Abiotic stress factors – cold, frost, drought, salinity – severely limit plant growth and development as well as the final yield in crops including cereals from the tribe Triticeae. Low temperatures, drought, and salinity represent stress factors associated with plant cell dehydration. Dehydration stress factors induce profound cellular response aimed at an elimination of water loss. Plant cell response to dehydration includes an accumulation of osmotically active compounds including hydrophilic proteins such as dehydrins. Several physiological studies focused on plant stress response have reported a positive relationship between the level of accumulation of dehydrin transcripts or proteins and plant stress tolerance (Kosová et al., 2010). More tolerant cultivars or genotypes usually reveal a higher level of dehydrin transcripts or proteins than the less tolerant ones although the relationship between plant stress tolerance and dehydrin transcript or protein accumulation is not always obvious (linear) due to a complex nature of plant stress tolerance mechanisms.

The aim of the study lies in a summary of the research focused on dehydrin transcript and protein accumulation in wheat and barley plants exposed to various abiotic stresses with a dehydrative component – cold, drought, salinity. We have tried to summarize recent results gained on cultivated common wheat (Triticum aestivum), durum wheat (T. durum) and barley (Hordeum vulgare) exposed to cold, frost, drought, and salt stress factors.

There has already been published a complete annotation of barley genome sequence (The International Barley Genome Sequencing Consortium, 2012) while common wheat (T. aestivum) complete genome sequencing is still in progress (). Barley (H. vulgare) represents the relatively most tolerant cultivated Triticeae species with respect to dehydrative stresses, especially drought and salinity (Colmer et al., 2006). There have been 13 dehydrin genes Dhn1 to Dhn13 identified in the barley genome (Tommasini et al., 2008). Common wheat genome is much larger than in barley due to its allohexaploid nature resulting in the presence of orthologs and also paralogs (Sarhan et al., 1997; Danyluk et al., 1998). Recently, Wang et al. (2014) have identified 54 dehydrin unigenes in common wheat genome by a search of wheat EST database.

Dehydrin structural types and expression patterns: both barley and wheat contain four (K_n, SK_n, Y_nSK_m, K_nS) out of five dehydrin structural types (Close, 1997) while they lack dehydrins of Y_nK_m type (Table 1). The largest group of dehydrins in barley (10 out of 13 Dhn genes) as well as in common wheat belong to Y_nSK_m type which encompass basic dehydrins induced by strong dehydration stresses (drought, salt, frost) as well as by abscisic acid (ABA) due to an occurrence of several ABRE elements in their promoters (Choi et al., 1999). Some of Y_nSK_m type dehydrins are embryo-specific such as barley Dhn12 (Choi and Close, 2000). K_n type dehydrins DHN5 in barley and several K_n type dehydrins (WCS120 family as well as low-molecular K_n type dehydrins such as WDHN13) in common wheat are induced by both cold and drought as well as by ABA (Choi et al., 1999; Tommasini et al., 2008; Wang et al., 2014). There has also been described an induction of barley Dhn5 by moderate levels of salicylic acid (SA) up to 0.25 mM while an inhibition by SA levels higher than 0.4 mM (Sun et al., 2009). For K_n type dehydrins with multiple copies of K-segment such as barley DHN5 (K₉) and wheat WCS120 (K₆), a cryoprotective activity has been reported (Houde et al., 1995; Bravo et al., 2003) similarly to Y_nK_m type dehydrins such as peach PCA60 (Wisniewski et al., 1999). Acidic SK₃ dehydrins include DHN8 in barley and WCOR410 family (WCOR410a,b,c) in common wheat and are induced predominantly by cold (Danyluk et al., 1994, 1998; Choi et al., 1999). Wheat SK_n type dehydrins can be induced by dehydration, ABA, methyl jasmonate (MeJA), gibberellins (GA), and SA (Zhu et al., 2014). KS type dehydrins include DHN13 in barley and at least one ortholog in wheat. KS type dehydrins seem to represent a specific group of small dehydrin proteins induced by chilling exclusively in reproductive tissues such as anthers (Rodriguez et al., 2005; Wang et al., 2014). Phylogenetic analysis of protein sequences of 13 Dhn genes in barley has revealed a high sequence similarity between the 10 dehydrin proteins of Y_nSK_m type (DHN1,2,3,4,6,7,9,10,11,12) while the remaining three dehydrin proteins DHN5 (K₉), DHN8 (SK₃), and DHN13 (KS) revealed significant differences with respect to Y_nSK_m type dehydrins as well as each other (Karami et al., 2013). DHN13 seems to be most distant with respect to the other barley dehydrin proteins both at protein sequence and expression levels (Karami et al., 2013).

