Drought stress substantially impacts the development and viability of Populus spp., which are essential for forestry and bioenergy production. This review summarizes and describes the functions of phytohormones, such as abscisic acid, auxins, and ethylene, in modulating physiological and molecular responses to water scarcity. Drought-induced ABA-mediated stomatal closure and root extension are essential adaptation processes. Furthermore, auxin–ABA (abscisic acid) interactions augment root flexibility, whereas ethylene regulates antioxidant defenses to alleviate oxidative stress. The advantageous function of endophytic bacteria, specifically plant growth-promoting rhizobacteria (PGPR), can augment drought resistance in spruce trees by enhancing nutrient absorption and stimulating root development. Structural adaptations encompass modifications in root architecture, including enhanced root length and density, which augment water uptake efficiency. Similarly, Arbuscular Mycorrhizal Fungi (AMF) significantly enhance stress resilience in forest trees. AMF establishes symbiotic relationships with plant roots, improving water and nutrient uptake, particularly phosphorus, during drought conditions. Furthermore, morphological alterations at the root–soil interface enhance interaction with soil moisture reserves. This review examines the complex mechanisms by which these hormones influence plant responses to water shortage, aiming to offer insights into prospective techniques for improving drought tolerance in common tree species and highlights the importance of hormone control in influencing the adaptive responses of prominent trees to drought stress, providing significant implications for research and practical applications in sustainable forestry and agriculture. These findings lay the groundwork for improving drought tolerance in Populus spp. by biotechnological means and by illuminating the complex hormonal networks that confer drought resistance.

Populus species, particularly P. trichocarpa, have long served as model trees for genomics research, owing to fully sequenced genomes. However, the high heterozygosity, and the presence of repetitive regions, including centromeres and ribosomal RNA gene clusters, have left 59 unresolved gaps, accounting for approximately 3.32% of the P. trichocarpa genome. In this study, the callus induction method was improved to derive a doubled haploid (DH) callus line from P. ussuriensis anthers. Leveraging long-read sequencing, we successfully assembled a nearly gap-free, telomere-to-telomere (T2T) P. ussuriensis genome spanning 412.13 Mb. This genome assembly contains only seven gaps and has a contig N50 length of 19.50 Mb. Annotation revealed 34,953 protein-coding genes in this genome, which is 465 more than that of P. trichocarpa. Notably, centromeric regions are characterized by higher-order repeats, we identified and annotated centromere regions in all DH genome chromosomes, a first for poplars. The derived DH genome exhibits high collinearity with P. trichocarpa and significantly fills gaps present in the latter's genome. This T2T P. ussuriensis reference genome will not only enhance our understanding of genome structure, and functions within the poplar genus but also provides valuable resources for poplar genomic and evolutionary studies.

Karyotype analysis and the investigation of chromosomal variations in Populus are challenging due to its small and morphologically similar chromosomes. Despite its utility in chromosome identification and karyotype evolutionary research, satellite DNA (satDNA) remains underutilized in Populus. In the present study, 12 satDNAs were identified from P. trichocarpa, and the copy numbers and chromosomal distributions of each satDNA were analyzed bioinformatically in the reference genomes of P. trichocarpa, P. simonii, and P. nigra. Ten satDNA probes for fluorescence in situ hybridization (FISH) were successfully developed and validated on chromosomes of P. xiaohei (poplar hybrid P. simonii × P. nigra). By integrating bioinformatic genomic satDNA distribution patterns with experimental FISH signals, we constructed a molecular karyotype of P. xiaohei. Comparative analysis revealed errors in current poplar genome assemblies. Comparative karyotype analysis of P. xiaohei and its doubled haploid (DH) lines revealed chromosomal variations in the DH lines relative to the donor tree. The results demonstrate that the newly developed satDNA probes constitute robust cytogenetic tools for detecting structural variations in Populus, while molecular karyotyping provides new insights into the genetic mechanisms underlying chromosome variations in P. xiaohei and the DH plants derived.