Ultraviolet Radiation as a Limiting Factor in Leaf Expansion and Development

Abstract

Reductions in leaf growth are a commonly observed response to ultraviolet radiation, but the underlying mechanisms remain poorly defined. This study examined the response of leaves exposed to a UV environment across a range of organizational scales, including leaf expansion rate, epidermal cell size and number, biomechanical properties, leaf-water relations and activity of cell-wall peroxidases. Two experimental approaches were used; Lettuce (Lactuca sativa L.) plants were propagated under (a) supplementary UV-B (9 kJ m )2 day )1) in controlled environment (CE) conditions, and (b) field conditions, where plants were placed under three horticultural films with differing UV transmissions. In both experiments, UV-B caused the greatest reductions in leaf expansion and final leaf size, with some reductions attributable to UV-A wavelengths. In supplementary UV-B conditions, adaxial cell size was reduced, while in field plants, both cell size and cell number were lower in an increased UV environment, as was the case with abaxial cells in CE plants. Although leaf turgor and leaf extensibility were not affected by UV wavelengths, breaking strain of leaf tissue was decreased under supplementary UV-B. Cell-wall peroxidase activity was increased in both supplementary UV conditions and in the field, where only a zero UV environment showed no upregulation of cell-wall peroxidase.

UV radiation reduces epidermal cell expansion inArabidopsis thalianaleaves without altering cellular microtubule organization

“…In more detail, the average cell area within the base, middle and top zones of the GFP-TUA6 leaf was systematically lower in UV-treated leaves from 8 days after UV-exposed and control plants of Lactuca sativa L. 7 and Pisum sativum 8 could be observed, reinforcing the idea that especially the modulation of cell wall properties is the main factor causing the observed UV-induced reduction in cell expansion. Some reports indicate differential expression of wall loosening enzymes like expansins or xyloglucan endotransglycosylase/hydrolases (XTHs), 9,10 or cell wall strengthening enzymes as particular peroxidases 7 after UV exposure. Another key event could involve UV-mediated changes in the phenylpropanoid pathway, which may cause changes in the lignin biosynthesis.…”

UV-B Inhibits Leaf Growth through Changes in Growth Regulating Factors and Gibberellin Levels

“…This decrease in the DEZ size correlates with a decrease in GA concentration by UV-B in this zone, which is regulated by the expression of GRF1 and possibly other transcription factors of the GRF family. UV-B also has been shown to inhibit cell expansion in several species (Gonzalez et al, 1998;Ruhland and Day, 2000;Ruhland et al, 2005;Wargent et al, 2009). In our experiments, the decrease in final leaf length and in leaf elongation rate in steady-state growth is not due to the reduction of mature cell size.…”

“…In this species, one of the most well-characterized physiological responses mediated by the UV-B photoreceptor UVR8 is the inhibition of hypocotyl growth (Kliebenstein et al, 2002). In addition, UVR8 controls leaf morphogenesis (Wargent et al, 2009). However, there are other UV-B-specific responses that occur independently of UVR8; some are related to UV-B-induced DNA damage, as the accumulation of photodimers can activate DNA damage response pathways that result in cell cycle arrest or programmed cell death in stem cells of the root apical meristem (Curtis and Hays, 2007;Furukawa et al, 2010).…”

“…Acute, stress-inducing UV-B conditions usually inhibit cell proliferation, while both cell proliferation and expansion are affected by lower doses and/or chronic UV-B treatments (Staxen and Bornman, 1994;Nogués et al, 1998;Laakso et al, 2000;Hopkins et al, 2002;Hofmann et al, 2003;Kakani et al, 2003;Hectors et al, 2007Hectors et al, , 2010Wargent et al, 2009;Robson and Aphalo, 2012). For example, the number of epidermal cells at maturity decreased after UV-B exposure in some species (Gonzalez et al, 1998;Hopkins et al, 2002); on the contrary, in lettuce (Lactuca sativa) leaves, UV-B reduced cell expansion rate and final leaf size (Wargent et al, 2009). In Arabidopsis, a number of UV-induced morphological changes have been described, including decreases in rosette diameter and inflorescence height (Hectors et al, 2007).…”

Repression of Growth Regulating Factors by the MicroRNA396 Inhibits Cell Proliferation by UV-B Radiation in Arabidopsis Leaves

“…However, available data about the effects of UV-B radiation on cell proliferation and cell expansion are contradictory, probably reflecting differences in experimental conditions. Acute, stress-inducing UV-B conditions cause necrosis and inhibit cell proliferation, while lower doses and/or chronic UV-B treatments affect both cell proliferation and expansion (Staxen and Bornman, 1994;Nogués et al, 1998;Laakso et al, 2000;Hopkins et al, 2002;Hofmann et al, 2003;Kakani et al, 2003;Hectors et al, 2007Hectors et al, , 2010Wargent et al, 2009a;Robson and Aphalo, 2012). For example, fewer epidermal cells were observed after UV-B exposure in some species (Gonzalez et al, 1998;Hopkins et al, 2002); however, stimulation of cell proliferation by UV-B radiation has also been reported in petunia (Petunia hybrida; Staxen and Bornman, 1994), and increases in leaf thickness attributed to increases in the number of parenchyma cells were reported in Brassica carinata and Medicago sativa (Bornman and Vogelmann, 1991).…”