Resistance-related physiological response of rice leaves to the compound stress of enhanced UV-B radiation and Magnaporthe oryzae

Abstract

An enhanced UV-B radiation (5.0 kJ m −2) was supplied before, during, and after Magnaporthe oryzae infection. The effects of single and compound stress of the UV-B radiation and M. oryzae on the resistance physiology and gene expression of rice leaves were examined. Results revealed that UV-B radiation given before M. oryzae infection (UV-B → M.) significantly increased the pathogenesisrelated proteins (PRs) activities of phenylalanine ammonialyase (PAL), lipoxygenase (LOX), chitinase (CHT), and β-1,3-glucanase, the resistance-related substances (flavonoids and total phenols) content, and resistance-related genes (OsPAL and OsCHT) expression, thereby improving the disease resistance of rice leaves. Simultaneous exposure to UV-B radiation and M. oryzae (UV-B/M.) significantly increased the OsLOX2 expression and the PRs activities. Exposure to UV-B radiation after M. oryzae infection (M. → UV-B) decreased the flavonoid content, did not improve the PRs activity, and increased OsLOX2 expression. Compound treatments of UV-B → M., UV-B/M., and M. → UV-B reduced the disease index by 62.3%, 40.2%, and 26.6%, respectively, indicating UV-B radiation inhibited the occurrence of M. oryzae disease, but its inhibitory effect weakened when it was provided after M. oryzae infection. Hence, rice responded to the compound stress of UV-B radiation and M. oryzae through a resistance-related physiological mechanism associated with the sequence of stress occurrence.

UV-B light and its application potential to reduce disease and pest incidence in crops

“…Concomitant, plants pre-treated with UV-B light before the inoculation showed significantly higher activity of PAL and CHT, a higher expression of OsPAL and OsCHT and a higher flavonoid and total phenol content compared to the UV-B treatment during and after inoculation. LOX activity was highest for the UV-B treatment during inoculation and ß−1,3-glucanase activity was similarly increased for the pre-treatment with UV-B and the irradiance during inoculation compared to UV-B irradiation after inoculation 114 . The positive effect of UV-B priming has also been demonstrated in broccoli sprouts, where UV-B (1 kJ m −2 ; 0.056 W m −2 ; 5 h; 280–360 nm) pre-treated plants were more resistant against aphids ( M. persicae) and caterpillars ( Pieris brassicae) compared to untreated plants 141 .…”

“…For example, LOX (lipoxygenase, part of the jasmonic acid biosynthesis pathway) activity increases in rice, and the jasmonic acid-responsive genes PR-4 ( PATHOGENESIS-RELATED PROTEIN 4 ) and BG3 ( BETA-1,3-GLUCANASE 3 ) are induced in broccoli after UV-B irradiation 113 , 114 , 141 . Moreover, UVR8 seems to be a key regulator of genes involved in JA biosynthesis ( AOS ( ALLENE OXIDE SYNTHASE), AOC1 ( ALLENE OXIDE CYCLASE1), AOC3 (ALLENE OXIDE CYCLASE3) and OXOPHYTODIENOATE REDUCTASE3 ), and signaling (transcription factor WKRY70 and JAZ1 ( JASMONATE ZIM DOMAIN1 )) in Arabidopsis under solar UV-B 149 .…”

“…UV-B light can induce pathogenesis-related proteins in various plants 46 , 158 , 159 . These compounds include e.g., OLPs (OSMOTIN-LIKE PROTEIN), chitinases, PDF1.2 (PLANT DEFENSIN 1.2) and PR-1, -2, -4 and, -5 (PATHOGENESIS-RELATED PROTEIN) 17 , 101 , 111 , 114 , 158 , 160 . For instance, OLP, a PR protein with antifungal activity, is induced in response to UV-B radiation in rice and ß-1.3-glucanase (PR-2) is triggered by UV-B in both rice and strawberry 114 , 160 and also in response to the pathogenic fungus Magnaporthe oryzae , causing rice blast 114 .…”

“…These compounds include e.g., OLPs (OSMOTIN-LIKE PROTEIN), chitinases, PDF1.2 (PLANT DEFENSIN 1.2) and PR-1, -2, -4 and, -5 (PATHOGENESIS-RELATED PROTEIN) 17 , 101 , 111 , 114 , 158 , 160 . For instance, OLP, a PR protein with antifungal activity, is induced in response to UV-B radiation in rice and ß-1.3-glucanase (PR-2) is triggered by UV-B in both rice and strawberry 114 , 160 and also in response to the pathogenic fungus Magnaporthe oryzae , causing rice blast 114 . However, OLP expression seems to be higher induced after a consecutive amount of days of UV-B radiation, while ß-1,3-glucanase is only highly expressed in the first day of UV-B radiation in strawberry 160 .…”

“…Meaning, we have few clues about how long the UV-B effect persists to reduce disease susceptibility and we cannot rule out an additional effect of UV-B treatment on the pathogen or pest. For instance, glasshouse grown rice (‘Baijiaolaojing’) was irradiated (5 kJ m −2 ; 0.198 W m −2 ; 7 h; 280–320 nm) 3 days prior, during, and 3 days after inoculation with the fungus M. oryzae 114 . All treatments resulted in a significantly increased resistance to the rice blast fungus compared to the untreated control.…”