Watching C4 photosynthesis evolve! Does Salsola divaricata, a C2 species on the Canary Islands, bridge the complex evolutionary transition between C3 and C4 photosynthesis?

C4光合作用(PS)是一个复杂的性状，其进化需要祖先C3谱系内叶片解剖和超微结构、途径生化和基因调控的深刻变化。C4PS在被子植物中出现了约65次，其中约15次起源于藜科。C4PS的进化过程仍然是一个谜。C4进化的概念模型总是倾向于从C3逐步过渡到C4，甘氨酸穿梭碳浓缩机制(C2PS)是关键步骤(最初由Monson等人提出)。(1984)；由Sage等人修改。2012年)。支持该模型的经验证据有一些局限性，最重要的是，从来没有考虑到群体水平上的表型可塑性和性状变异的缺陷。为了验证当前的C4进化模型，这项赠款建议研究加那利群岛的C2种Salsola divaricata，以及摩洛哥西南部的两个密切相关的C3种S.verticillata和S.gionomaschala，它们似乎存在C4光合作用进化的假说早期阶段。这项研究包括使用下一代测序方法对摩洛哥西南部16个种群及其近亲的遗传分化进行评估，通过测序、当地气候和土壤参数进行基因分型，以及根据该模型调查与C4相关的性状。这些是C2和C4蛋白的定量分析，碳补偿点的测量，C2和C4酶在叶片横截面和束鞘超微结构中的表达模式。对具有偏离性状表达的选定样本的转录组分析将进一步揭示基因调控水平上的分化和/或可塑性。大花菜C4性状的表型变异可能代表了C4进化的不同遗传固定进化状态，从而支持了当前的模型，也可能是一个复杂生理综合征的广泛表型可塑性的有趣案例，并刺激了对当前模型的根本重新评估，使其转向一个包含表型可塑性的概念。了解高度复杂的C4PS的进化，不仅是解开一个令人感兴趣的生物学问题，也是当前将C4途径导入水稻等C3作物的研究热点。

C4 photosynthesis (PS) is a complex trait and its evolution requires profound changes of leaf anatomy and ultrastructure, pathway biochemistry and gene regulation within the ancestral C3 lineage. C4 PS arose c. 65 times in the angiosperms with one hotspot of c. 15 origins in the Chenopodiaceae. The processes of how C4 PS evolved are still a mystery. Conceptual models of C4 evolution always favoured a gradual, stepwise transition from C3 to C4 with the glycine shuttle carbon concentrating mechanisms (C2 PS) as the key step (originally proposed by Monson et al. (1984); modified by Sage et al. 2012). The empirical evidence that supports the model has a number of limitations, most importantly the flaw that phenotypic plasticity and variation of traits at the population level has never been taken into account. To test the current model of C4 evolution this grant proposal suggests to study the C2 species Salsola divaricata from the Canary Islands and the two closely related C3 species S. verticillata and S. gymnomaschala from SW Morocco in which hypothetical early phases of C4 photosynthesis evolution appear to be present. The study includes an assessment of the genetic differentiation among 16 populations of Salsola divaricata and its closest relatives from SW Morocco using the next generation sequencing method genotyping by sequencing, local climate and edaphic parameters and a survey of C4-relevant traits as proposed by the model. These are quantitative assays of C2 and C4 proteins, carbon compensation point measurements, expression patterns of C2 and C4 enzymes in leaf cross sections and bundle sheath ultrastructure. A transcriptome analysis of selected samples with deviating trait expression will furthermore reveal differentiation and/or plasticity at the gene regulatory level. The phenotypic variation of C4 traits in Salsola divaricata might represent distinct genetically fixed evolutionary states of C4 evolution and therefore support the current model or it might be a fascinating case of broad phenotypic plasticity of a complex physiological syndrome and stimulate a fundamental re-evaluation of the current model towards a concept that includes phenotypic plasticity. To understand the evolution of the highly complex trait C4 PS is not only to disentangle a fascinating biological problem of broad interest but also a hot topic in current research efforts to introduce the C4 pathway into C3 crops such as rice.