Effects of light and UV-C radiation on the transcriptional activity of COP1 and HY5 gene homologues in barley.

Photomorphogenic regulators COP1 (Constitutive Photomorphogenic 1) and HY5 (Elongated Hypocotyl 5) play a key role in plant development by guiding the transition from dark to light growth. In Arabidopsis they are also implicated in the transcriptional control of photolyase genes. Here we characterize the transcript abundance of COP1 and HY5 gene homologues in barley in relation to light-grown conditions and UV-damage response. Etiolated and green 6-day-old seedlings were UV-C irradiated and exposed to light or kept in darkness. The abundance of barley COP1 and HY5 transcripts was assessed by real-time RT-PCR. In etiolated leaves we found several-fold lower levels of COP1 transcripts which reached the levels of the green ones after 1 h of light exposure. Barley HY5 transcripts were very low in the dark-grown seedlings and after 1 h of illumination they increased drastically to levels significantly exceeding those measured in the green leaves. Both genes were upregulated by light in the irradiated plants as well, but to a lesser extent compared with their controls, probably due to the presence of non-repaired DNA damage in the etiolated leaves soon after irradiation. The enhanced transcription of barley COP1 under light is unexpected in view of the well-known function of COP1 as a negative regulator of plant photomorphogenesis but conforms to the positive role reported for AtCOP1 in UV-B signalling. HY5 is recognized as a stimulator of light-inducible genes and our data support such a role for the barley HY5 homologue as well. Our study shows that, in barley seedlings, the regulation of COP1 and HY5 gene expression is achieved through light-positive transcriptional modulation, suggesting that both genes contribute to the de-etiolation phase in barley. According to our knowledge, this is the first quantitation of the COP1 and HY5 mRNAs in barley that also regards the UV-damage response of this crop.

The effect of chloramphenicol, actinomycin D and 5-bromouracil on the synthesis of photosynthetic pigments

The present study concerned the effect of chloramphenicol (100 μg/ml), actinomycin D (30 μg/ml), and 5-bromouracil (190 μg/ml) on the accumulation of chlorophyll α, chlorophyll b, β-carotene and four fractions of xanthophylls (with the domination of: lutein, zeaxanthin, violaxanthin and neoxanthin) in the primary bean leaves. The pigment content was determined in etiolated leaves after exposure to light for different lengths of time. It results from this study that chloramphenicol inhibits β-carotene synthesis more than do other pigments. The formation of xanthophylls and chlorophyll b is relatively less sensitive to the action of this antibiotic. Actinomycin D is also a somewhat more effective inhibitor of the accumulation of β-carotene than other pigments. In 5-bromouracil-treated leaves the accumulation of all carotenoids is inhibited almost to the same extent. These results suggest that the accumulation of chlorophyll b and xanthophylls is a little less dependent upon the activity of 70 S ribosomes in chloroplasts than the accumulation of chlorophyll α and β-carotene.

First Report of Xanthomonas translucens Causing Etiolation on Creeping Bentgrass Turf in Illinois, Kentucky, and North Carolina

Symptoms of etiolation, which is an abnormal elongation and yellowing of tillers, have been observed on creeping bentgrass [Agrostis stolonifera L. (CBG)] putting greens for decades; however, symptoms are typically transient and non-problematic. Reports of etiolation have become more frequent recently and research supports the involvement of bacteria (1). During stressful summer periods in 2011 and 2012, 62 CBG putting green samples were submitted to the NCSU Turf Clinic exhibiting symptoms of etiolation, chlorosis, and/or general decline. Microscopic examination of stem and leaf tissue often showed bacterial streaming from the xylem tissue. Symptomatic tissue was surface disinfested in sodium hypochlorite (10% Clorox) for 5 min, blotted dry, and rinsed in sterile dH2O. Disinfested tissue was placed in a small drop of sterile dH2O on a glass microscope slide and cut to allow bacteria to stream into the water for 2 min. The resulting bacterial suspension was streaked onto three nutrient agar (NA) plates and incubated at 30°C overnight. Bacterial colonies varied in morphology and those present in the greatest number based on morphology were re-streaked to isolate individual colonies. Bacterial isolates were tentatively identified to species using rDNA sequencing of 16S and ITS regions (3). Sequencing results showed isolates obtained from 6 locations (in Illinois, Kentucky, and North Carolina) having a positive match (≥99% 16S and ≥93% ITS) to Xanthomonas translucens (GenBank accessions AY572961, HM181927, JX976312, AY253329, and AB680445). Additional research is needed to confirm pathovar designation as X. translucens isolates were similar to both poae and graminis pathovars. A representative isolate (LW10-12A) was also examined for carbon source utilization using the BIOLOG 3rd Gen Microplate (Biolog Inc., Hayward, CA) resulting in a positive identification of X. translucens. Isolate LW10-12A was used to inoculate 6-week-old seeded creeping bentgrass cv. A1 plants maintained at 1 cm height in 3.5 cm diameter containers. Scissors were dipped in a cell suspension (~109 CFU ml−1 in sterile dH2O) and used to cut healthy CBG plants at 1 cm height and the remaining suspension was applied to the foliage until runoff using an atomizer bottle. Non-inoculated plants were cut and misted using sterile dH2O. After inoculation, plants were placed in a sealed clear plastic Camwear container (Cambro Co., Huntington Beach, CA) for 48 h and then transferred to the growth chamber bench (30°C) receiving irrigation twice daily with dH2O. Etiolation was rated within each of the four replicates by counting the number of etiolated leaves that were easily observed as significantly higher than the rest of the turf canopy. Plants inoculated with X. translucens exhibited etiolation of the youngest leaf within 48 h, whereas the non-inoculated plants did not. Symptoms were similar to observations in the field, as etiolated leaves were chlorotic and easily extracted from the turf surface. Microscopic examination showed bacterial streaming and identification of bacteria, using the previously described methods, was positive for X. translucens. Etiolation symptoms persisted over multiple weeks, but a decline in turf quality was not observed. Etiolation has been previously suggested as a precursor to bacterial wilt, caused by X. translucens pv. poae, on annual bluegrass [Poa annua L. f. reptans (Hausskn) T. Koyama] (2) and Acidovorax avenae has also been shown to produce etiolation on CBG (1). To our knowledge, this is the first confirmation of X. translucens as a cause of etiolation in CBG. References: (1) P. R. Giordano et al. Plant Dis. 96:1736, 2012. (2) N. A. Mitkowski et al. Plant Dis. 89:469, 2005. (3) N. W. Schaad et al. Lab. Guide for Ident. of Plant Path Bac., 2001.