**3. Dehydrins in orthodox and recalcitrant seeds**

The categories "orthodox" and "recalcitrant" seeds are used to describe the storage behavior of seeds. Orthodox seeds undergo maturation drying and are shed from the parent plant at low moisture contents. During maturation, they acquire desiccation tolerance, allowing them to be dried without irreversible damage. Because of this ability, seeds can be stored for long periods in cold and dry vaults. On the other hand, recalcitrant seeds do not undergo maturation drying and are shed at relatively high moisture contents.

of heat-stable dehydrin-like proteins in plant mitochondria. The most tolerant winter wheat and rye accumulate more of the heat-stable dehydrins than maize. Cold-induced accumulation of the heat-stable mitochondrial dehydrin-like proteins in all species studied was accompanied

Dehydrins have been most extensively studied in relation to drought and cold stresses [29– 35]. Some experimental studies provide evidence that dehydrins contribute to freezing stress tolerance in plants and suggest that this could be partly due to their protective effect on membranes [36]. Dehydrins stabilize plant plasma and organellar membranes in conditions of

The hypothesis that dehydrins have detergent and chaperone-like properties and may interact with compatible solutes to serve as structural stabilizers of macromolecules under conditions

Numerous *in vitro* functions have been described and proposed for dehydrins, including cryoprotection of lactate dehydrogenase (LDH), cryoprotection of purified protoplasts and chloroplasts, prevention of water loss, binding of excess ions, binding of nucleic acids, prevention of protein aggregation at elevated temperatures, and prevention of ice crystal growth. RAB18 (Y2SK2) accumulates in response to the phytohormone abscisic acid (ABA), drought, and low temperature [22]. LTI29 and COR47 (SK3) accumulate primarily in response to low temperature but also to ABA and salt stress [22]. LTI30 (K6) accumulates mainly under cold stress [22]. ERD14 (SK2) is present in non-stressed plants although the protein level is upregulated by stress, particularly drought stress [22]. Dehydrin XERO (YSK2) mRNA has

Of these functions, the most extensively studied has been the cryoprotection of LDH, where it has been shown that dehydrins are more effective than small molecules such as sucrose at

The following are the functions of dehydrins with experimental evidence: binding to water and ions, binding to phospholipids, radical scavenging, phosphorylation, binding to calcium,

DHN genes are also expressed significantly in seeds toward the end of maturation, a period when the seed undergoes a developmentally programmed reduction in water content [22, 40]. The LEA/dehydrin proteins have been estimated to comprise up to 4% of the total seed

The categories "orthodox" and "recalcitrant" seeds are used to describe the storage behavior of seeds. Orthodox seeds undergo maturation drying and are shed from the parent plant at low moisture contents. During maturation, they acquire desiccation tolerance, allowing them to be dried without irreversible damage. Because of this ability, seeds can be stored for long

protection of enzymes, binding to cytoskeletons, and binding to nucleic acids [17].

stress, and further zinc may be an important co-factor in stabilization [37, 38].

of water deficit [10] is now experimentally evident [17, 39].

been found to be constitutively expressed [39].

protein [7].

protecting LDH activity from freeze-thaw damage [39].

**3. Dehydrins in orthodox and recalcitrant seeds**

by increasing of plant cryotolerance [15].

320 Abiotic and Biotic Stress in Plants - Recent Advances and Future Perspectives

It is believed that, in orthodox seeds, dehydrins favor the development of tolerance to osmotic stress at seed dehydration during their maturation [2, 6].

The lack of resistance of recalcitrant seeds to drying was thought to be the result of the absence of dehydrins [9, 41]. Subsequent studies, however, demonstrated that dehydrins are present in the fraction of heat-stable proteins in recalcitrant seeds of many woody species of the temperate climatic zone [41–43], including those of horse chestnut, but they were not found in species inhabiting humid tropics. This raises the question of the function performed by dehydrins in recalcitrant seeds.

In this connection, the investigation of dehydrin functions, properties, and distribution in recalcitrant seeds becomes actual. The data available so far indicate that dehydrins are present in some but not all recalcitrant seed species. They appear in response to low-temperature stress, an increase in the ABA content, and natural or artificial limited dehydration [42, 44–46].

Two tropical recalcitrant species exhibited a differential capacity to produce dehydrin-related proteins during seed maturation [43]. Dehydrins were present in axes and cotyledons of *Castanospermum australe* seeds during mid-maturation and at maturity. However, in *Trichilia dregeana*, no dehydrin-related polypeptides were detected in the mature seed. During the development of *C. australe* seeds, the nature of the dehydrin-related polypeptides accumulated in the cotyledons and axis changed and new polypeptides were detected in the mature seeds. The dehydrins present in cotyledons of mature seeds (31, 37, and 40 kDa) were still detectable after germination (i.e., in untreated seedlings) [43].

Kalemba and Pukacka [47] have compared mature and dried seeds from three species of the Acer genus, which differed in desiccation tolerance. Seeds of three Acer species—*Acer plata‐ noides* L. (Norway maple, orthodox), *Acer pseudoplatanus* L. (sycamore, recalcitrant), and *Acer saccharinum* (silver maple, recalcitrant)—harvested during various cropping years were compared and analyzed to determine whether a genetic or an environmental influence dominated the regulation of dehydrin protein expression. The authors compared the appear‐ ance of dehydrins and small heat shock proteins in seedlots originating from cropping years that differed in weather conditions, which were monitored in detail during seed develop‐ ment.The experiments showedthatthreemaindehydrinswithapproximatemolecularweights of 46, 35, and 23 kDa were characteristic of all examined Acer species seeds. The three pro‐ teins were present in two seedlots of the orthodox Norway maple seeds and were noted either individually or together in all seedlots of recalcitrant Acer seeds. The modulation of dehydrin expression by environmental factors such as developmental heat sum and rainfall is sup‐ posed [47].

The presence of dehydrins alone in recalcitrant seeds is not sufficient to prevent desiccation injury [42, 9].

In two papers published recently [48, 49], the authors suggest an interesting point of view and "paradigm change": LEA proteins are synthesized as response to drought stress, which takes place at the end of maturation (of orthodox seeds). But it is known that dehydrins/LEA proteins are synthesized before maturation drying [6]. And maturation drying is genetically deter‐ mined in orthodox seeds. During embryogenesis and maturation drying of orthodox seeds, dehydrins are synthesized and seeds may survive dry storage. "Typical" recalcitrant seeds [48, 49] such as *Avicennia* and *Brunguiera* have no dehydrins and high water content and they do not survive water loss during storage. Intermediate seeds (*Coffea, Barringtinia*) have no dehydrins during embryogenesis but have genes of dehydrins and may synthesize dehydrins after partial water loss in storage, so these seeds may survive [48, 49]. And "atypical" recalci‐ trant seeds (*Camellia, Castanea, Euterpe, Quercus*) accumulate some dehydrins at the end of embryogenesis and undergo a weak maturation drying and can be stored for a limited time [48, 49]. So, the role of dehydrins in recalcitrant seeds is not clear, and further comprehensive studies are required.
