Low red/far-red ratios reduce Arabidopsis resistance to Botrytis cinerea and jasmonate responses via a COI1-JAZ10-dependent, salicylic acid-independent mechanism

Abstract

Light is an important modulator of plant immune responses. Here, we show that inactivation of the photoreceptor phytochrome B (phyB) by a low red/far-red ratio (R:FR), which is a signal of competition in plant canopies, down-regulates the expression of defense markers induced by the necrotrophic fungus Botrytis cinerea, including the genes that encode the transcription factor ETHYLENE RESPONSE FACTOR1 (ERF1) and the plant defensin PLANT DEFENSIN1.2 (PDF1.2). This effect of low R:FR correlated with a reduced sensitivity to jasmonate (JA), thus resembling the antagonistic effects of salicylic acid (SA) on JA responses. Low R:FR failed to depress PDF1.2 mRNA levels in a transgenic line in which PDF1.2 transcription was up-regulated by constitutive expression of ERF1 in a coronatine insensitive1 (coi1) mutant background (35S::ERF1/coi1). These results suggest that the low R:FR effect, in contrast to the SA effect, requires a functional SCFCOI1-JASMONATE ZIM-DOMAIN (JAZ) JA receptor module. Furthermore, the effect of low R:FR depressing the JA response was conserved in mutants impaired in SA signaling (sid2-1 and npr1-1). Plant exposure to low R:FR ratios and the phyB mutation markedly increased plant susceptibility to B. cinerea; the effect of low R:FR was (1) independent of the activation of the shade-avoidance syndrome, (2) conserved in the sid2-1 and npr1-1 mutants, and (3) absent in two RNA interference lines disrupted for the expression of the JAZ10 gene. Collectively, our results suggest that low R:FR ratios depress Arabidopsis (Arabidopsis thaliana) immune responses against necrotrophic microorganisms via a SA-independent mechanism that requires the JAZ10 transcriptional repressor and that this effect may increase plant susceptibility to fungal infection in dense canopies.

Figure 1.

Supplemental FR radiation, which mimics the effects of neighbor proximity on the light environment and phyB status, down-regulates the expression of Arabidopsis defense-related genes induced by B. cinerea. A, Effect of FR on the response of ERF1 to B. cinerea infection. B, Effect of FR on the response of PDF1.2 to B. cinerea infection. C, Effect of FR on the response of ORA59 to B. cinerea infection. D, Effect of FR on the response of HEL to B. cinerea infection. Expression levels were measured 24 h after inoculation of 4-week-old, soil-grown Arabidopsis plants with a 5-μL drop of B. cinerea spore suspension and are expressed relative to the healthy control under ambient light conditions. Amb, Ambient light; Bc, B. cinerea; FR, low R:FR. Error bars indicate se (n = 6 replicates). The significance of the FR-B. cinerea interaction term (FR*Bc) is shown in each panel; different letters indicate significant differences between treatment means.

Figure 2.

Phytochrome inactivation by FR radiation reduces the expression of JA response marker genes and the accumulation of leaf phenolics and anthocyanins. A, Interactive effects of MeJA and FR on the expression of ERF1. B, Interactive effects of MeJA and FR on the expression of PDF1.2. C, Interactive effects of MeJA and FR on the expression of ASA1. D, Interactive effects of MeJA and FR on the accumulation of soluble leaf phenolics. E, Interactive effects of MeJA and FR on anthocyanin accumulation. Response levels were measured 3 h (genes) or 72 h (leaf metabolites) after spraying 3-week-old, soil-grown Col-0 Arabidopsis plants with a 200 μm solution of MeJA and are given relative to the Col-0 control under ambient light conditions. Amb, Ambient light; FR, low R:FR; FW, fresh weight. Error bars indicate se (n = 3 replicates). Within each panel, different letters indicate significant differences between treatment means.

Figure 3.

The effect of FR down-regulating PDF1.2 responses to JA is conserved in the sid2-1 mutants. A, Interactive effects of MeJA and FR on the expression of PDF1.2 in Col-0 plants. B, Interactive effects of MeJA and FR on the expression of PDF1.2 in the sid2-1 mutant. Expression levels were measured 6 h after spraying 3-week-old, soil-grown Arabidopsis plants with a 200 μm solution of MeJA and are given relative to the Col-0 control under ambient light conditions. Amb, Ambient light; FR, low R:FR. Error bars indicate se (n = 4 replicates). The FR-MeJA interaction term (FR*MeJA) was statistically significant for all genotypes; within each panel, different letters indicate significant differences between treatment means.

