Growth-limiting drought stress induces time-of-day dependent transcriptome and physiological responses in hybrid poplar

Episodic drought stress negatively impacts the health of long-lived trees. Understanding the genetic and molecular mechanisms that underpin response to drought stress is requisite for selecting or enhancing climate change resilience. Here we aim to establish standardized drought stress protocols for transcriptome studies in poplar trees, to determine how hybrid poplars respond to prolonged and uniform exposure to drought; to determine if the responses to moderate and more severe growth-limiting drought stresses were qualitatively or quantitatively different; and, to determine how response to drought changes throughout the day. We established hybrid poplar trees (Populus x 'Okanese') from unrooted stem cutting with abundant soil moisture for six weeks. We then withheld water to establish three soil water contents reflecting well-watered, moderate, and severe growth-limiting drought conditions. Plants were rewatered as needed for three weeks to maintain the soil water conditions. The mild and severe drought treatments elicited distinct changes in growth and development, photosynthetic rates and global transcriptomic changes. Notably, the time of day of sampling was strongest signal in the transcriptome data and it quantitatively and qualitatively affected drought responsive changes in gene expression. These analyses emphasize the complex nature of drought regulation in long-lived trees. Overall design: Examination of 3 water deficit treatments of Okanese poplar seedling leaves at 2 times of day; includes 18 samples: 3 replicates each of 3 treatments at 2 timepoints

间歇性干旱胁迫会对长寿树木的健康造成负面影响。阐明介导干旱胁迫响应的遗传与分子机制，是筛选或提升树木应对气候变化韧性的必要前提。本研究旨在建立适用于杨树转录组（Transcriptome）研究的标准化干旱胁迫处理方案，以达成三大研究目标：明确杂交杨树在长期均匀干旱胁迫下的响应模式；探究中度与重度生长限制性干旱胁迫的响应在性质与程度上是否存在差异；解析干旱响应随昼夜节律的动态变化。我们以未生根茎段扦插获得杂交杨树（Populus x 'Okanese'）种苗，先在土壤含水量充足的条件下培养六周。随后停止浇水，设置三种土壤含水量梯度，分别对应充分供水、中度生长限制性干旱以及重度生长限制性干旱胁迫环境。后续按需补水并维持三周，以稳定各处理组的土壤水分条件。轻度与重度干旱胁迫处理均引发了杨树生长发育、光合速率以及全基因组转录组表达谱的显著变化。值得注意的是，采样时刻是转录组数据中最显著的影响因子，其在表达量与调控模式两方面均对干旱响应基因的表达产生了显著影响。本研究结果凸显了长寿树木中干旱调控机制的复杂性。实验设计概要：对欧卡尼斯杨幼苗叶片开展三种水分亏缺处理下的取样分析，覆盖两个采样时刻；共包含18个样本：三种处理各设置3次生物学重复，每个处理在两个采样时刻分别取样。