Assessment of Phenylpropanoids in Response to UV

The Williams variety of soybean was used for assessment of individual phenylpropanoids. On Day 7 after planting in darkness, separate growth trays were individually irradiated with 300 nm, 317 nm, 368 nm or Untreated. After irradiation, seedlings were returned to darkness until harvest of the primary leaf material 24 hours later. An analysis of materials produced in response to UV is shown in Table 17.3. All chemicals listed on the left were standardized with purified compounds in order to properly identify metabolites. Data indicate that 300 nm and 317 nm (both in UV-B region) seem to suppress the development of several groups of protective pigments (i.e., anthocyanins, synapic acid, flavanones) and unexpectedly 300 nm does not induce visible synthesis of quercetin. Both 300 nm and 317 nm treatments reduced the production of Phe itself; the amino acid that is the precursor to the phenylpropanoid pigments. This may be evidence that young germinating seeds and young seedlings cannot synthesize new pigments due to suppression of Phe production. For many higher plants, the most abundant screening compound for broad UV is the structure quercetin, identified as the main inducible pigment for UV-protection in many plant species (Sheahan and Rechnitz, 1993; reviewed in Rozema et al., 2002). It is likely to be very important in the very young seedling due to the reserves of quercetin compounds in the Arabidopsis seed (reviewed in Lepiniec et al., 2006). We may not observe changes in quercetin if there are sufficient seed reserves for our brief treatments in seedlings treated with 300 nm. Quercetin is not only an efficient absorber of UV-B, but also has strong antioxidant capabilities, likely to assist the seedling in times of stress (reviewed in Winkel-Shirley, 2002). In plants where it has been examined, quercetin is the most efficient and prolific and abundant absorber of UV-B (Sheahan and Rechnitz, 1993; Rozema et al., 2002).

Table 17.3 Metabolome analysis of 8-day-old soybean etiolated seedling primary leaves treated with UV-B or UV-A radiation. Seedlings of Williams were grown for 7 days in complete darkness, treated with wavelengths shown (Untreated = mock pulse), then primary leaves harvested 24 h later to be ground in liquid nitrogen for metabolome analysis (see Methods). Malonyl dichloride is unchanged under different light/radiation regimes and is used herein as a negative control. + increase; x decrease; — no change compared to Untreated level. Untreated Plant Material = Value found in dark grown seedlings never exposed to UV, where the value is set to value 1.0

Table 17.3 Metabolome analysis of 8-day-old soybean etiolated seedling primary leaves treated with UV-B or UV-A radiation. Seedlings of Williams were grown for 7 days in complete darkness, treated with wavelengths shown (Untreated = mock pulse), then primary leaves harvested 24 h later to be ground in liquid nitrogen for metabolome analysis (see Methods). Malonyl dichloride is unchanged under different light/radiation regimes and is used herein as a negative control. + increase; x decrease; — no change compared to Untreated level. Untreated Plant Material = Value found in dark grown seedlings never exposed to UV, where the value is set to value 1.0

Metabolome compounds and substances

Dark

300 nm

317 nm

368 nm

Anthocyanins

1.0

x

-

+

p-coumaric Acid

1.0

-

+

+

3-Hydroxycinnamic Acid

1.0

+

+

+

Flavanone

1.0

x

-

-

Sinapic Acid

1.0

x

+

-

Quercetin

1.0

-

+

+

Malonyl Dichloride (control)

1.0

-

-

-

Phenylalanine

1.0

x

-

+

UV-B and UV-A wavelengths appear to induce or reduce the different phenylpropanoids possible in soybean (Table 17.3), which may indicate that UV-B and UV-A are using different signaling pathways to regulate pigment production. If there is significant damage to the developing plastids of the young plants, it is possible that appearance or disappearance of particular pigments occurs as an alarm response to deleterious effects of UV, like DNA alteration. Phe is directly responsible for making many pigments in dark-grown Arabidopsis, as evidenced by genetic and developmental studies (Warpeha et al., 2008). The same dependence on Phe may occur for soybean. However, currently under investigation, we do not know the exact quantity of pigments stored in soybean seed, nor do we have information on the exact Phe budget for the dormant seed as it converts to a germinating seed. Once these values are established, it will be easier to establish where and at what time soybean controls its pigment-synthesis and pigment-deployment responses.

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