MISCELLANEOUS Methyl Jasmonate Counteract UV-B Stress in Barley Seedlings I. Fedina 1 , D. Nedeva 1 , K. Georgieva 1 & M. Velitchkova 2 1 Academic Metodi Popov Institute of Plant Physiology, Bulgarian Academy of Sciences, Sofia, Bulgaria 2 Institute of Biophysics, Bulgarian Academy of Sciences, Sofia, Bulgaria Introduction The responses of plants to abiotic stresses depend on many factors, but phytohormones are thought to be among the most important endogenous substances involved in the mechanisms of tolerance or susceptibility of various plant species (Nayyar and Chander 2004, Kumar et al. 2008). Jasmonic acid (JA) and its methyl ester (MeJA) are endogenous growth substances identified in many plant species, eliciting a wide variety of physio- logical and developmental responses (Staswick 1992, Sembdner and Parthier 1993, Kang et al. 2005). Jasmo- nates are one of the simplest non-traditional plant hormones with diverse roles and functions including a potential role in plant defense as part of the complex signaling pathways (Creelman and Mullet 1997). They modulate expression of numerous genes and influence specific aspects of plant growth, development and responses to environmental stresses. JA and MeJA accumulation in plants increased during biotic and abiotic stresses (Creelman and Mullet 1995, Mackerness et al. 2001). JA is derived from linolenic acid via octa- decanoid pathway (Reinbothe et al. 1994). Ultraviolet B (UV-B) irradiation results in the perturbation of plant membranes and/or the activation of lipases, which causes the release the signal-transduction pathway that activates stress inducible genes; DNA damages (Hidema et al. 2000) might be the initial signal that activates the octa- decanoid pathway (Vick and Zimmerman 1984, Hofmann et al. 2006). Thus, the response to UV-B radiation may be regulated by JA derived from linolenic acid through the octadecanoid pathway. The level of JA increased dramatically on UV-B exposure indicating a possible role for this compound in UV-B-mediated responses and for regulation of gene expression and subsequent resistance/ tolerance to UV-B (Mackerness et al. 1999). It has been shown that exogenous application of JA methyl ester (MeJA) improves the tolerance of plants to drought, Keywords antioxidant enzymes; chlorophyll fluorescence; flavonoids; methyl jasmonate; oxygen evolution; ultraviolet B radiation Correspondence I. Fedina Academic Metodi Popov Institute of Plant Physiology, Bulgarian Academy of Sciences, Academic Georgi Bonchev Street, Building 21, Sofia 1113, Bulgaria Tel.: +359 2 979 26 20 Fax: +359 2 873 99 52 Email: fedina@obzor.bio21.bas.bg Accepted December 17, 2008 doi:10.1111/j.1439-037X.2008.00358.x Abstract The role of exogenously applied phytohormone methyl jasmonate (MeJA) in counteracting the ultraviolet B (UV-B) stress in barley seedlings was investi- gated. Barley seedlings (Hordeum vulgare L., cv. Alfa) 4 days old were supplied with 5 · 10 )5 m MeJA through the roots for 3 days and then exposed for 2 days for 5 h per day to UV-B (312 nm, biological effectiveness of UV-B radi- ation 28.8 kJ m )2 day )1 ). The rate of 14 CO 2 fixation, PSI and PSII activities and chlorophyll content decreased, but flavonoids, H 2 O 2 , malondialdehyde, proline and UV-B induced compounds increased after UV-B treatment. The rate of photosynthetic oxygen evolution was more strongly inhibited by UV-B-irradia- tion than PSI and PSII efficiency. MeJA itself increased the content of free proline, which acts as a stress protector due to its radical scavenging ability. Increased superoxide dismutase, catalase and peroxidase (POX) activities in the leaves and in the roots and the POX isoforms induction revealed the MeJA involvement in plant tolerance to oxidative stress caused by UV-B irradiation. It was shown that pre-treatment with MeJA counteracted UV-B stress. There- fore, it was suggested that MeJA could acts as a mediator in plant defense responses to UV-B irradiation by enhancing the activity of antioxidant system and free radical scavenging capability of plant cells. J. Agronomy & Crop Science (2009) ISSN 0931-2250 204 ª 2009 Blackwell Verlag, 195 (2009) 204–212