Establishing a foothold in the desert: a transcriptomic view of survival from the extremophyte, Haloxylon ammodendron

Drought is an inevitable stress almost all terrestrial plants face in their life cycles. Desert dwelling plants show extreme adaptations to drought but their genomes are largely unexplored compared to drought sensitive model plants generally studied to understand plant drought tolerance. Haloxylon ammodendron is a pioneer species extremely tolerant to drought and capable of colonizing desert sand dunes. Seedling establishment is the most critical development stage in the survival of H. ammodendron. H. ammodendron seedlings are able to withstand high light, and low temperature stresses characteristic of temperate desert environments in addition to drought. We have investigated the genome-wide transcript responses under induced drought stress during early seedling establishment to identify prevailing basal and induced gene clusters that likely contribute to survival and stress adapted growth in H. ammodendron. We find staggering support for drought response transcript accumulation together with other transcripts that may transform the cellular expression space into a preadapted state for salt, light, osmotic, and temperature stress tolerance. While transcript accumulation is excessive for genes associated with abiotic stress tolerance under an induced drought treatment, H. ammodendron seems to enhance biotic stress tolerance simultaneously by down-regulation of several genes that would be found at an up-regulated state during pathogen entry in susceptible plants. We detected enriched basal level transcript allocation that suggests preadaptation to abiotic stresses as well as pathogen defense in H. ammodendron when compared to other Amaranthaceae family transcriptomes under stress neutral conditions. Amaranthaceae is one of the most enriched plant families for extremophytes. We found transcripts that are generally maintained at low levels and some induced only under abiotic stress in Arabidopsis thaliana to be highly expressed under basal conditions in the Amaranthaceae transcriptomes including H. ammodendron. These could be novel candidates to expand or initiate discovery of new stress adaptive gene networks and mechanisms naturally selected in extremophytes that allow survival under environmental stresses. RNA from Haloxylon ammodendron whole seedlings grown for seven days under control and 5% PEG treatments for Illumina paired-end sequencing.

干旱胁迫几乎是所有陆生植物在其生命周期中必然面临的逆境胁迫。与常用于解析植物耐旱分子机制的干旱敏感型模式植物相比，荒漠生植物的耐旱适应性基因组研究仍存在大量未被探索的空白。梭梭（Haloxylon ammodendron）是一种极端耐旱的先锋物种，能够定植荒漠沙丘。幼苗建成是梭梭种群存活过程中最为关键的发育阶段。梭梭幼苗除可耐受干旱胁迫外，还能抵御温带荒漠环境特有的强光与低温胁迫。本研究针对梭梭幼苗早期建成阶段的诱导干旱胁迫开展全基因组转录组（transcriptome）响应分析，旨在筛选可能参与梭梭存活与胁迫适应性生长的核心基础表达与诱导表达基因簇（gene cluster）。研究结果显示，干旱响应转录本大量积累的同时，另有一批转录本可将细胞表达谱重塑为适应盐、强光、渗透与温度胁迫的预适应状态，该现象得到了显著的证据支持。在诱导干旱处理下，与非生物胁迫耐受相关的基因转录本大量积累的同时，梭梭还可通过下调若干在感病植物病原菌入侵时会上调表达的基因，同步增强生物胁迫耐受能力。与胁迫中性条件下的其他苋科（Amaranthaceae）植物转录组相比，梭梭的基础转录本分配模式显著富集，这表明其本身已预适应非生物胁迫与病原菌防御。苋科是嗜极植物（extremophytes）分布最为丰富的植物科之一。本研究发现，在模式植物拟南芥（Arabidopsis thaliana）中通常维持低水平表达、仅在非生物胁迫下才会被诱导的转录本，在包括梭梭在内的苋科植物的基础转录组中均呈现高表达状态。这些转录本可作为新型候选基因，助力拓展乃至发掘嗜极植物中自然选择形成的、可耐受环境胁迫的新型胁迫适应性基因网络与调控机制。本研究的实验材料为在对照与5%聚乙二醇（PEG）处理条件下培养7天的梭梭全株幼苗，所提取的RNA将用于Illumina双端测序（Illumina paired-end sequencing）。