# Order in Chaos: Lesser-Conserved and Repeat Structures in Dehydrins
## 1 Introduction  
Dehydrins (Dhns) are a family of intrinsically disordered plant proteins that are closely associated with the tolerance and survival of dehydration, freezing, and other dehydrative stresses. They are classified as a subset of late embryogenesis abundant (LEA) proteins that were first discovered and described in orthodox (dehydration tolerant) seeds [1]. Dhns were first named and had their general characteristics described over 25 years ago [2, 3]. They have been found in all major land plant groups from bryophytes to angiosperm crown groups, and in most if not all plant tissues from roots and seeds to buds and leaves. They are highly hydrophilic, enriched in glycine and polar residues, heat stable, and largely unstructured in aqueous solution, as indicated by CD spectra. Some are constitutively expressed, but expression may increase in response to stress, while others seem to be produced mainly in response to specific stresses [4]. There is now an extensive body of scientific literature on Dhn occurrence, localization, response to stress, and, to a lesser extent, function [5, 6, 7, 8, 9]. The central identifying feature of Dhns is the 15-residue K segment, with the consensus sequence, usually given as EKKGIMDKIKEKLPG, that occurs in anywhere from 1 to asmany as 15 copies and is generally well-conserved in amino acid identity or properties across the entire spectrum of land plants. In a nonpolar environment, the K segment folds into an amphipathic a-helix. K segments have been shown to bind to membranes, suggesting that Dhns help prevent dehydration-induced denaturation of these critical structures [10; 11]. While some Dhns contain only one or more K segments, the following four other conserved segment types may occur in various arrangements in different subgroups of Dhns:  
* Y segments, originally described by the 7-residue consensus sequence (V/T)DEYGNP [3], typically occur in 1 to 4 copies N-terminal to other conserved segments, but more uncommonly in as many as 36 copies and rarely between other conserved segments. [PERSON] and [PERSON] [12] recently noted that the Y segment consensus can be extended to include an eighth, nonpolar residue, usually V or I. * F segments, first described in 2017 [13] and confirmed in a MEME analysis of 426 Dhns in 53 angiosperm and 3 gymnosperm genomes [14], vary around the conserved core sequence DRGLFDFLGKK and typically occur in a single copy N terminal to other conserved segments. An uncommon variant, GCGMFDFLKK, here called the GCvariant, is known to occur only in rosd and a few asterid species [13]. * H segments, first described in 2021 [12], with the 15-residue consensus M(A/S)GIIHKI EE(T/K/A)LH(I/M)G, occur at the N terminus of Dhns with a terminal S segment. * S segments consist of strings of 3 to 13 serine residues, typically interrupted by 1 or a few single D or E residues, with additional conserved prefix or suffix strings that vary somewhat in different types of Dhns [13]. They typically occur in one of two positions, either between Y or F segments and K segments or at the C terminus of the Dhns with N-terminal H segments [12]. Based on the number and position of the five conserved segment types, Dhns are classified into different groups, with new variants added to the list with the recent discoveries of the F and H segments. The common types are Kn, SKn, HKnS, YnKn, YnSKn, and FSKn, with any other arrangements being relatively uncommon. A phylogenetic analysis of 305 Dhn amino acid sequences from 56 land plant genomes shows that Dhns fall into 3 orthologous groups [12], designated F-, H-, or Y-Dhns, here called Dhn orders and named F(S)Kn, (H)KnS, and Yn(S)Kn, respectively, with parentheses referring to the absence of diagnostic segments in some individual Dhns. Some Kn and SKn Dhns can be classified into one of these orders by phylogenetic analysis [12].