ORS1, an H₂O₂-responsive NAC transcription factor, controls senescence in Arabidopsis thaliana

Abstract

We report here that ORS1, a previously uncharacterized member of the NAC transcription factor family, controls leaf senescence in Arabidopsis thaliana. Overexpression of ORS1 accelerates senescence in transgenic plants, whereas its inhibition delays it. Genes acting downstream of ORS1 were identified by global expression analysis using transgenic plants producing dexamethasone-inducible ORS1-GR fusion protein. Of the 42 up-regulated genes, 30 (~70%) were previously shown to be up-regulated during age-dependent senescence. We also observed that 32 (~76%) of the ORS1-dependent genes were induced by long-term (4 d), but not short-term (6 h) salinity stress (150 mM NaCl). Furthermore, expression of 16 and 24 genes, respectively, was induced after 1 and 5 h of treatment with hydrogen peroxide (H₂O₂), a reactive oxygen species known to accumulate during salinity stress. ORS1 itself was found to be rapidly and strongly induced by H₂O₂ treatment in both leaves and roots. Using in vitro binding site selection, we determined the preferred binding motif of ORS1 and found it to be present in half of the ORS1-dependent genes. ORS1 is a paralog of ORE1/ANAC092/AtNAC2, a previously reported regulator of leaf senescence. Phylogenetic footprinting revealed evolutionary conservation of the ORS1 and ORE1 promoter sequences in different Brassicaceae species, indicating strong positive selection acting on both genes. We conclude that ORS1, similarly to ORE1, triggers expression of senescence-associated genes through a regulatory network that may involve cross-talk with salt- and H₂O₂-dependent signaling pathways.

Figure 1.

Delayed Senescence in ors1-1 Mutant and ORS1 RNAi Lines. (A) T-DNA is inserted in exon III of ORS1. (B) Absence of ORS1 transcript (arrow) in ors1-1 mutant plants, shown by RT–PCR with primers annealing to the start and stop regions of the coding segment. Forward (F) and reverse (R) primer positions are indicated by arrows in (A). WT, wild-type (Col-0); M, molecular size marker. (C) Ors1-1 mutant showing delayed senescence, 55 d after sowing (DAS) at long-day conditions and 120 DAS at short-day conditions. (D) Chlorophyll content of the five biggest leaves from plants shown in (C). (E) SAG12 expression in mutants and wild-type plants shown in (C) determined by qRT–PCR. Note that one cycle difference in the qRT–PCR corresponds to a two-fold difference in gene expression. (F) ORS1 expression in the five biggest leaves of early- (Lip-0 and Col-0) and late- (N13) senescent accessions at different days after sowing (DAS), determined by qRT–PCR. The Y-axis indicates 40-ΔC_t, where ΔC_t is equal to C_{t gene_of_interest} – C_{t reference_gene_UBQ10}. Data are means of three independent experiments ± SD. Differences in expression levels are significant for all comparisons between N13 and Lip-0 or Col-0, respectively (Student's t-test, p < 0.05) with the following exception: 30 DAS, N13 vs. Col-0. (G) Delayed senescence in ORS1 RNAi line, 40 DAS.

Figure 2.

ORS1 Overexpression Plants. (A) Northern blot analysis of plants transformed with the 35S:ORS1 construct. Numbers indicate individual transformants. C, control (untransformed) plant. Blots were hybridized with ³²P-labeled ORS1 cDNA probe. (B) Early senescence in ORS1 overexpression lines at 35 DAS compared to an empty vector (EV) control plant. (C) Chlorophyll content of the first six leaves of 35S:ORS1 plants in comparison to EV lines. (D) Percentage of survived leaves. (E) Ion leakage. (F) SAG12 expression in 35S:ORS1 and EV lines, determined by qRT–PCR. Data in (C)–(F) were obtained from plants at 35 DAS; means ± SD of at least three replicates. Plants were grown under long-day conditions (16 h/8 h, light/dark). (G) Chlorophyll content in rosette leaves of ors1-1, ORS1–RNAi, EV, and 35S:ORS1 lines. Leaves were detached from 5-week-old plants, placed on moist filter paper in Petri dishes, and kept in the dark for 4 d. Means ± SD of three replicates. Chlorophyll levels in ors1-1 versus EV and 35S:ORS1 lines were significantly different (Student’s t-test, p < 0.05).

Figure 3.

ORS1 Promoter-Driven GUS Expression. (A) Arabidopsis. (a) GUS expression in 10-day-old seedling. (b) Leaves of a 25-day-old seedling. Note GUS staining in the leaf tip. (c) GUS staining in young and old leaves from a soil-grown ∼5-week-old plant. (d) Immature flower. (e) Mature flower with GUS staining in stamens and the floral abscission zone (arrow). (f) Roots. (g) Anther and mature silique. (B) Tobacco. (a) Leaf with GUS staining in the tip region. (b) Immature flower. (c) Mature flower with GUS staining in petals. (d) Young (left) and mature (right) stamens. (e) Root. Incubation time in GUS staining solution was less than 1 h in all cases.

Figure 4.

H₂O₂-Dependent ORS1 Expression. GUS activity in 2-week-old Arabidopsis seedlings transformed with Prom_ORS1:GUS and Prom_ORS1del: GUS constructs, treated for 1 h with 10 mM H₂O₂ compared to control. (A) Histochemical assay. GUS staining was performed for 30 min. (B) 4-Methylumbelliferyl-beta-D-glucuronide (4-MUG) assay. MUG activity is given as relative value, where the activity of H₂O₂-treated plants was compared to control condition.

Figure 5.

Conserved Non-Coding Sequences in Promoters of ORS1 and ORE1 Orthologs. Promoter sequences were obtained from Arabidopsis thaliana, Arabidopsis arenosa, Capsella rubella, Raphanus sativus, and Brassica oleracea. (A) ORE1 orthologous promoters. Minimal motif size (MMS): 10; maximal number of mutations accepted within the motifs (MNM): 0. Fully conserved sequences in ORE1 promoters are shown above the alignments. Boxes shown in color indicate known cis-acting elements (>4 bp long) deposited in the PLACE database. 1, W box, wounding, salicylic acid and stress response element; 2, GT-1 binding site, light-regulated transcription; 3, WRKY binding site; 4, heat shock response element; 5, HD-Zip protein binding element. (B) ORE1 orthologous promoters. MMS: 12; MNM: 2. (C) ORS1 orthologous promoters. MMS: 10; MNM: 0. Fully conserved sequences in ORS1 promoters are indicated. Boxes shown in color indicate known cis-acting elements (>4 bp long). 6, ASF-1 binding site, transcriptional activation by auxin and/or salicylic acid; 7, ATMYB2 binding site, water stress response element; 8 and 3, WRKY binding site; 9, W box, wounding, salicylic acid and stress response element; 10, cis-elements for ethylene and circadian regulation. (D) ORS1 orthologous promoters. MMS: 12; MNM: 2. Gray lines connect identical conserved non-coding sequences; blocks shown in green highlight coding sequences (CDS) of ORS1 and ORE1 orthologs.

Figure 6.

Deletion of Conserved Non-Coding Sequences in the ORS1 Promoter. (A) Representation of highly conserved sequences in the proximal part of the ORS1 promoter using WebLogo software (http://weblogo.berkeley.edu/). (B) ORS1-204 promoter-driven GUS expression. Note the almost complete absence of expression in senescent leaves and the abscission zone of mature flowers (encircled). GUS activity in senescent leaves is still driven by the –230-bp promoter deletion harboring CNS1 (shown on the left) and the flower abscission zone (not shown).