Hou‐Ling Wang

Summary Drought, a primary abiotic stress, seriously affects plant growth and productivity. Stomata play a vital role in regulating gas exchange and drought adaptation. However, limited knowledge exists of the molecular mechanisms underlying stomatal movement in trees. Here, Pe CHYR 1 , a ubiquitin E3 ligase, was isolated from Populus euphratica , a model of stress adaptation in forest trees. Pe CHYR 1 was preferentially expressed in young leaves and was significantly induced by ABA (abscisic acid) and dehydration treatments. To study the potential biological functions of Pe CHYR 1 , transgenic poplar 84K ( Populus alba × Populus glandulosa ) plants overexpressing Pe CHYR 1 were generated. Pe CHYR 1 overexpression significantly enhanced H 2 O 2 production and reduced stomatal aperture. Transgenic lines exhibited increased sensitivity to exogenous ABA and greater drought tolerance than that of WT (wild‐type) controls. Moreover, up‐regulation of Pe CHYR 1 promoted stomatal closure and decreased transpiration, resulting in strongly elevated WUE (water use efficiency). When exposed to drought stress, transgenic poplar maintained higher photosynthetic activity and biomass accumulation. Taken together, these results suggest that Pe CHYR 1 plays a crucial role in enhancing drought tolerance via ABA ‐induced stomatal closure caused by hydrogen peroxide (H 2 O 2 ) production in transgenic poplar plants.

Plants face a relentless onslaught from a diverse array of pathogens in their natural environment, to which they have evolved a myriad of strategies that unfold across various temporal scales. Cell surface pattern recognition receptors (PRRs) detect conserved elicitors from pathogens or endogenous molecules released during pathogen invasion, initiating the first line of defence in plants, known as pattern-triggered immunity (PTI), which imparts a baseline level of disease resistance. Inside host cells, pathogen effectors are sensed by the nucleotide-binding/leucine-rich repeat (NLR) receptors, which then activate the second line of defence: effector-triggered immunity (ETI), offering a more potent and enduring defence mechanism. Moreover, PTI and ETI collaborate synergistically to bolster disease resistance and collectively trigger a cascade of downstream defence responses. This article provides a comprehensive review of plant defence responses, offering an overview of the stepwise activation of plant immunity and the interactions between PTI-ETI synergistic signal transduction.

During leaf senescence, the final stage of leaf development, nutrients are recycled from leaves to other organs, and therefore proper control of senescence is thus critical for plant fitness. Although substantial progress has been achieved in understanding leaf senescence in annual plants, the molecular factors that control leaf senescence in perennial woody plants are largely unknown. Using RNA sequencing, we obtained a high-resolution temporal profile of gene expression during autumn leaf senescence in poplar (Populus tomentosa). Identification of hub transcription factors (TFs) by co-expression network analysis of genes revealed that senescence-associated NAC family TFs (Sen-NAC TFs) regulate autumn leaf senescence. Age-dependent alternative splicing (AS) caused an intron retention (IR) event in the pre-mRNA encoding PtRD26, a NAC-TF. This produced a truncated protein PtRD26IR, which functions as a dominant-negative regulator of senescence by interacting with multiple hub Sen-NAC TFs, thereby repressing their DNA-binding activities. Functional analysis of senescence-associated splicing factors identified two U2 auxiliary factors that are involved in AS of PtRD26IR. Correspondingly, silencing of these factors decreased PtRD26IR transcript abundance and induced early senescence. We propose that an age-dependent increase of IR splice variants derived from Sen-NAC TFs is a regulatory program to fine tune the molecular mechanisms that regulate leaf senescence in trees.

In the present study, PeSTZ1, a cysteine-2/histidine-2-type zinc finger transcription factor, was isolated from the desert poplar, Populus euphratica, which serves as a model stress adaptation system for trees. PeSTZ1 was preferentially expressed in the young stems and was significantly up-regulated during chilling and freezing treatments. PeSTZ1 was localized to the nucleus and bound specifically to the PeAPX2 promoter. To examine the potential functions of PeSTZ1, we overexpressed it in poplar 84K hybrids (Populus alba × Populus glandulosa), which are known to be stress-sensitive. Upon exposure to freezing stress, transgenic poplars maintained higher photosynthetic activity and dissipated more excess light energy (in the form of heat) than wild-type poplars. Thus, PeSTZ1 functions as a transcription activator to enhance freezing tolerance without sacrificing growth. Under freezing stress, PeSTZ1 acts upstream of ASCORBATE PEROXIDASE2 (PeAPX2) and directly regulates its expression by binding to its promoter. Activated PeAPX2 promotes cytosolic APX that scavenges reactive oxygen species (ROS) under cold stress. PeSTZ1 may operate in parallel with C-REPEAT-BINDING FACTORS to regulate COLD-REGULATED gene expression. Moreover, PeSTZ1 up-regulation reduces malondialdehyde and ROS accumulation by activating the antioxidant system. Taken together, these results suggested that overexpressing PeSTZ1 in 84K poplar enhances freezing tolerance through the modulation of ROS scavenging via the direct regulation of PeAPX2 expression.

Populus euphratica is the only arboreal species that is established in the world's largest shifting-sand desert in China and is well-adapted to the extreme desert environment, so it is widely considered a model system for researching into abiotic stress resistance of woody plants. However, few P. euphratica reference genes (RGs) have been identified for quantitative real-time polymerase chain reaction (qRT-PCR) until now. Validation of suitable RGs is essential for gene expression normalization research. In this study, we screened 16 endogenous candidate RGs in P. euphratica leaves in six abiotic stress treatments, including abscisic acid (ABA), cold, dehydration, drought, short-duration salt (SS) and long-duration salt (LS) treatments, each with 6 treatment gradients. After calculation of PCR efficiencies, three different software tools, NormFinder, geNorm and BestKeeper, were employed to analyze the qRT-PCR data systematically, and the outputs were merged by means of a non-weighted unsupervised rank aggregation method. The genes selected as optimal for gene expression analysis of the six treatments were RPL17 (ribosomal protein L17) in ABA, EF1α (elongation factor-1 alpha) in cold, HIS (histone superfamily protein H3) in dehydration, GIIα in drought and SS, and TUB (tubulin) in LS. The expression of 60S (the 60S ribosomal protein) varied the least during all treatments. To illustrate the suitability of these RGs, the relative quantifications of three stress-inducible genes, PePYL1, PeSCOF-1 and PeSCL7 were investigated with different RGs. The results, calculated using qBasePlus software, showed that compared with the least-appropriate RGs, the expression profiles normalized by the recommended RGs were closer to expectations. Our study provided an important RG application guideline for P. euphratica gene expression characterization.