Expression profiling of developing barley main shoot apices and leaves during the vegetative phase and early inflorescence development in response to the photoperiod and natural allelic variation at Ppd-H1

The timing of reproductive development determines spike architecture and thus yield in temperate grasses such as barley (Hordeum vulgare L.). Reproductive development in barley is controlled by the photoperiod response gene Ppd-H1 which accelerates flowering time under long-day (LD) conditions. A natural mutation in Ppd-H1 prevalent in spring barley causes a reduced photoperiod response, and thus, late flowering under LD. However, it is not very well understood how LD and Ppd-H1 control pre-anthesis development, and thus spike architecture and yield in barley. We performed a detailed morphological analysis of the pre-anthesis development in the spring barley cultivar Scarlett and its wild barley derived introgression line S42-IL107, carrying the photoperiod responsive Ppd-H1 allele. Characterization of shoot apex development in these genotypes indicated that floral transition and initiation of floral primordia occurred under LD (16h light/ 8h dark) and short day (SD, 8h light/ 16h dark) conditions, while inflorescence and seed development strictly required LDs. Additionally, a fast photoperiod response in the presence of the dominant Ppd-H1 allele promoted floret fertility under LDs. To characterize the effects of the photoperiod and allelic variation at Ppd-H1 on gene expression during pre-anthesis development we performed RNA sequencing of leaves and developing main shoot apices during the vegetative phase and early stages of inflorescence development in Scarlett and S42-IL107 grown under SD and LD conditions. Main shoot apices of both genotypes were sampled at defined developmental stages, i.e. Waddington stage W0.5, W1.0, W2.0 and W3.5, respectively. Leaf samples were harvested from plants before (W1.0) and after floral transition (W2.0). We identified genes that were specifically regulated at floral transition independent of day-length and Ppd-H1 and thus may serve as markers for the staging of floral transition. Furthermore, we identified transcripts differentially expressed between photoperiods and between genotypes in leaves and in shoot apices. This set of transcripts might act as candidates downstream of Ppd-H1 and are correlated with the promotion of shoot apex development and higher floret fertility under LD and in the presence of the photoperiod responsive Ppd-H1 allele.

生殖发育的时序决定了温带禾本科植物（如大麦（Hordeum vulgare L.））的穗部结构，进而影响其产量。大麦的生殖发育由光周期响应基因Ppd-H1调控，该基因在长日照（long-day, LD）条件下可加速开花进程。春大麦中广泛存在的Ppd-H1自然突变会削弱光周期响应能力，进而导致长日照条件下开花延迟。然而，目前学界对长日照与Ppd-H1如何调控大麦花前发育，进而影响穗部结构与产量的机制仍不甚明晰。本研究对春大麦品种Scarlett及其携带光周期响应型Ppd-H1等位基因的野生大麦衍生渐渗系S42-IL107的花前发育开展了详细的形态学分析。对这两种基因型材料的茎尖发育特征进行表征后发现，在长日照（16小时光照/8小时黑暗）与短日照（short-day, SD，8小时光照/16小时黑暗）条件下，均可发生花转型与花原基起始过程；而花序与种子发育则严格依赖长日照条件。此外，在显性Ppd-H1等位基因存在时，快速的光周期响应可提升长日照条件下的小花育性。为解析光周期与Ppd-H1等位基因变异对花前发育过程中基因表达的调控作用，本研究对在短日照与长日照条件下培养的Scarlett与S42-IL107，分别在营养生长期及花序发育早期阶段的叶片与发育中的主茎尖进行了RNA测序（RNA sequencing）。两种基因型的主茎尖均在明确的发育阶段采样，即沃丁顿分期（Waddington stage）W0.5、W1.0、W2.0与W3.5阶段。叶片样本则采集于花转型前（W1.0）与花转型后（W2.0）的植株。本研究鉴定出一批在花转型阶段特异性调控、且不受日照时长与Ppd-H1基因型影响的基因，这些基因可作为花转型分期的标记基因。此外，我们还鉴定出在不同光周期条件下以及不同基因型的叶片与茎尖中差异表达的转录本。这类转录本可作为Ppd-H1下游的候选调控因子，且与长日照条件下及携带光周期响应型Ppd-H1等位基因时的茎尖发育促进与更高小花育性呈显著相关。