Effector XopQ-induced stromule formation in Nicotiana benthamiana depends on ETI signaling components ADR1 and NRG1

In Nicotiana benthamiana, the expression of the Xanthomonas effector XANTHOMONAS OUTER PROTEIN Q (XopQ) triggers RECOGNITION OF XOPQ1 (ROQ1)-dependent effector-triggered immunity (ETI) responses accompanied by the accumulation of plastids around the nucleus and the formation of stromules. Both plastid clustering and stromules were proposed to contribute to ETI-related hypersensitive cell death and thereby to plant immunity. Whether these reactions are directly connected to ETI signaling events has not been tested. Here, we utilized transient expression experiments to determine whether XopQ-triggered plastid reactions are a result of XopQ perception by the immune receptor ROQ1 or a consequence of XopQ virulence activity. We found that N. benthamiana mutants lacking ROQ1, ENHANCED DISEASE SUSCEPTIBILITY 1, or the helper NUCLEOTIDE-BINDING LEUCINE-RICH REPEAT IMMUNE RECEPTORS (NLRs) N-REQUIRED GENE 1 (NRG1) and ACTIVATED DISEASE RESISTANCE GENE 1 (ADR1), fail to elicit XopQ-dependent host cell death and stromule formation. Mutants lacking only NRG1 lost XopQ-dependent cell death but retained some stromule induction that was abolished in the nrg1_adr1 double mutant. This analysis aligns XopQ-triggered stromules with the ETI signaling cascade but not to host programmed cell death. Furthermore, data reveal that XopQ-triggered plastid clustering is not strictly linked to stromule formation during ETI. Our data suggest that stromule formation, in contrast to chloroplast perinuclear dynamics, is an integral part of the N. benthamiana ETI response and that both NRG1 and ADR1 hNLRs play a role in this ETI response.

LC-MS Based Draft Map of the Arabidopsis thaliana Nuclear Proteome and Protein Import in Pattern Triggered Immunity

Despite its central role as the ark of genetic information and gene expression the plant nucleus is surprisingly understudied. We isolated nuclei from the Arabidopsis thaliana dark grown cell culture left untreated and treated with flg22 and nlp20, two elicitors of pattern triggered immunity (PTI) in plants, respectively. An liquid chromatography mass spectrometry (LC-MS) based discovery proteomics approach was used to measure the nuclear proteome fractions. An enrichment score based on the relative abundance of cytoplasmic, mitochondrial and Golgi markers in the nuclear protein fraction allowed us to curate the nuclear proteome producing high quality catalogs of around 3,000 nuclear proteins under untreated and both PTI conditions. The measurements also covered low abundant proteins including more than 100 transcription factors and transcriptional co-activators. We disclose a list of several hundred potentially dual targeted proteins including proteins not yet found before for further study. Protein import into the nucleus in plant immunity is known. Here we sought to gain a broader impression of this phenomenon employing our proteomics data and found 157 and 73 proteins to possibly be imported into the nucleus upon stimulus with flg22 and nlp20, respectively. Furthermore, the abundance of 93 proteins changed significantly in the nucleus following elicitation of immunity. These results suggest promiscuous ribosome assembly and a role of prohibitins and cytochrome C in the nucleus in PTI.

Shaping plastid stromules-principles of in vitro membrane tubulation applied in planta

Plastids undergo drastic shape changes under stress, including the formation of stroma-filled tubules, or `stromules'. Stromules are dynamic, and may extend, branch and retract within minutes. There are two prerequisites for stromule extension: excess plastid membrane and a force(s) that shapes the membrane into a tubule. In vitro studies provide insight into the basic molecular machinery for tubulation, and are often cited when discussing stromule formation. In this review, we evaluate in vitro modes of tubulation in the context of stromule dynamics, and find that most mechanisms fail to explain stromule morphology and behavior observed in planta. Current data support a model of stromule formation relying on pulling motors (myosins and kinesins) and cytoskeleton (actin and microtubules).

Induction of stromule formation by extracellular sucrose and glucose in epidermal leaf tissue of Arabidopsis thaliana

BACKGROUND

Stromules are dynamic tubular structures emerging from the surface of plastids that are filled with stroma. Despite considerable progress in understanding the importance of certain cytoskeleton elements and motor proteins for stromule maintenance, their function within the plant cell is still unknown. It has been suggested that stromules facilitate the exchange of metabolites and/or signals between plastids and other cell compartments by increasing the cytosolically exposed plastid surface area but experimental evidence for the involvement of stromules in metabolic processes is not available. The frequent occurrence of stromules in both sink tissues and heterotrophic cell cultures suggests that the presence of carbohydrates in the extracellular space is a possible trigger of stromule formation. We have examined this hypothesis with induction experiments using the upper epidermis from rosette leaves of Arabidopsis thaliana as a model system.

RESULTS

We found that the stromule frequency rises significantly if either sucrose or glucose is applied to the apoplast by vacuum infiltration. In contrast, neither fructose nor sorbitol or mannitol are capable of inducing stromule formation which rules out the hypothesis that stromule induction is merely the result of changes in the osmotic conditions. Stromule formation depends on translational activity in the cytosol, whereas protein synthesis within the plastids is not required. Lastly, stromule induction is not restricted to the plastids of the upper epidermis but is similarly observed also with chloroplasts of the palisade parenchyma.

CONCLUSIONS

The establishment of an experimental system allowing the reproducible induction of stromules by vacuum infiltration of leaf tissue provides a suitable tool for the systematic analysis of conditions and requirements leading to the formation of these dynamic organelle structures. The applicability of the approach is demonstrated here by analyzing the influence of apoplastic sugar solutions on stromule formation. We found that only a subset of sugars generated in the primary metabolism of plants induce stromule formation, which is furthermore dependent on cytosolic translational activity. This suggests regulation of stromule formation by sugar sensing mechanisms and a possible role of stromules in carbohydrate metabolism and metabolite exchange.

In vivo transport of folded EGFP by the DeltapH/TAT-dependent pathway in chloroplasts of Arabidopsis thaliana

Among the protein translocation pathways of the thylakoid membrane in chloroplasts, the DeltapH/TAT pathway is unique in several aspects. In vitro transport assays with isolated chloroplasts or thylakoids have defined the trans-thylakoidal proton gradient as the sole requirement for effecting transport. From these studies, evidence has also accumulated indicating that, in contrast to the remaining protein transport pathways present in the thylakoid membrane, the DeltapH/TAT pathway is able to mediate the transport of folded proteins. The present work has established a novel approach to demonstrate the transport of folded proteins by this pathway in vivo. For this purpose, Arabidopsis thaliana plants were stably transformed with gene constructs expressing enhanced green fluorescent protein (EGFP) alone or fused to the transit peptides of different chloroplast proteins under the control of the 35S CAMV promoter. The intracellular and intraorganellar distribution of EGFP in the resulting transformants showed that while all the chloroplast transit peptides efficiently mediated the transport of EGFP into plastids, only those specific for the DeltapH/TAT pathway were able to direct the protein into the thylakoid lumen as well. This could be demonstrated both by fluorescence and immunoelectron microscopy. Analysis of isolated and fractionated chloroplasts using western blot and spectrofluorometric assays confirmed the presence of folded EGFP solely within the thylakoid lumen of these lines. These results strongly suggest that the protein adopts a folded state in the chloroplast stroma and thus, can only be translocated further into the chloroplast lumen by the DeltapH/TAT pathway.