Inhibition of hypocotyl elongation by ultraviolet‐B radiation in de‐etiolating tomato seedlings. II. Time‐course, comparison with flavonoid responses and adaptive significance

Abstract

S, 1995, Inhihition of hypocotyl ekmgatioB by ukraviolel-B radiation in de-eliolating tomato seedlings, 11. Time-course, comparison with flavonoid responses and adaptive significance. -Physiol. Plant. 93: 591-601.UV-B radiation inhibits hypocotyl elongation in etiolated tomato {Lycopersicon escuientum Mill. cv. Alisa Craig) seedlings acting through a photoreceptor system with peak apparent effectiveness around 300 nm. In order to further characterize the response and gain insight into its potential ecological significance, the time-course of inhibition was measured and compared with the time-course of flavonoid accumulation in the same seedlings, W^hen a background of strong (> 620 jjmol m--^ s-') white light (WL) was supplemented v^ith low irradiance UV-B (~ 3 pmol m-s-'), suhstantial (5 0%) inhihiti(m of elongation occurred within 3 h of the light treatment. The magnitude of UV-B-induced elongation inhibition was similar in wild type (WT) and uu-mutant seedlings, in spite of the large differences between genotypes in rate and temporal pattern of elongation. In comparison to the effect of UV-B on elongation, induction of llavsmoid accumulation in WT and au seedlings undergoing de-etiolation was a much slower response. Several UV'-ahsorbing compounds appeared to he specifically induced by light, and some of them accumulated faster under the WL -f UV-B treatment than under WL alone. However, there was litlie or no detectable effect of WL on flavonoid levels until up to 3 h of treatment, and the specific L'V-B effect was measurable only after 6 h of continuous treatment. Indeed. UV-B-screening properties of crude alcoholic extracts were not different between WL and WL + UV-B treatments until after 9 or 24 h. When the light treatments were applied to seedlings thai v, ere jus( breaking through the soil surface. UV-B v\as found to consistently retard seedling emergence. These results suggest that the rapid inhibition of elongation in de-etiolating seedlings is an evolved response lo UV-B. which ma\ ser\'e to minimize seedling exposure lo sunlight until protective pigmentation responses (triggered by WL and UV-B) have taken place in the seedlings" epidermis.

An unidentified ultraviolet-B-specific photoreceptor mediates transcriptional activation of the cyclobutane pyrimidine dimer photolyase gene in plants

“…For example, little UVB reaches the ground in the morning or late in the afternoon when the visible light is abundant. This lack of correlation has often been mentioned as an ecological justiWcation for the evolution of a UVBspeciWc receptor system (Ballare et al 1995b). It is interesting that plants increase the CPD photolyase activity in response to long-wavelength UVB (300-315 nm) rather than the DNA-damaging short-wavelength UVB ( < 300 nm).…”

Research advances on the biological effects of elevated ultraviolet-B radiation on terrestrial plants

“…Plants have developed natural adaptations such as anatomical, morphological and biochemical changes, which help to reduce UV-B penetration into plant tissues, and to alleviate the detrimental effect of UV-B radiation. It has been reported (Ballaré et al 1995) that the inhibition of hypocotyl elongation and retardation of plumular hook opening in emerging seedlings increased amount of UV-B-absorbing compounds in the epidermis, thereby reducing the damaging impact of UV-B when the seedling eventually is exposed to full sunlight. Similarly, the reduced leaf area expansion and increased leaf thickness induced by elevated UV-B radiation may lengthen the optical path between the leaf epidermis and sensitive cellular sites in the mesophyll.…”

Photomorphogenic responses to UV radiation IV: a comparative study of UVB effects on growth and pigment accumulation in etiolated and de-etiolated hypocotyls of wild-type andaureamutant of tomatoLycopersicon esculentumMill)

“…Therefore the idea proposed by BALLARE et al (1995) that "accumulation of UV -absorbing flavonoids in tomato is not dependent on the availability of phytochrome type I (phyA)" appears rather unjustified. In contrast the same LPS light irradiation affected the accumulation of UV -absorbing compounds induced by UV radiation in a similar way in the au mutant and wild type tomato.…”

“…Numerous UV -absorbing compounds have been proposed as potential photo receptors in UV -induced photomorphogenesis: DNA, phytochromes, flavins and pterins, aromatic residues of proteins, auction (see BALLARE et al, 1995, for the relevant literature). Consequently the protective mechanisms activated by plants to avoid UVinduced damages are still far from being clarified (STAPLETON, 1992).…”