The role of ABA in the freezing injury avoidance in two Hypericum species differing in frost tolerance and potential to synthesize hypericins
Cold stress is a major environmental factor that limits the distribution of plants and determines the spectrum and amount of secondary metabolites with a protective function. The most studied representative of the genus Hypericum, H. perforatum L. (St. John’s wort), is known as a producer of the photodynamic pigment hypericin, the unique bioactive compound structurally belonging to naphtodianthrones. In relation to the cosmopolitan distribution, we hypothesised that low temperature stress could increase the content of naphtodianthrones as a part of the adaptive mechanisms. Two strategies in preventing the freezing injury in the genus Hypericum were defined. Based on a frost-killing temperature (LT50) in untreated (control) plants and more than a 10 °C decrease in LT50 in cold-acclimated plants, we demonstrated the freezing tolerance for H. perforatum. On the contrary, the freezing avoidance was preferable in H. canariense—the species endemic to (sub)tropical Canary Islands and Madeira. The freezing tolerance/avoidance was related to the course of ABA accumulation/depletion in H. perforatum/H. canariense under a subfreezing temperature of −4 °C; however, the effect of dehydration or application of 76 μM ABA on the level of endogenous ABA was comparable. The 48-h exposure of H. perforatum control plants to −4 °C resulted in a 1.6-fold increase in the content of naphtodianthrones, along with the 1.5-fold increase of ABA. On the contrary, neither dehydration nor exogenous ABA stimulated the biosynthesis of these compounds. Our findings indicate possible integration of ABA signalling into naphtodianthrones biosynthesis under subfreezing conditions; this mechanism could be modified by plant tolerance to cold environments.
KeywordsNaphtodianthrones Abscisic acid Cold acclimation LT50 HPLC
This work was supported by the Slovak Research and Development Agency under contracts No. APVV 0040-10, SK-BG 0012-10, the Scientific Grant Agency of the Slovak Ministry of Education No. 1/142/11, the Czech Science Foundation (Nr. 14-34792S) and by the Ministry of Education, Youth and Sports of the Czech Republic (The National Program for Sustainability I, Nr. LO1204).
Conflict of interest
The authors declare that they have no conflict of interest.
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