Convergent responses to stress. Solar ultraviolet-B radiation and Manduca sexta herbivory elicit overlapping transcriptional responses in field-grown plants of Nicotiana longiflora

Abstract

The effects of solar ultraviolet (UV)-B (280-315 nm) on plants have been studied intensively over the last 2 decades in connection with research on the biological impacts of stratospheric ozone depletion. However, the molecular mechanisms that mediate plant responses to solar (ambient) UV-B and their interactions with response mechanisms activated by other stressors remain for the most part unclear. Using a microarray enriched in wound- and insect-responsive sequences, we examined expression responses of 241 genes to ambient UV-B in field-grown plants of Nicotiana longiflora Cav. Approximately 20% of the sequences represented on the array showed differential expression in response to solar UV-B. The expression responses to UV-B had parallels with those elicited by simulated Manduca sexta herbivory. The most obvious similarities were: (a) down-regulation of several photosynthesis-related genes, and (b) up-regulation of genes involved in fatty acid metabolism and oxylipin biosynthesis such as HPL (hydroperoxide lyase), alpha-DIOX (alpha-dioxygenase), LOX (13-lipoxygenase), and AOS (allene oxide synthase). Genes encoding a WRKY transcription factor, a ferredoxin-dependent glutamate-synthase, and several other insect-responsive genes of unknown function were also similarly regulated by UV-B and insect herbivory treatments. Our results suggest that UV-B and caterpillar herbivory activate common regulatory elements and provide a platform for understanding the mechanisms of UV-B impacts on insect herbivory that have been documented in recent field studies.

Figure 1.

Effect of chronic exposure to ambient UV-B on the abundance of transcripts of insectresponsive genes in field-grown N. longiflora plants. Plants were grown in the field for 3 weeks in replicated plots under filters that were fully transparent to the solar spectrum (nearambient UV-B treatment) or selectively attenuated the UV-B component of sunlight (attenuated UV-B treatment; see “Materials and Methods” for details). The expression ratio (ER) indicates transcript abundance in the nearambient UV-B treatment relative the attenuated UV-B treatment. The arrayed genes were clustered into 13 functional groups according to the predicted function of the gene product. For a list of UV-B-responsive genes, see Table I. The complete list of the responses of all arrayed genes is available as Supplemental Table I (http://www.plantphysiol.org).

Figure 2.

Comparison between the effects of solar UV-B in N. longiflora and simulated ambient UV-B in N. attenuata on the expression of insect-responsive genes. The data for N. longiflora were obtained from Figure 1. The data for N. attenuata were obtained in a completely independent greenhouse experiment under simulated-ambient UV-B (see “Materials and Methods” for details). The graph shows the responses to UV-B in N. attenuata of all those genes that were significantly affected by UV-B in N. longiflora: photosynthesis-related genes (♦), N metabolism (▪), carbohydrate and cell wall metabolism (×), membrane transport (*), stress response and signaling (•), oxylipin synthesis (▵), PI (○), pathogen-responsive (▾), M. sexta-responsive genes of unknown function (□), and regurgitate-responsive genes of unknown function (▴). The complete list of the responses of all arrayed genes is available as Supplemental Table I (http://www.plantphysiol.org).

Figure 3.

Comparison between the effects of solar UV-B and a simulated caterpillar treatment in N. longiflora (for details, see “Materials and Methods”). The graph shows the responses to the simulated caterpillar treatment (wounding + M. sexta regurgitate relative to the intact control, both under attenuated UV-B) of genes that were significantly affected by UV-B in field-grown plants. A, Responses in functional categories 1 to 10: photosynthesis-related genes (♦), N metabolism (▪), carbohydrate and cell wall metabolism (XET; ×), membrane transport (*), stress response and signaling (WRKY-2; •), oxylipin synthesis (▵), PI (○), and pathogen responsive (PR1; ▾). B, Functional category 11, i.e. M. sexta-regulated genes of unknown function (□). C, Functional category 12, i.e. regurgitate-responsive genes of unknown function (▴). The complete list of the responses of all arrayed genes is available as Supplemental Table I (http://www.plantphysiol.org).

Figure 4.

Effects of simulated ambient UV-B (greenhouse experiment) on the kinetics of PI activity changes in response to a simulated caterpillar treatment (wounding + M. sexta regurgitate) applied at time = 0 to N. longiflora and N. attenuata plants. PI activity levels in the non-wounded controls were always below 0.008 mg per milligram of protein and are not shown for clarity. A significant UV-B effect on PI levels (in N. attenuata) is denoted by asterisks (P < 0.05); bars indicate ±1 se (n = 3 plants per sampling). N. longiflora produced very low PI levels, and there was a trend for reduced PI activities in UV-B-exposed plants (P = 0.3 at time = 48 h). The white and dashed segments below the x axis represent day and night, respectively.

Figure 5.

Effects of simulated ambient UV-B (greenhouse experiment) on the growth (weight gain) of M. sexta caterpillars. The experiment had a duration of 1 week. Bars indicate ±1 se (n = 10 [N. attenuata] and 8 [N. longiflora] replicate plants, each with two caterpillars).