James R. Shinkle

Comparative Photobiology of Growth Responses to Two UV-B Wavebands and UV-C in Dim-red-light- and White-light-grown Cucumber (Cucumis sativus) Seedlings: Physiological Evidence for Photoreactivation†

Abstract We examined the influence of short-term exposures of different UV wavebands on the elongation and phototropic curvature of hypocotyls of cucumbers (Cucumis sativus L.) grown in white light (WL) and dim red light (DRL). We evaluated (1) whether different wavebands within the ultraviolet B (UV-B) region elicit different responses; (2) the hypocotyl elongation response elicited by ultraviolet C (UV-C); (3) whether irradiation with blue light–enriched white light (B/WL) given simultaneous with UV-B treatments reversed the effect of UV in a manner indicative of photoreactivation; and (4) whether responses in WL-grown plants were similar to those grown in DRL. Responses to brief (1–100 min) irradiations with three different UV wavebands all induced inhibition of elongation measured after 24 h. When WL-grown seedlings were irradiated with light containing proportionally greater short wavelength UV-B (37% of UV-B between 280 and 300 nm), inhibition of hypocotyl elongation was induced at a threshold of 0.5 kJ m−2, whereas exposure to UV-B including only wavelengths longer than 290 nm (and only 8% of UV-B between 290 and 300 nm) induced inhibition of hypocotyl elongation at a threshold of 1.6 kJ m−2. The UV-C treatment induced reduction in elongation at a threshold of <0.01 kJ m−2 for DRL-grown plants and <0.03 kJ m−2 for WL-grown plants. B/WL caused 50% reversal of the short-wavelength UV-B–induced inhibition of elongation in DRL-grown seedlings but did not reverse the effect of long-wavelength UV-B. B/WL caused 30% reversal of the UV-C–induced inhibition of elongation in WL-grown seedlings but did not affect the response to short-wavelength UV-B. Short-wavelength UV-B also induced positive phototropic curvature in both types of seedlings, and this was reversed 60% or completely in DRL-grown and WL-grown seedlings, respectively. The similarity of responses between the etiolated (DRL-grown) and de-etiolated (WL-grown) seedlings indicates that the short-wavelength specific response may be relevant to natural light environments, and the apparent photoreactivation implicates DNA damage as the sensory mechanism for the response.

Phytochrome Regulation of Plant Development at the Whole Plant, Physiological, and Molecular Levels

Of the many environmental factors which play a role in the survival, growth, and reproduction of green plants, light is among the most crucial. Not only does it drive the process of photosynthesis, through which its energy is transduced through the photosynthetic pigments into usable chemical form, but it also provides vital environmental information which can affect seed germination, leaf growth, stem growth, flowering, and a host of other processes. Such effects of light, in which it provides an environmental cue to trigger a given response, rather than providing a direct energy source for the response itself, collectively define the field of photomorphogenesis. An excellent series of review articles on photomorphogenesis was recently edited by Shropshire and Mohr (1983).

Photomorphogenic regulation of increases in UV-absorbing pigments in cucumber (Cucumis sativus) and Arabidopsis thaliana seedlings induced by different UV-B and UV-C wavebands.

Brief (1-100 min) irradiations with three different ultraviolet-B (UV-B) and ultraviolet-C (UV-C) wave bands induced increases the UV-absorbing pigments extracted from cucumber (Cucumis sativus L.) and Arabidopsis. Spectra of methanol/1% HCl extracts from cucumber hypocotyl segments spanning 250-400 nm showed a single defined peak at 317 nm. When seedlings were irradiated with 5 kJ m(-2) UV-B radiation containing proportionally greater short wavelength UV-B (37% of UV-B between 280 and 300 nm; full-spectrum UV-B, FS-UVB), tissue extracts taken 24 h after irradiation showed an overall increase in absorption (91% increase at 317 nm) with a second defined peak at 263 nm. Irradiation with 1.1 kJ m(-2) UV-C (254 nm) caused similar changes. In contrast, seedlings irradiated with 5 kJ m(-2) UV-B including only wavelengths longer than 290 nm (8% of UV-B between 290 and 300 nm; long-wavelength UV-B, LW-UVB) resulted only in a general increase in absorption (80% at 317 nm). The increases in absorption were detectable as early as 3 h after irradiation with FS-UVB and UV-C, while the response to LW-UVB was first detectable at 6 h after irradiation. In extracts from whole Arabidopsis seedlings, 5 kJ m(-2) LW-UVB caused only a 20% increase in total absorption. Irradiation with 5 kJ m(-2) FS-UVB caused the appearance of a new peak at 270 nm and a concomitant increase in absorption of 72%. The induction of this new peak was observed in seedlings carrying the fah1 mutation which disrupts the pathway for sinapate synthesis. The results are in agreement with previously published data on stem elongation indicating the existence of two response pathways within the UV-B, one operating at longer wavelengths (>300 nm) and another specifically activated by short wavelength UV-B (<300 nm and also by UV-C).

Evidence of physiological phototropin1 (phot1) action in response to UV-C illumination

Stem growth kinetics were measured in cucumber (Cucumis sativus) and Arabidopsis thaliana using highly-sensitive monitoring with 5-minute resolution, in darkness and in response to a short, single pulse of UV-C illumination. The results show that UV-C, like blue light, induces a rapid decrease in seedling growth rate. The fluence-response kinetics and time course were similar to the phototropin1 mediated response observed following a blue pulse. Arabidopsis seedlings were used to assess the genetic mechanism of this response. The phot1 mutant exhibited defects in stem growth rate inhibition, with sustained growth inhibition completely absent following specific treatments. The cryptochrome and phytochrome mutants exhibited responses comparable to wild type, suggesting that these receptor classes do not contribute to this response. The work demonstrates in two species that UV-C has an effect on a rapid plant photomorphogenic response and that the response is partially mediated by the phot1 photoreceptor.

Adapting a Photochemical Reactor to the Study of UV Ecology in Vineyard Yeast

In the vineyard, many genera of yeast can be present on the grapes, but they are eventually outcompeted by Saccharomyces yeast as fermentation progresses. A selective pressure that has the potential to affect the composition of the non-Saccharomyces community is UV light, particularly in places with very high levels of UV-B (280 to 315 nm) radiation, like New Zealand. Understanding this ecology could conceivably be very important because of growing evidence that non-Saccharomyces yeast present on grapes in vineyards can play a role in the fermentation process and terroir. Thus, understanding how UV light can affect yeast community composition represents a step toward understanding how specific taste signatures derived from certain microbial communities are produced. To fully understand these processes, overall UV sensitivity to wavelengths encountered in the vineyard would need to be characterized for each member of the non-Saccharomyces vineyard yeast community. Problematically, traditional UV-sensitivity assays have used instruments that only emit at 254 nm (UV-C), a wavelength filtered out by the ozone layer in natural environments. Here, we present a method that allows experimental determination of UV-B sensitivity in yeast (and presumably, sensitivity to several other wavelengths) that uses a Rayonet RPR-100 photochemical reactor. This method outperforms traditional methods of irradiation in both ecological relevance and tight control of wavelength and flux. With our protocol, better understanding of the ecological processes that drive community structure in vineyards, and therefore also microbial terroir, can be achieved.