Transcriptomic Response to High Light in the Charophyte Klebsormidium nitens

During the evolution of life on Earth, the conquest of land by plants played a pivotal role producing a boost in land biomass, a substantial drop in atmospheric CO2, an increase in oxygen and the emergence of new terrestrial habitats facilitating land colonization by animals. Therefore, the characterization of the molecular mechanisms that allowed plant terrestralization is a cornerstone in evolutionary studies. Viridiplantae or the green lineage is divided into two clades Chlorophyta or green microalgae and Streptophyta that in turn splits into Embryophyta or land plants and Charophyta. The latest are mainly considered aquatic algae although some facultative terrestrial species has been identified. Charophyta are generally accepted as the extant algal species most closely related to current land plants and, therefore, they are used in evolutionary studies on plant terrestralization. High light irradiance was one of the major stressors that ancestral charophytic algae needed to overcome during the transition from aquatic to terrestrial environments. In this study, we have chosen the facultative terrestrial early charophytic alga Klebsormidium nitens to perform an integrative transcriptomic and metabolomic analysis under high light in order to unveil key mechanisms involved in the early steps of plants terrestralization. We found a fast chloroplast retrograde signaling possibly mediated by reactive oxygen species and the inositol polyphosphate 1-phosphatase (SAL1) and 3-phosphoadenosine-5-phosphate (PAP) pathways inducing gene expression and accumulation of specific metabolites. Systems used by both Chlorophyta and Embryophyta were activated such as the xanthophyll cycle with an accumulation of zeaxanthin and protein folding and repair mechanisms constituted by NADPH-dependent thioredoxin reductases, thioredoxin-disulfide reductases and peroxiredoxins. Similarly, cyclic electron flow, specifically the pathway dependent on Proton Gradient Regulation 5, was strongly activated under high light. We detected a simultaneous co-activation of the non-photochemical quenching mechanisms based on LHC-like Stress Related protein and the photosystem II subunit S that are specific to Chlorophyta and Embryophyta respectively. Exclusive Embryophyta systems for the synthesis, sensing and response to the phytohormone auxin were also activated under high light in Klebsormidium leading to an increase in auxin content with the concomitant accumulation of amino acids such as tryptophan, histidine and phenylalanine.

在地球生命演化历程中，植物登陆事件发挥了关键作用：它不仅推动陆地生物量大幅提升，显著降低了大气二氧化碳浓度，增加了氧气含量，还催生了全新的陆地生境，为动物登陆奠定了基础。因此，解析植物登陆的分子机制，是演化生物学研究的核心基石之一。绿色植物界（Viridiplantae，又称绿色演化支）可分为两大演化支：绿藻门（Chlorophyta，即绿色微藻）与链型植物门（Streptophyta）；后者又进一步划分为有胚植物（Embryophyta，即陆地植物）与轮藻门（Charophyta）。轮藻门多被视为水生藻类，但目前已发现部分兼性陆生物种。学界普遍认为，现存轮藻门藻类是与当代陆地植物亲缘关系最近的藻类类群，因此轮藻被广泛应用于植物登陆相关的演化生物学研究。在从水生到陆生的过渡过程中，高光辐照是祖先轮藻需要应对的主要环境胁迫因子之一。本研究选取兼性陆生的早期轮藻光滑克莱伯藻（Klebsormidium nitens）作为研究对象，在高光胁迫条件下开展整合转录组与代谢组分析，以揭示植物登陆早期阶段的关键分子机制。研究发现，一条由活性氧（reactive oxygen species, ROS）、肌醇多磷酸1-磷酸酶（SAL1）及3-磷酸腺苷-5-磷酸（PAP）通路介导的快速叶绿体逆行信号被激活，诱导了特定基因的表达与代谢物的积累。绿藻门与有胚植物共有的调控系统被激活，包括伴随玉米黄质（zeaxanthin）积累的叶黄素循环，以及由NADPH依赖型硫氧还蛋白还原酶、硫氧还蛋白二硫键还原酶与过氧化物还原酶组成的蛋白质折叠与修复机制。类似地，高光胁迫下依赖质子梯度调控蛋白5（PGR5）的循环电子流通路被显著激活。研究同时检测到两类分别属于绿藻门与有胚植物的非光化学淬灭机制被共同激活：基于类LHC胁迫相关蛋白的淬灭通路（绿藻门特有）与基于光系统II亚基S（PsbS）的淬灭通路（有胚植物特有）。高光胁迫下，克莱伯藻中还激活了仅存在于有胚植物的生长素合成、感知与响应调控系统，这使得生长素含量升高，并伴随色氨酸、组氨酸与苯丙氨酸等氨基酸的积累。