Institute for Plant Biotechnology, Department of Genetics, Stellenbosch University, Private Bag X1, Matieland, Stellenbosch 7602, South Africa.
Department of Forestry and Wood Sciences, Stellenbosch University, Bosman St, Stellenbosch 7599, South Africa.
Tree Physiol. 2024 Jul 2;44(7). doi: 10.1093/treephys/tpae068.
Future climatic scenarios forecast increasingly frequent droughts that will pose substantial consequences on tree mortality. In light of this, drought-tolerant eucalypts have been propagated; however, the severity of these conditions will invoke adaptive responses, impacting the commercially valuable wood properties. To determine what mechanisms govern the wood anatomical adaptive response, highly controlled drought experiments were conducted in Eucalyptus grandis W. Hill ex Maiden, with the tree physiology and transcriptome closely monitored. In response to water deficit, E. grandis displays an isohydric stomatal response to conserve water and enable stem growth to continue, albeit at a reduced rate. Maintaining gaseous exchange is likely a critical short-term response that drives the formation of hydraulically safer xylem. For instance, the development of significantly smaller fibers and vessels was found to increase cellular density, thereby promoting drought tolerance through improved functional redundancy, as well as implosion and cavitation resistance. The transcriptome was explored to identify the molecular mechanisms responsible for controlling xylem cell size during prolonged water deficit. Downregulation of genes associated with cell wall remodeling and the biosynthesis of cellulose, hemicellulose and pectin appeared to coincide with a reduction in cellular enlargement during drought. Furthermore, transcript levels of NAC and MYB transcription factors, vital for cell wall component biosynthesis, were reduced, while those linked to lignification increased. The upregulation of EgCAD and various peroxidases under water deficit did not correlate with an increased lignin composition. However, with the elevated cellular density, a higher lignin content per xylem cross-sectional area was observed, potentially enhancing hydraulic safety. These results support the requirement for higher density, drought-adapted wood as a long-term adaptive response in E. grandis, which is largely influenced by the isohydric stomatal response coupled with cellular expansion-related molecular processes.
未来的气候情景预测干旱将更加频繁，这将对树木死亡率造成重大影响。有鉴于此，已经繁殖了耐旱桉树；然而，这些条件的严重程度将引发适应性反应，影响具有商业价值的木材特性。为了确定哪些机制控制木材解剖结构的适应性反应，在巨桉中进行了高度受控的干旱实验，密切监测树木生理学和转录组。在水分亏缺的情况下，巨桉表现出等水合作用的气孔反应，以节约用水并使茎继续生长，尽管生长速度降低。维持气体交换可能是一个关键的短期反应，它驱动形成水力更安全的木质部。例如，发现显著较小的纤维和导管的发育增加了细胞密度，从而通过提高功能冗余度以及抗内爆和空化性来提高耐旱性。探索转录组以鉴定在长期水分亏缺期间控制木质部细胞大小的分子机制。与细胞壁重塑和纤维素、半纤维素和果胶生物合成相关的基因下调似乎与干旱期间细胞增大减少同时发生。此外，细胞壁成分生物合成所必需的 NAC 和 MYB 转录因子的转录水平降低，而与木质化相关的转录水平增加。木质素含量增加，而与木质素合成相关的转录水平增加。EgCAD 和各种过氧化物酶在水分亏缺下的上调与木质素组成的增加没有相关性。然而，随着细胞密度的增加，观察到每木质部横截面积的木质素含量更高，可能增强水力安全性。这些结果支持在巨桉中需要更高密度、耐旱性的木材作为长期适应性反应，这主要受到等水合作用的气孔反应以及与细胞扩张相关的分子过程的影响。
