Molecular signatures in the induction of UV protection by chlorogenic acid accumulation in sunflower leaves

Phenolic compounds act as UV-screening compounds in higher plants and their accumulation is induced not only by UV-B radiation but also by
other environmental factors such as high photon flux density (PFD) or low temperature. While in most plants these compounds belong to both
groups, flavonoids and hydroxycinnamic acid derivatives (HCAs), sunflower (Helianthus annuus L.) leaves contain only compounds of the latter
group, mainly chlorogenic acid (CGA) derivatives. In order to better understand the specific signalling pathways that dynamically induce CGA
accumulation while avoiding the induction of flavonoids, we exposed young sunflower plants to sudden changes in environmental
conditions by
transferring them from low PFD, 21°C, to either high PFD (350 μmol m-2 s-1) or low temperature (9°C) or UV-B radiation (50 mW m-2).
After transfer, epidermal UV-A absorbance, indicating the formation of CGA derivatives, increased already after 24 h in all conditions, correlating
well with the results of HPLC analyses. Six h (except for low temperature), 12 h, 24 h and 72 h after the transfer, samples were taken from the
youngest mature leaf and RNA was extracted for RNAseq analysis. Especially the first time points after the transfer (6 h and 12 h) showed a
higher number of differentially expressed genes in all three environmental conditions compared to the initial time point before the transfer, whereas
later time points (24 h and 72 h) came closer to the initial pattern of gene expression. At 12 h and 24 h after transfer, 302 and 50 differentially
expressed genes, respectively, were shared by all three environmental conditions. However, the differential expression of all these genes was
in the same direction for exposure to high PFD and low temperature, whereas there was a surprisingly high discrepancy between induction by
UV-B and high PFD. 183 of the 302 genes differentially regulated after 12 h were downregulated by UV-B radiation but upregulated by high
PFD. When focusing on hydroxycinnamoyl quinate transferases (HQT), which are central enzymes in the biosynthesis of CGA, HQT2 and
HQT3 were highly expressed under all conditions. The nature of the differentially expressed genes will be further analysed to provide a deeper
understanding of the molecular mechanisms underlying the UV protection mechanism in sunflower based on accumulation of hydroxycinnamic
acid conjugates.
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