Journal of Experimental Botany, Vol. 55, No. 396, pp. 507-516, February 1, 2004
© 2004 Oxford University Press
Plants and the Environment |
Photoperiod and temperature differentially regulate the expression of two dehydrin genes during overwintering of birch (Betula pubescens Ehrh.)
Received 8 April 2003; Accepted 21 October 2003
1 Department of Biosciences, Division of Genetics, and Institute of Biotechnology, University of Helsinki, PO Box 56, FIN-00014 Helsinki, Finland
2 Department of Plant Sciences, Agricultural University of Norway, PO Box 5022, N-1432 Ås, Norway
3 Finnish Forest Research Institute, Vantaa Research Centre, PO Box 18, FIN-01301 Vantaa, Finland
4 Finnish Forest Research Institute, Punkaharju Research Station, Finlandiantie 18, FIN-58450 Punkaharju, Finland
* To whom correspondence should be addressed. Fax: +358 9 19159079. E-mail: tapio.palva{at}helsinki.fi
Abbreviations: ABA, abscisic acid; ANOVA, analysis of variance; DHN, dehydrin protein; DHN, dehydrin gene; DW, dry weight; FW, fresh weight; HSD, honestly significant difference; LEA, late embryogenesis abundant; LD, long daylength; LT, low temperature; LT50, temperature of 50% lethality; SD, short daylength; WS, water stress.
The overwintering of trees in northern areas depends on processes regulated by photoperiod and temperature. To identify the physiological and genetic factors involved in this environmental control, three latitudinal ecotypes of pubescent birch (Betula pubescens Ehrh.) growing in a common garden experiment were used. Each ecotype responded to the shortening of the photoperiod according to its specific critical daylength, resulting in the induction of freezing tolerance and dehydration of buds first in the northern ecotype, followed by the central and southern ecotypes, respectively. By contrast, there was no clear difference in the timing of dormancy release, bud rehydration, and deacclimation in the spring, suggesting that these traits were controlled mainly by temperature. To elucidate the role of dehydrins (DHN) in the overwintering process, two DHN genomic clones were isolated from pubescent birch and expression of the corresponding genes, both in field and under controlled conditions, was characterized. BpuDhn1 was found to encode an YnKn-type of basic DHN, while BpuDhn2 encoded an acidic, SKn-type of DHN. In field-grown trees the level of BpuDhn1 increased in buds during the autumn, while the level of BpuDhn2 was highest during the coldest winter months. Under controlled conditions BpuDhn1 increased in response to the combined effect of short daylength and low, non-freezing temperatures whereas the expression of BpuDhn2 was mainly controlled by low temperature while photoperiod had less effect on its expression. These results suggest that DHNs participate in the sensitive environmental regulation of the overwintering process in birch.
Key words: Birch, dehydrins, dormancy, ecotype, freezing tolerance.
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