Evolution of the modular, disordered stress proteins known as dehydrins

Dehydrins, plant proteins that are upregulated during dehydration stress conditions, have modular sequences that can contain three conserved motifs (the Y-, S-, and K-segments). The presence and order of these motifs are used to classify dehydrins into one of five architectures: Kn, SKn, KnS, YnKn, and YnSKn, where the subscript n describes the number of copies of that motif. In this study, an architectural and phylogenetic analysis was performed on 426 dehydrin sequences that were identified in 53 angiosperm and 3 gymnosperm genomes. It was found that angiosperms contained all five architectures, while gymnosperms only contained Kn and SKn dehydrins. This suggests that the ancestral dehydrin in spermatophytes was either Kn or SKn, and the Y-segment containing dehydrins first arose in angiosperms. A high-level split between the YnSKn dehydrins from either the Kn or SKn dehydrins could not be confidently identified, however, two lower level architectural divisions appear to have occurred after different duplication events. The first likely occurred after a whole genome duplication, resulting in the duplication of a Y3SK2 dehydrin; the duplicate subsequently lost an S- and K- segment to become a Y3K1 dehydrin. The second split occurred after a tandem duplication of a Y1SK2 dehydrin, where the duplicate lost both the Y- and S- segment and gained four K-segments, resulting in a K6 dehydrin. We suggest that the newly arisen Y3K1 dehydrin is possibly on its way to pseudogenization, while the newly arisen K6 dehydrin developed a novel function in cold protection.脱水素（Dehydrins）是一类在脱水胁迫条件下表达量上调的植物蛋白，其序列具有模块化结构，可包含三类保守基序：Y片段、S片段与K片段。基于上述基序的存在与否及其排列顺序，可将脱水素划分为五大结构类型：Kn、SKn、KnS、YnKn及YnSKn，其中下标n代表对应基序的拷贝数量。本研究针对从53个被子植物（angiosperm）基因组与3个裸子植物（gymnosperm）基因组中鉴定得到的426条脱水素序列，开展了结构与系统发育分析。研究发现，被子植物拥有全部五种结构类型的脱水素，而裸子植物仅存在Kn与SKn型脱水素。这表明种子植物的祖先脱水素应为Kn或SKn型，而携带Y片段的脱水素最早起源于被子植物类群。尽管无法明确界定YnSKn型脱水素与Kn/SKn型脱水素间的高级分化分支，但两次不同的复制事件后均发生了低阶的结构分化：第一次大概率为全基因组复制（whole genome duplication）事件，原始的Y3SK2型脱水素发生复制后，其中一个拷贝丢失了一段S片段与一段K片段，最终演变为Y3K1型脱水素；第二次分化则源于Y1SK2型脱水素的串联重复（tandem duplication），其中一个拷贝丢失了Y片段与S片段，并新增了四段K片段，最终形成K6型脱水素。我们推测，新产生的Y3K1型脱水素可能正走向假基因化（pseudogenization），而新生的K6型脱水素则演化出了低温保护的全新功能。