Abstract
Main conclusionEpigenetic memory affects the timing of bud burst phenology and the expression of bud burst-related genes in genetically identical Norway spruce epitypes in a manner usually associated with ecotypes.In Norway spruce, a temperature-dependent epigenetic memory established during embryogenesis affects the timing of bud burst and bud set in a reproducible and predictable manner. We hypothesize that the clinal variation in these phenological traits, which is associated with adaptation to growth under frost-free conditions, has an epigenetic component. In Norway spruce, dehydrins (DHNs) have been associated with extreme frost tolerance. DHN transcript levels decrease gradually prior to flushing, a time when trees are highly sensitive to frost. Furthermore, EARLY BUD BREAK 1 genes (EBB1) and the FT-TFL1-LIKE 2-gene (PaFTL2) were previously suggested to be implied in control of bud phenology. Here we report an analysis of transcript levels of 12 DHNs, 3 EBB1 genes and FTL2 in epitypes of the same genotype generated at different epitype-inducing temperatures, before and during spring bud burst. Earlier flushing of epitypes originating from embryos developed at 18 °C as compared to 28 °C, was associated with differential expression of these genes between epitypes and between buds and last year’s needles. The majority of these genes showed significantly different expressions between epitypes in at least one time point. The general trend in DHN expression pattern in buds showed the expected reduction in transcript levels when approaching flushing, whereas, surprisingly, transcript levels peaked later in needles, mainly at the moment of bud burst. Collectively, our results demonstrate that the epigenetic memory of temperature during embryogenesis affects bud burst phenology and expression of the bud burst-related DHN, EBB1 and FTL2 genes in genetically identical Norway spruce epitypes.
Highlights
Changes in global climate have been considered to represent a significant challenge for sufficiently rapid adaptation of species subjected to strong seasonal environmental stresses
Main conclusion Epigenetic memory affects the timing of bud burst phenology and the expression of bud burstrelated genes in genetically identical Norway spruce epitypes in a manner usually associated with ecotypes
Timing of bud burst differs between genetically identical Norway spruce epitypes The epitypes generated from one single genotype examined in this work, were generated by somatic embryogenesis at two epitype-inducing temperatures (Fig. 1) as shown by Kvaalen and Johnsen (2008)
Summary
Changes in global climate have been considered to represent a significant challenge for sufficiently rapid adaptation of species subjected to strong seasonal environmental stresses. Plants can cope with these stressful conditions and Planta (2017) 246:553–566 become more resistant to future exposures through an epigenetic memory mechanism. Epigenetic mechanisms that generate or remove epigenetic marks play an important role in plasticity responses to the environment and contribute to stress memory and adaptation in plants (Sahu et al 2013; Thellier and Luttge 2013; Baulcombe and Dean 2014; Avramova 2015; Crisp et al 2016). Epigenetic mechanisms could modulate the development, morphology and physiology of an organism, contributing to an adaptive capacity of species such as forest trees (Brautigam et al 2013; Pikaard and Mittelsten Scheid 2014). Stress-induced epigenetic modifications are reversible but can be mitotically and meiotically transmitted in the form of heritable epialleles (Iwasaki and Paszkowski 2014)
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