| Summary: | <i>R. raciborskii</i> is known for growing under wide ranges of temperature and light. In temperate regions, however, low temperature and high light may serve as a stressful condition for invading tropical populations. The genetic basis of <i>R. raciborskii’s</i> adaptation to this combination of stresses are unknown. In this study, the growth rate and the expression of genes that may be crucial in the response to the chill/light stress of two <i>R. raciborskii</i> strains (differing in their climatic origin and toxicity) exposed to low temperature and high light were examined. Results showed that AMU-DH-30, a non CYN (cylindrospermopsin) producing strain isolated from the temperate region, exhibited under stress the upregulation of genes involved in the protein translation (<i>rbp1, nusG, hflX</i>), membrane fluidity (<i>desA</i>), photosynthetic activity (<i>ccr2</i> and <i>ftsH</i>), and the accumulation of compatible solutes (<i>asd</i>). In contrast, a CYN producing Australian strain CS-505 was not able to adapt quickly and to continue growth during stress conditions. Intriguingly, CS-505 and AMU-DH-30 had a similar ability to resume growth when the stress conditions subsided. Moreover, in strain CS-505 the <i>cyrB</i> gene was significantly upregulated under the stress conditions. The presented results shed new light on the possible mechanisms involved in the response of <i>R.</i><i>raciborskii</i> to chill/light stress.
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