Jasmonate-dependent and -independent pathways mediate specific effects of solar ultraviolet B radiation on leaf phenolics and antiherbivore defense

Abstract

Ultraviolet B (UV-B) radiation, a very small fraction of the daylight spectrum, elicits changes in plant secondary metabolism that have large effects on plant-insect interactions. The signal transduction pathways that mediate these specific effects of solar UV-B are not known. We examined the role of jasmonate signaling by measuring responses to UV-B in wild-type and transgenic jasmonate-deficient Nicotiana attenuata plants in which a lipoxygenase gene (NaLOX3) was silenced (as-lox). In wild-type plants, UV-B failed to elicit the accumulation of jasmonic acid (JA) or the bioactive JA-isoleucine conjugate but amplified the response of jasmonate-inducible genes, such as trypsin proteinase inhibitor (TPI), to wounding and methyl jasmonate, and increased the accumulation of several phenylpropanoid derivatives. Some of these phenolic responses (accumulation of caffeoyl-polyamine conjugates) were completely lacking in as-lox plants, whereas others (accumulation of rutin and chlorogenic acid) were similar in both genotypes. In open field conditions, as-lox plants received more insect damage than wild-type plants, as expected, but the dramatic increase in resistance to herbivory elicited by UV-B exposure, which was highly significant in wild-type plants, did not occur in as-lox plants. We conclude that solar UV-B (1) uses jasmonate-dependent and -independent pathways in the elicitation of phenolic compounds, and (2) increases sensitivity to jasmonates, leading to enhanced expression of wound-response genes (TPI). The lack of UV-B-induced antiherbivore protection in as-lox plants suggests that jasmonate signaling plays a central role in the mechanisms by which solar UV-B increases resistance to insect herbivores in the field.

Figure 1.

Effects of UV-B and simulated herbivory on the accumulation of soluble phenolic compounds in N. attenuata plants grown in the glasshouse. The experimental treatments resulted from a factorial combination of UV-B and simulated herbivory: C, natural daylight; UV, natural daylight supplemented with UV-B radiation; H, natural daylight plus simulated herbivory (wounds treated with S. frugiperda regurgitate); UVH, UV radiation plus simulated herbivory (for details, see “Materials and Methods”). Leaf samples were taken 72 h after the H treatment. Each bar represents the mean + se (n = 6 individual plants). G, Genotype; WT, wild type. P values for significant interactions are shown (three-way ANOVA; for the full ANOVA table and Tukey comparisons for interaction terms, see Supplemental Table S1).

Figure 2.

Effects of solar UV-B on RF_UV, which is used as an indicator of epidermal transmittance to UV radiation. Plants were grown in the field under near-ambient (UV) or attenuated (C) UV-B radiation using stretch or clear polyester filters, respectively. Each bar represents the mean + se (n = 5; each biological replicate is a pool of three individual plants). G, Genotype; WT, wild type. P values for the relevant terms are shown (two-way ANOVA; for the full ANOVA table, see Supplemental Table S2).

Figure 3.

Effects of UV-B and simulated herbivory on the accumulation of individual phenolic compounds in N. attenuata plants grown in the glasshouse. The experimental treatments resulted from a factorial combination of UV-B and simulated herbivory: C, natural daylight; UV, natural daylight supplemented with UV-B radiation; H, natural daylight plus simulated herbivory (wounds treated with S. frugiperda regurgitate); UVH, UV radiation plus simulated herbivory (for details, see “Materials and Methods”). Leaf samples were taken 72 h after the H treatment. Each bar represents the mean + se (n = 6 individual plants). G, Genotype; WT, wild type. P values for the relevant terms of the factorial model are shown (for the full ANOVA results and Tukey comparisons, see Supplemental Table S3).

Figure 4.

Effects of UV-B and simulated herbivory on TPI gene expression in N. attenuata plants grown in the glasshouse. The experimental treatments resulted from a factorial combination of UV-B and simulated herbivory: C, natural daylight; UV, natural daylight supplemented with UV-B radiation; H, natural daylight plus simulated herbivory (wounds treated with S. frugiperda regurgitate); UVH, UV radiation plus simulated herbivory (for details, see “Materials and Methods”). TPI gene expression was measured by qPCR 24 h after the H treatment. Each bar represents the mean + se (n = 3; each biological replicate is a pool of three individual plants). WT, Wild type. The P value for the UV × H interaction term is indicated. Different letters indicate significant differences between means (P < 0.05, Tukey test). For the full ANOVA results and Tukey comparisons, see Supplemental Table S4.

Figure 5.

Effects of UV-B and simulated herbivory on jasmonate accumulation in N. attenuata plants grown in the glasshouse. The experimental treatments resulted from a factorial combination of UV-B and simulated herbivory: C, natural daylight; UV, natural daylight supplemented with UV-B radiation; H, natural daylight plus simulated herbivory (wounds treated with S. frugiperda regurgitate); UVH, UV radiation plus simulated herbivory (for details, see “Materials and Methods”). Samples for jasmonate determinations were obtained 1 and 2 h after the H treatment. Each bar represents the mean + se (n = 3; each biological replicate is a pool of 10 individual plants). P values for the relevant terms of the model are shown (for the full ANOVA results, see Supplemental Table S5).

Figure 6.

Effects of UV-B on plant sensitivity to jasmonate. The experimental treatments resulted from a factorial combination of UV-B and MeJA applications: C, natural daylight; UV, natural daylight supplemented with UV-B radiation (for details, see “Materials and Methods”). Samples for qPCR analysis were obtained 24 h after MeJA application. Expression data are normalized to the expression level detected in the control × 0 μm MeJA combination. Thin bars indicate ±1 se (n = 3; each biological replicate is a pool of three individual plants). The P value for the UV × MeJA interaction is indicated (for the full ANOVA results, see Supplemental Table S6).

Figure 7.

UV-B exposure in the field reduces natural herbivory in wild-type plants but not in as-lox plants. Plants were grown in the field under near-ambient (UV) or attenuated (C) UV-B radiation using stretch or clear polyester filters, respectively. Leaf damage in the field was predominantly caused by thrips (T. tabaci and Frankliniella spp.) and is expressed as a percentage of total leaf area. Each bar represents the mean + se (n = 4 independent blocks of three plants each). WT, Wild type. Different letters indicate significant differences between means (P < 0.05, Tukey test). For the full ANOVA results and Tukey comparisons, see Supplemental Table S7.

Figure 8.

Model of the proposed interaction between UV-B and jasmonate signaling in the induction of plant defenses. UV-B induces accumulation of flavonoids through a mechanism that does not require jasmonate synthesis and is not activated in response to herbivory or jasmonate treatment. In contrast, UV-B induces the accumulation of polyamine (PA) adducts via a mechanism that is completely dependent on LOX3 activity and is also activated by MeJA. UV-B does not promote TPI gene expression, which is induced by jasmonate and herbivory, but greatly enhances the response to herbivory by increasing plant sensitivity to jasmonate. The jasmonate-dependent effects of solar UV-B on plant defense are functionally significant, determining the antiherbivore effects of UV-B radiation in the field. Solid arrows indicate synthesis or transformation, and dotted arrows indicate controls. TF, Transcription factors.