (TALEs) such as AvrBs3 are transcription activators that bind to the promoter of their target genes. TALEs contain a central domain with 17.5 repeats, a C-terminal nuclear localization signal (NLS) and the transcriptional activation domain. The DNA-binding specificity of TALEs is dependent on a 2-amino-acid motif within each repeat. As an example, the consensus UPA box and the corresponding 2-amino-acid motifs for AvrBs3 are illustrated, with the TALE DNA code shown below. Figure is adapted from Boch and Bonas (2010).
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