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Following the recognition of pathogen-encoded effectors, plant TIR-NB-LRR immune receptors induce defense signaling by a largely unknown mechanism. We identify a novel and conserved role for the SQUAMOSA PROMOTER BINDING PROTEIN (SBP)-domain transcription factor SPL6 in enabling the activation of the defense transcriptome following its association with a nuclear-localized immune receptor. During an active immune response, the Nicotiana TIR-NB-LRR N immune receptor associates with NbSPL6 within distinct nuclear compartments. NbSPL6 is essential for the N-mediated resistance to Tobacco mosaic virus. Similarly, the presumed Arabidopsis ortholog AtSPL6 is required for the resistance mediated by the TIR-NB-LRR RPS4 against Pseudomonas syringae carrying the avrRps4 effector. Transcriptome analysis indicates that AtSPL6 positively regulates a subset of defense genes. A pathogen-activated nuclear-localized TIR-NB-LRR like N can therefore regulate defense genes through SPL6 in a mechanism analogous to the induction of MHC genes by mammalian immune receptors like CIITA and NLRC5.

Vascular phloem loading has long been recognized as an essential step in the establishment of a systemic virus infection. In this study, an interaction between the replication protein of tobacco mosaic virus (TMV) and phloem-specific auxin/indole acetic acid (Aux/IAA) transcriptional regulators was found to modulate virus phloem loading in an age-dependent manner. Promoter expression studies show that in mature tissues TMV 126/183-kDa–interacting Aux/IAAs predominantly express and accumulate within the nuclei of phloem companion cells (CCs). Furthermore, CC Aux/IAA nuclear localization is disrupted upon infection with an interacting virus. In situ analysis of virus spread shows that the inability to disrupt Aux/IAA CC nuclear localization correlates with a reduced ability to load into the vascular tissue. Subsequent systemic movement assays also demonstrate that a virus capable of disrupting Aux/IAA localization is significantly more competitive atmoving out of older plant tissues than a noninteracting virus. Similarly, CC expression and overaccumulation of a degradation-resistant Aux/IAA-interacting protein was found to inhibit TMV accumulation and phloem loading selectively in flowering plants. Transcriptional expression studies demonstrate a role for Aux/IAA-interacting proteins in the regulation of salicylic and jasmonic acid host defense responses as well as virus-specific movement factors, including pectin methylesterase, that are involved in regulating plasmodesmata size-exclusion limits and promoting virus cell-to-cell movement. Combined, these findings indicate that TMV directs the reprogramming of auxin-regulated gene expression within the vascular phloem of mature tissues as a means to enhance phloem loading and systemic spread.

Following the recognition of pathogen-encoded effectors, plant TIR-NB-LRR immune receptors induce defense signaling by a largely unknown mechanism. We identify a novel and conserved role for the SQUAMOSA PROMOTER BINDING PROTEIN (SBP)-domain transcription factor SPL6 in enabling the activation of the defense transcriptome following its association with a nuclear-localized immune receptor. During an active immune response, the Nicotiana TIR-NB-LRR N immune receptor associates with NbSPL6 within distinct nuclear compartments. NbSPL6 is essential for the N-mediated resistance to Tobacco mosaic virus. Similarly, the presumed Arabidopsis ortholog AtSPL6 is required for the resistance mediated by the TIR-NB-LRR RPS4 against Pseudomonas syringae carrying the avrRps4 effector. Transcriptome analysis indicates that AtSPL6 positively regulates a subset of defense genes. A pathogen-activated nuclear-localized TIR-NB-LRR like N can therefore regulate defense genes through SPL6 in a mechanism analogous to the induction of MHC genes by mammalian immune receptors like CIITA and NLRC5.

Tobacco Mosaic Virus and Arabidopsis thaliana serve as ideal model systems to study the molecular aspects of virus-host interactions. Using this system, an interaction between the helicase domain within TMV replicase protein and an auxin responsive protein, IAA26 was identified. IAA26 is a member of the Aux/IAA family of transcription factors that function as repressors in signaling pathways controlled by the phytohormone auxin. Characterization of the interaction was carried out utilizing a helicase mutant defective in its interaction with IAA26 and by creating transgenic plants silenced for IAA26 expression. These studies suggest that the interaction was not essential for either viral replication or movement but promoted the development of disease symptoms. Cellular co-localization studies revealed that in TMV infected tissue, the nuclear localization and stability of IAA26 was compromised and the protein was relocalized to distinct cytoplasmic vesicles in association with the viral replicase. In keeping with its role as a transcription factor, the alterations in IAA26 function should lead to misregulation of downstream auxin responsive genes and this is supported by the fact that ∼30% of the Arabidopsis genes displaying transcriptional alterations to TMV could be linked to the auxin signaling pathway. Aux/IAA family members share significant sequence and functional homology, and an additional interaction screen identified two more Arabidopsis Aux/IAA proteins, IAA27 and IAA18 and a putative tomato Aux/IAA protein, LeIAA26 that could interact with TMV helicase. The nuclear localization of these three proteins was disrupted by TMV and alterations in LeIAA26 levels induced virus infection-like symptoms in tomato. Additionally, transgenic plants over-expressing a proteolysis resistant mutant of IAA26 showed abnormal developmental phenotype, the severity of which was abrogated during TMV infection which blocked nuclear accumulation of the protein. Taken together, these findings suggest that TMV induced disease symptoms can partially be explained by the ability of the virus to disrupt the functioning of interacting Aux/IAA proteins within susceptible hosts. The significance of such interactions is yet to be determined but it appears that they may be advantageous to the virus while infecting tissues that are in a developmentally static stage.