The hnRNP-Q protein LIF2 participates in the plant immune response

Abstract

Eukaryotes have evolved complex defense pathways to combat invading pathogens. Here, we investigated the role of the Arabidopsis thaliana heterogeneous nuclear ribonucleoprotein (hnRNP-Q) LIF2 in the plant innate immune response. We show that LIF2 loss-of-function in A. thaliana leads to changes in the basal expression of the salicylic acid (SA)- and jasmonic acid (JA)- dependent defense marker genes PR1 and PDF1.2, respectively. Whereas the expression of genes involved in SA and JA biosynthesis and signaling was also affected in the lif2-1 mutant, no change in SA and JA hormonal contents was detected. In addition, the composition of glucosinolates, a class of defense-related secondary metabolites, was altered in the lif2-1 mutant in the absence of pathogen challenge. The lif2-1 mutant exhibited reduced susceptibility to the hemi-biotrophic pathogen Pseudomonas syringae and the necrotrophic ascomycete Botrytis cinerea. Furthermore, the lif2-1 sid2-2 double mutant was less susceptible than the wild type to P. syringae infection, suggesting that the lif2 response to pathogens was independent of SA accumulation. Together, our data suggest that lif2-1 exhibits a basal primed defense state, resulting from complex deregulation of gene expression, which leads to increased resistance to pathogens with various infection strategies. Therefore, LIF2 may function as a suppressor of cell-autonomous immunity. Similar to its human homolog, NSAP1/SYNCRIP, a trans-acting factor involved in both cellular processes and the viral life cycle, LIF2 may regulate the conflicting aspects of development and defense programs, suggesting that a conserved evolutionary trade-off between growth and defense pathways exists in eukaryotes.

Figure 1. JA- and SA-dependent signaling pathways in the lif2-1 mutant.

(A) Expression levels of genes involved in the JA-dependent signaling pathway (LOX3, AOS, AOC, OPR3, COI1, and JAR1) and of a JA-responsive marker gene (PDF1.2) in the rosette leaves of seven-week-old wild-type (WT) and lif2-1 plants. (B) Expression levels of genes of the SA-dependent signaling pathway (EDS1, PAD4, and ICS1) and of SA-responsive marker genes (NPR1 and PR1) in seven-week-old wild-type (WT) and lif2-1 rosette leaves. CC-NB-LRR and TIR-NB-LRR are disease resistance proteins with coiled-coil (CC), nucleotide-binding (NB), leucine-rich repeat (LRR), or Toll-Interleukin Receptor (TIR) domains. (C) Quantification of phytohormone contents in the rosette leaves of three-week-old plants using HPLC-electrospray-MS/MS. The amount of JA was expressed as a ratio of peak areas (209>62/214>62) per fresh weight (FW). The amount of other hormones was expressed in ng/FW. The bars represent standard deviation.

Figure 2. The lif2 mutants are less susceptible to P. syringae infection.

(A) Bacterial growth of the virulent DC3000 strain in wild-type (WT) and lif2-1 rosette leaves at 24 hours post-inoculation (hpi). (B) Rosette leaves imaged 5 days post-inoculation (dpi) with the virulent DC3000 strain. The lif2-1 and lif2-3 plants had similar responses, whereas the complemented lif2-1 mutant (lif2-c) behaved similarly to WT plants. Four independent experiments were performed with similar results. (C) Bacterial growth of the avirulent DC3000 avrRpm1 strain in rosette leaves at 24 hpi. For bacterial growth experiments, each data point represents the mean value from at least thirty leaves. Similar results were obtained in two independent experiments. The bars represent standard deviation. (Student's t-test, * p<0,05).

Figure 3. Expression of genes involved in the JA- and SA-dependent signaling pathways in response to P. syringae inoculation.

Seven-week-old rosette leaves of wild-type (WT) and lif2-1 mutant plants were inoculated with buffer (mock) or with the bacterial pathogen DC3000. Leaves were collected at 24 hpi and gene expression was assessed by quantitative RT-PCR. The expression of genes involved in (A) JA- and (B) SA-dependent signaling pathways is shown. WT mock (white), WT DC3000 (light grey), lif2-1 mock (grey), and lif2-1 DC3000 (black).

Figure 4. The response of the lif2-1 sid2-2 double mutant to P. syringae inoculation.

(A) Seven-week-old plants grown in short-day conditions. Scale bar, 1 cm. (B) DC3000 bacterial growth at 24 hpi. Each inoculated leaf was ground in MgCl₂ and the bacterial suspension was then diluted and plated on solid medium. Twenty-four leaves were analysed per genotype. Stars indicate a significant difference from wild-type (WT) plants (Mann and Whitney, * p<0.05). Experiments were repeated twice and gave similar results. (C) The relative expression of PR1 in untreated rosette leaves of seven-week-old plants.

Figure 5. LIF2 is involved in the plant defense response to S. sclerotiorum.

Leaves of four-week-old plants were inoculated with S. sclerotiorum strain S55. The lif2 alleles and the complemented lif2-c line in the Col-0 background were analysed. A. thaliana Col-0, Rubezhnoe-1 (Rbz-1) (more resistant than Col-0), and Shahdara (Sha) (more susceptible than Col-0) accessions were used as controls. (A) Symptoms at 7 dpi. (B) The disease score was evaluated for each line at 7 dpi. Means and standard deviations were based on at least twenty plants per line. A significant difference in susceptibility relative to that of the Col-0 accession is indicated with an asterisk (Kruskal and Wallis's test, * p<0.05).

Figure 6. LIF2 is involved in the plant's susceptibility to B. cinerea.

(A-C) Six-week-old plants were infected with a mycelium plug of the virulent B. cinerea B0510 (A-B) and BD90 (C) strains. (A) Symptoms at 3 dpi with the B0510 strain. (B-C) Lesion diameters were measured at 1 to 3 dpi. Stars indicate a significant difference from wild-type leaves on the corresponding day (Mann and Whitney's test, with a p-value of <0.05). (D) Expression of marker genes involved in the defense response to B. cinerea.

Figure 7. Glucosinolate (GLS) contents in lif2-1 and lhp1-1 young seedlings.

The GLSs were quantified in lif2-1 (A) and lhp1-1 (B) using ESI-HPLC-MS. The full names of the GLSs are listed in Table S1.

Figure 8. Stress-response genes deregulated in the lif2 transcriptome.

(A) The distribution of deregulated TFs in lif2. (B) Deregulated genes belonging to the GO term “Negative regulators of defence response”. The log ratio and the p-value were extracted from our CATMA transcriptome data. (C) Hierarchical clustering analysis performed using the MultiExperiment Viewer application, the Pearson correlation as current metric, and complete linkage clustering as the linkage method. The expression profiles of 38 deregulated TFs and 6 negative regulators in different biotic conditions were used in this analysis. Three gene clusters (I to III) were identified.

Figure 9. Model for the functions of the hnRNP-Q LIF2 protein.

LIF2 may regulate the balance between development and plant immunity by minimizing the energy cost of plant defense.