Diversity of a cytokinin dehydrogenase gene in wild and cultivated barley

The cytokinin dehydrogenase gene HvCKX2.1 is the regulatory target for the most abundant heterochromatic small RNAs in drought-stressed barley caryopses. We investigated the diversity of HvCKX2.1 in 228 barley landraces and 216 wild accessions and identified 14 haplotypes, five of these with ten or more members, coding for four different protein variants. The third largest haplotype was abundant in wild accessions (51 members), but absent from the landrace collection. Protein structure predictions indicated that the amino acid substitution specific to haplotype 3 could result in a change in the functional properties of the HvCKX2.1 protein. Haplotypes 1–3 have overlapping geographical distributions in the wild population, but the average rainfall amounts at the collection sites for haplotype 3 plants are significantly higher during November to February compared to the equivalent data for plants of haplotypes 1 and 2. We argue that the likelihood that haplotype 3 plants were excluded from landraces by sampling bias that occurred when the first wild barley plants were taken into cultivation is low, and that it is reasonable to suggest that plants with haplotype 3 are absent from the crop because these plants were less suited to the artificial conditions associated with cultivation. Although the cytokinin signalling pathway influences many aspects of plant development, the identified role of HvCKX2.1 in the drought response raises the possibility that the particular aspect of cultivation that mitigated against haplotype 3 relates in some way to water utilization. Our results therefore highlight the possibility that water utilization properties should be looked on as a possible component of the suite of physiological adaptations accompanying the domestication and subsequent evolution of cultivated barley.

细胞分裂素脱氢酶（cytokinin dehydrogenase）基因HvCKX2.1是干旱胁迫大麦颖果中丰度最高的异染色质小RNA的调控靶标。我们对228份大麦地方品种与216份野生大麦种质的HvCKX2.1基因多样性开展研究，共鉴定出14种单倍型（haplotype），其中5种单倍型包含10个及以上成员，编码4种不同的蛋白质变体（protein variant）。第三大的单倍型在野生种质中富集（含51个成员），但未在大麦地方品种集合中被检测到。蛋白质结构预测结果显示，仅存在于单倍型3的氨基酸替换（amino acid substitution）可能会改变HvCKX2.1蛋白的功能特性。单倍型1至3在野生种群中存在重叠的地理分布范围，但单倍型3植株采样点的11月至次年2月平均降雨量，显著高于单倍型1与2植株的对应数据。我们认为，单倍型3植株因早期野生大麦驯化阶段产生的采样偏差而被排除在地方品种之外的可能性较低；更合理的解释是，携带单倍型3的植株难以适应栽培相关的人工生长条件，因此未被纳入栽培大麦种群。尽管细胞分裂素信号通路可调控植物的诸多发育过程，但HvCKX2.1在干旱响应中的功能提示：可能存在某一与栽培相关的选择因子，通过与水分利用相关的途径对单倍型3产生了抑制作用。因此，本研究结果表明，水分利用特性或许是栽培大麦驯化及后续演化过程中伴随的一系列生理适应性状的组成部分之一。