Interplay among Nitric Oxide and UVR8 in plants exposed to UV-B radiation

FERNÁNDEZ, MARÍA BELÉN* ; LATORRE, LUCAS; CASSIA, RAUL

Congreso; 19th Congress of the European Society for Photobiology; 2021

The ultraviolet- B (UV-B) resistance locus 8 (UVR8) is the only known UV-B photoreceptor in plants1 and is conserved from Chlorophytes to angiosperms2. After UV-B irradiation, UVR8 turns from an inactive homodimer to an active monomer regulating the expression of several genes involved in tolerance and acclimation3. Nitric oxide (NO) is an UV-B downstream gaseous signal that increases in response to UV-B and attenuates its damaging effects on plants4. The aim of this work was to analyse the interplay between NO and UVR8 in the UVR8 signalling pathway of plants exposed to UV-B. To this end, we first analysed the expression of NIA1 (nitrate reductase 1, a NO enzymatic source) in three-week-old control and UV-B irradiated (3.47 m−2 s−1, for 2h) Arabidopsis Col 0 (WT) and uvr8-1 null mutant plants using RT-qPCR. Results obtained show that NIA1 was upregulated after UV-B radiation in WT plants, however, this response was impaired in mutants. This indicates that expression of NIA1 after UV-B treatment depends on the UVR8 signalling pathway, and thus, NO increase through this enzyme may be UVR8 dependent. We also analysed if the expression of PHR1 (cyclobutane pirimidine dimers photolyase involved in DNA repair), IAA19 (an auxin responsive gene) and UVR8 are regulated by UV-B and UVR8. RT-qPCR results demonstrated that PHR1 was upregulated whereas IAA19 and UVR8 were downregulated after UV-B treatment, and all responses were abolished in mutants. This confirms that regulation of these genes occurs through the UVR8- dependent pathway. IAA19 expression is repressed under UV-B due to MYB77, BIM1and BES1 transcription factors (TFs) sequestration by UVR85. In order to determine if similar regulation occurs for the UVR8 gene, in silico analysis of the predicted UVR8 promoter using PlantPAN database revealed the presence of binding motifs for these TFs, supporting its negative regulation after UV-B. We also analysed possible UV-B and NO co-regulation of the UVR8 signalling pathway components, using publicly available microarray and RNAseq databases from Arabidopsis plants treated with NO. This analysis revealed co- regulation by NO and UV-B of several genes involved in this pathway. We found that CHS, IAA2 and GA2OX (GA2-oxidase) are upregulated in Arabidopsis leaves both by UV-B and exogenous application of NO. On the other hand, other genes were negatively co- regulated by UV-B and NO, such as the growth promoting genes PRE1 (paclobutrazol resistance 1) and EXP1 (expansin b1). Finally, prediction analysis of S-nitrosylated proteins from the UVR8 signalling pathway and cysteine conservation using multiple sequence alignment revealed that COP1, PHR1, IAA14 and NIA1 could be targets of this post-translational modification. In conclusion, we report an interplay between UVR8 and NO in the UVR8 signalling pathway as: 1) NO increment depends on UVR8 2) and NO regulates the components from the UVR8 signalling pathway at the gene transcription and post-translational levels under UV-B irradiation.