Figure 4.

The effect of FR down-regulating PDF1.2 expression requires COI1. A, Interactive effects of MeJA and FR on the expression of PDF1.2 in Col-0 plants. B, Interactive effects of MeJA and FR on the expression of PDF1.2 in the 35S::ERF1-coi1.1 line. C, Interactive effects of MeJA and FR on the expression of PDF1.2 in the 35S::ERF1-COI1.1 line. Expression levels were measured 6 h after spraying 3-week-old, soil-grown Col-0, 35S::ERF1/coi1-1, and 35S::ERF1/COI1-1 Arabidopsis plants with a 200 μm solution of MeJA and are expressed relative to the Col-0 control under ambient light conditions. Amb, Ambient light; FR, low R:FR. Error bars indicate se (n = 3 replicates). When the FR-MeJA interaction term (FR*MeJA) was statistically significant, different letters indicate significant differences between treatment means.

Figure 5.

Phytochrome inactivation by a FR treatment that simulates the proximity of neighboring plants decreases Arabidopsis resistance to B. cinerea. A, FR effect in the Col-0 wild type. B, FR effect in the phyB mutant, which displays constitutive SAS morphology. C, FR effect in the sav3-2 mutant, which fails to induce the SAS morphology in response to supplemental FR. The bars indicate the frequency distribution of disease symptoms 2 d after inoculation of plants of comparable leaf sizes (see “Materials and Methods”). Disease rating is expressed as the fraction of leaves falling in the following classes: I, no visible disease symptoms; II, nonspreading lesion; III, spreading lesion with less than the 30% of the leaf area; IV, spreading lesion with more than 30% of the leaf area, with additional chlorosis and leaf collapse. Statistical analysis of the disease response is based on a χ² comparison between the light treatments for each genotype. For each genotype, asterisks indicate significant (*** P < 0.001) differences between light treatments; ns = not significant. Am, Ambient; FR, Far-red. [See online article for color version of this figure.]

Figure 6.

Inactivation of the cry1 photoreceptor (either by growing plants under PAR depleted in the blue component of the light spectrum or by the cry1 mutation) induces a SAS phenotype that resembles the effect of phyB inactivation on plant morphology but does not reduce plant resistance to B. cinerea. A, Effect of blue light depletion in the Col-0 wild type. B, Effect of blue light depletion in the cry1 mutant. Notice also that cry1 is as resistant as Col-0 under ambient light (P = 0.69). C, Constitutive expression of the SAS phenotype in phyB and cry1 plants. The bars indicate the frequency distribution of disease symptoms 3 d after inoculation of plants of comparable leaf sizes (for details, see Fig. 5). Am, Ambient light; -B, blue light attenuation. [See online article for color version of this figure.]

Figure 7.

The effect of phyB inactivation by FR radiation reducing Arabidopsis resistance to B. cinerea is functionally connected with the phyB effects on JA signaling. A, FR effect in the Col-0 wild type. B, FR effect in the jar1-1 mutant, which is deficient in the formation of the JA-Ile bioactive conjugate. The bars indicate the frequency distribution of disease symptoms 2 d after inoculation of plants of comparable leaf sizes (for details, see Fig. 5). *** P < 0.001; ns = not significant. Am, Ambient; FR, Far-red. [See online article for color version of this figure.]

Figure 8.

The effect of phyB inactivation by FR radiation on Arabidopsis resistance to B. cinerea is independent of SA signaling. A and C, FR effect in the Col-0 wild type. B, FR effect in the sid2-1 mutant, which is deficient in isochorismate synthase 1. D, FR effect in the npr1-1 mutant, which is deficient in SA signaling. The bars indicate the frequency distribution of disease symptoms 2 d after inoculation of plants of comparable leaf sizes (** P < 0.01, *** P < 0.001; for details, see Fig. 5). Am, Ambient; FR, Far-red. [See online article for color version of this figure.]

Figure 9.

The effect of phyB inactivation by FR radiation decreasing Arabidopsis resistance to B. cinerea requires JAZ10. A, FR effect in the Col-0 wild type. B, FR effect in two independent RNAi lines silenced for the expression of the JAZ10 gene. The bars indicate the frequency distribution of disease symptoms 2 d after inoculation of plants of comparable leaf sizes. Asterisks indicate significant (** P < 0.01) differences between light treatments; ns = not significant (for details, see Fig. 5). Am, Ambient; FR, Far-red. [See online article for color version of this figure.]