Suppression of Xo1-Mediated Disease Resistance in Rice by a Truncated, Non-DNA-Binding TAL Effector of Xanthomonas oryzae

Abstract

Delivered into plant cells by type III secretion from pathogenic Xanthomonas species, TAL (transcription activator-like) effectors are nuclear-localized, DNA-binding proteins that directly activate specific host genes. Targets include genes important for disease, genes that confer resistance, and genes inconsequential to the host-pathogen interaction. TAL effector specificity is encoded by polymorphic repeats of 33–35 amino acids that interact one-to-one with nucleotides in the recognition site. Activity depends also on N-terminal sequences important for DNA binding and C-terminal nuclear localization signals (NLS) and an acidic activation domain (AD). Coding sequences missing much of the N- and C-terminal regions due to conserved, in-frame deletions are present and annotated as pseudogenes in sequenced strains of Xanthomonas oryzae pv. oryzicola (Xoc) and pv. oryzae (Xoo), which cause bacterial leaf streak and bacterial blight of rice, respectively. Here we provide evidence that these sequences encode proteins we call “truncTALEs,” for “truncated TAL effectors.” We show that truncTALE Tal2h of Xoc strain BLS256, and by correlation truncTALEs in other strains, specifically suppress resistance mediated by the Xo1 locus recently described in the heirloom rice variety Carolina Gold. Xo1-mediated resistance is triggered by different TAL effectors from diverse X. oryzae strains, irrespective of their DNA binding specificity, and does not require the AD. This implies a direct protein-protein rather than protein-DNA interaction. Similarly, truncTALEs exhibit diverse predicted DNA recognition specificities. And, in vitro, Tal2h did not bind any of several potential recognition sites. Further, a single candidate NLS sequence in Tal2h was dispensable for resistance suppression. Many truncTALEs have one 28 aa repeat, a length not observed previously. Tested in an engineered TAL effector, this repeat required a single base pair deletion in the DNA, suggesting that it or a neighbor disengages. The presence of the 28 aa repeat, however, was not required for resistance suppression. TruncTALEs expand the paradigm for TAL effector-mediated effects on plants. We propose that Tal2h and other truncTALEs act as dominant negative ligands for an immune receptor encoded by the Xo1 locus, likely a nucleotide binding, leucine-rich repeat protein. Understanding truncTALE function and distribution will inform strategies for disease control.

Effector Mimics and Integrated Decoys, the Never-Ending Arms Race between Rice and Xanthomonas oryzae

“…oryzicola ( Xoc ) because TALE-derived (truncTALES and iTALES) effectors can suppress this resistance suggesting they might act as effector mimics. In the case of Xo1 this suppression is independent from DNA binding at least for the Tal2h truncTALE (Read et al, 2016). It will be interesting to determine whether Xa1 inhibition by iTALES is independent of DNA binding as well, or if its suppression is by a different mechanism.…”

“…In response to these defense systems, pathogens have evolved strategies to avoid recognition and suppress plant defenses (Brown and Tellier, 2011). Three recent reports dealing with the resistance of rice to Xanthomonas oryzae have added a new twist to our understanding of this fascinating co-evolutionary arms race (Ji et al, 2016; Read et al, 2016; Triplett et al, 2016). They show that pathogens also develop sophisticated effector mimics to trick recognition.…”

“…Remarkably, two other groups reported the recognition of several TALE effectors by the Xo1 locus a yet uncharacterized gene, which the authors argue to be likely a NLR protein (Read et al, 2016; Triplett et al, 2016). This rather non-specific recognition of TALEs does not lead to broad-spectrum resistance to Xoo and X. oryzae pv.…”

“…However, it appears that at least 3.5 central repeats regardless of their RVDs are required to trigger both Xa1 and Xo1 resistance (Ji et al, 2016; Triplett et al, 2016). Additionally, in the case of Xo1, TALE recognition and activation of resistance does not require the activation domain and is independent of DNA-binding (Read et al, 2016; Triplett et al, 2016). As a consequence, Xa1 recognizes an entire effector family, and not as other NLRs, only individual effectors in a very specific manner.…”

“…The second major discovery of these studies is that truncated TALE gene variants, previously considered as pseudogenes, designated as truncTALEs (Read et al, 2016) can act as interfering TALEs (iTALEs; Ji et al, 2016). Thus, some truncTALEs can block Xa1 - and Xo1 -mediated recognition of full-length TALEs, hence acting as iTALEs and suppressing resistance.…”