Dehydrins are intracellular proteins which accumulate to high levels in cell cytoplasm but they can also be found in several organelles. Dehydrins containing a stretch of serine residues, the S-segment, can be regulated via phosphorylation of the S-segment. Phosphorylated S-segment has been reported to function as a nuclear localization signal (NLS) for DHN5 (YSK₂) in durum wheat (Brini et al., 2007a). However, nuclear localization has also been reported for dehydrin proteins lacking the S-segment such as WCS120 in cold-treated wheat crown tissue (Houde et al., 1995). At tissue and organ level, comparison of vegetative organs (leaves, crowns, roots) revealed the relatively highest dehydrin protein accumulation in crowns followed by leaves and relatively lowest accumulation in roots when expressed per unit amount of organ fresh weight in salt-treated barley plants (Kosová et al., 2013a). Similarly, Houde et al. (1995) have also found the relatively highest accumulation of WCS120 proteins in wheat crown tissue upon cold since crowns represent the crucial organ for the whole plant survival. At tissue level, immunoanalyses have revealed an enhanced accumulation of dehydrins in xylem cells associated with vessel elements both in wheat (Houde et al., 1995; Danyluk et al., 1998) and barley (Bravo et al., 1999).

Dehydrins have been studied in many plants during the past two decades (Rorat, 2006; Battaglia et al., 2008; Kosová et al., 2010). Cereals from the tribe Triticeae belong to the most studied plants due to their economic importance and adverse effects of abiotic stresses on their productivity. Dehydrins in barley and wheat have already been well characterized at gene and transcript levels (primary sequence, promoter analysis, induction by several factors; Karami et al., 2013; Wang et al., 2014); however, dehydrin proteins, not genes or transcripts, are directly involved in an acquisition of stress tolerance, and, therefore, it is necessary to study dehydrin role in plant stress response at protein level. Dehydrin transcripts respond more rapidly to changes in environment while proteins are more conservative. Comparative studies focused on parallel dynamics of WCS120 transcript and protein during cold acclimation of common wheat have revealed a much faster induction of WCS120 transcript upon cold reaching a peak at ca 2 days after an onset of cold treatment while WCS120 protein accumulation reached a peak at ca 21 days after an onset of cold (Ganeshan et al., 2008). Similarly, dehydrin transcript degradation after a temperature increase in a deacclimation treatment is much more rapid than the corresponding protein since dehydrin protein WDHN13 was detected on the immunoblots even at 3 days after the onset of deacclimation while transcript was barely detectable at 1 day of deacclimation (Ohno et al., 2003). Therefore, it is necessary to study the dynamics of dehydrin responses to abiotic stresses at protein level since several post-transcriptional control mechanisms (alternative start codons, natural antisense transcripts) may significantly affect final protein expression. The study at protein level may be complicated by some methodological problems such as a high similarity of barley Y_nSK_m dehydrin protein sequences and thus problems with mass spectrometry identification. Protein post-translational modification such as phosphorylation also significantly affects protein subcellular localization and its final function. There has been found a correlation between dehydrin protein relative accumulation (WCS120, DHN5) and plant maximum acquired frost tolerance in plants subjected to cold (Vítámvás et al., 2007; Kosová et al., 2008b) and even in plants subjected only to mild cold when the plants were grown under controlled temperature (Vítámvás et al., 2010; Kosová et al., 2013b). In field samples, the relationships seem to be more complex and a sum of all detected dehydrin proteins rather than a single protein seems to represent a reliable marker for selection of frost-tolerant materials.

There have already been published several studies on dehydrins in wheat and barley plants. Dehydrins have been well characterized at protein sequence, promoter structure and short-term transcript induction. However, a long-term dynamics of transcripts and proteins with respect to an acquisition of an enhanced stress tolerance as well as dehydrin protein post-translational modifications, cellular localization and their physico-biochemical functions in stress-treated plants remain largely unresolved and represent currently the main challenge to understand dehydrin role in plant stress response.

Klára Kosová has outlined the idea and prepared the text of a brief summary on current state of dehydrin research in stress-treated wheat and barley plants. Pavel Vítámvás and Ilja T. Prášil contributed to preparation, drafting, critical reading, and publication of the manuscript.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.