A Powerful Method for Studying Protein-Protein Interactions in Plants: Coimmunoprecipitation (Co-IP) Assay

PagKNAT2/6b promotes shoot branching by attenuating auxin-strigolactone signalling in poplar

Shoot branching from axillary bud (AB) directly determines plant architecture. However, the mechanism through which AB remains dormant or emerges to form branches as plants grow remains largely unknown. Here, the auxin-strigolactone (IAA-SL) pathway was first shown to regulate shoot branching in poplar, and we found that PagKNAT2/6b could modulate this pathway. PagKNAT2/6b was expressed mainly in the shoot apical meristem and AB and was induced by shoot apex damage. PagKNAT2/6b overexpressing poplar plants (PagKNAT2/6b OE) exhibited multiple branches that mimicked the branching phenotype of nontransgenic plants after decapitation treatment, while compared with nontransgenic controls, PagKNAT2/6b antisense transgenic poplar and Pagknat2/6b mutant lines exhibited a significantly decreased number of branches after shoot apex damage treatment. In addition, we found that PagKNAT2/6b directly inhibits the expression of the key IAA synthesis gene PagYUC6a, which is specifically expressed in the shoot apex. Moreover, overexpression of PagYUC6a in the PagKNAT2/6b OE background reduced the number of branches after shoot apex damage treatment. Overall, we conclude that PagKNAT2/6b responds to shoot apical injury and regulates shoot branching through the IAA-SL pathway. These findings may provide a theoretical basis and candidate genes for genetic engineering to create new forest tree species with different crown types.

Proxitome profiling reveals a conserved SGT1-NSL1 signaling module that activates NLR-mediated immunity

Suppressor of G2 allele of skp1 (SGT1) is a highly conserved eukaryotic protein that plays a vital role in growth, development, and immunity in both animals and plants. Although some SGT1 interactors have been identified, the molecular regulatory network of SGT1 remains unclear. SGT1 serves as a co-chaperone to stabilize protein complexes such as the nucleotide-binding leucine-rich repeat (NLR) class of immune receptors, thereby positively regulating plant immunity. SGT1 has also been found to be associated with the SKP1-Cullin-F-box (SCF) E3 ubiquitin ligase complex. However, whether SGT1 targets immune repressors to coordinate plant immune activation remains elusive. In this study, we constructed a toolbox for TurboID- and split-TurboID-based proximity labeling (PL) assays in Nicotiana benthamiana and used the PL toolbox to explore the SGT1 interactome during pre- and post-immune activation. The comprehensive SGT1 interactome network we identified highlights a dynamic shift from proteins associated with plant development to those linked with plant immune responses. We found that SGT1 interacts with Necrotic Spotted Lesion 1 (NSL1), which negatively regulates salicylic acid-mediated defense by interfering with the nucleocytoplasmic trafficking of non-expressor of pathogenesis-related genes 1 (NPR1) during N NLR-mediated response to tobacco mosaic virus. SGT1 promotes the SCF-dependent degradation of NSL1 to facilitate immune activation, while salicylate-induced protein kinase-mediated phosphorylation of SGT1 further potentiates this process. Besides N NLR, NSL1 also functions in several other NLR-mediated immunity. Collectively, our study unveils the regulatory landscape of SGT1 and reveals a novel SGT1-NSL1 signaling module that orchestrates plant innate immunity.

Tobacco mosaic virus hijacks its coat protein-interacting protein IP-L to inhibit NbCML30, a calmodulin-like protein, to enhance its infection

Calcium is an important plant immune signal that is essential for activating host resistance, but how RNA viruses manipulate calcium signals to promote their infections remains largely unknown. Here, we demonstrated that tobacco mosaic virus (TMV) coat protein (CP)-interacting protein L (IP-L) associates with calmodulin-like protein 30 (NbCML30) in the cytoplasm and nucleus, and can suppress its expression at the nucleic acid and protein levels. NbCML30, which lacks the EF-hand conserved domain and cannot bind to Ca²⁺ , was located in the cytoplasm and nucleus and was downregulated by TMV infection. NbCML30 silencing promoted TMV infection, while its overexpression inhibited TMV infection by activating Ca²⁺ -dependent oxidative stress in plants. NbCML30-mediated resistance to TMV mainly depends on IP-L regulation as the facilitation of TMV infection by silencing NbCML30 was canceled by co-silencing NbCML30 and IP-L. Overall, these findings indicate that in the absence of any reported silencing suppressor activity, TMV CP manipulates IP-L to inhibit NbCML30, influencing its Ca²⁺ -dependent role in the oxidative stress response. These results lay a theoretical foundation that will enable us to engineer tobacco (Nicotiana spp.) with improved TMV resistance in the future.

Palmitoylation of γb protein directs a dynamic switch between Barley stripe mosaic virus replication and movement

Viral replication and movement are intimately linked; however, the molecular mechanisms regulating the transition between replication and subsequent movement remain largely unknown. We previously demonstrated that the Barley stripe mosaic virus (BSMV) γb protein promotes viral replication and movement by interacting with the αa replicase and TGB1 movement proteins. Here, we found that γb is palmitoylated at Cys-10, Cys-19, and Cys-60 in Nicotiana benthamiana, which supports BSMV infection. Intriguingly, non-palmitoylated γb is anchored to chloroplast replication sites and enhances BSMV replication, whereas palmitoylated γb protein recruits TGB1 to the chloroplasts and forms viral replication-movement intermediate complexes. At the late stages of replication, γb interacts with NbPAT15 and NbPAT21 and is palmitoylated at the chloroplast periphery, thereby shifting viral replication to intracellular and intercellular movement. We also show that palmitoylated γb promotes virus cell-to-cell movement by interacting with NbREM1 to inhibit callose deposition at the plasmodesmata. Altogether, our experiments reveal a model whereby palmitoylation of γb directs a dynamic switch between BSMV replication and movement events during infection.

Barley stripe mosaic virus γb protein targets thioredoxin h-type 1 to dampen salicylic acid-mediated defenses

Salicylic acid (SA) acts as a signaling molecule to perceive and defend against pathogen infections. Accordingly, pathogens evolve versatile strategies to disrupt the SA-mediated signal transduction, and how plant viruses manipulate the SA-dependent defense responses requires further characterization. Here, we show that barley stripe mosaic virus (BSMV) infection activates the SA-mediated defense signaling pathway and upregulates the expression of Nicotiana benthamiana thioredoxin h-type 1 (NbTRXh1). The γb protein interacts directly with NbTRXh1 in vivo and in vitro. The overexpression of NbTRXh1, but not a reductase-defective mutant, impedes BSMV infection, whereas low NbTRXh1 expression level results in increased viral accumulation. Similar with its orthologs in Arabidopsis (Arabidopsis thaliana), NbTRXh1 also plays an essential role in SA signaling transduction in N. benthamiana. To counteract NbTRXh1-mediated defenses, the BSMV γb protein targets NbTRXh1 to dampen its reductase activity, thereby impairing downstream SA defense gene expression to optimize viral cell-to-cell movement. We also found that NbTRXh1-mediated resistance defends against lychnis ringspot virus, beet black scorch virus, and beet necrotic yellow vein virus. Taken together, our results reveal a role for the multifunctional γb protein in counteracting plant defense responses and an expanded broad-spectrum antibiotic role of the SA signaling pathway.