Leaf senescence, the last stage of leaf development, is a type of postmitotic senescence and is characterized by the functional transition from nutrient assimilation to nutrient remobilization which is essential for plants’ fitness. The initiation and progression of leaf senescence are regulated by a variety of internal and external factors such as age, phytohormones, and environmental stresses. Significant breakthroughs in dissecting the molecular mechanisms underpinning leaf senescence have benefited from the identification of senescence-altered mutants through forward genetic screening and functional assessment of hundreds ofsenescence-associated genes(SAGs)viareverse genetic research in model plantArabidopsis thalianaas well as in crop plants. Leaf senescence involves highly complex genetic programs that are tightly tuned by multiple layers of regulation, including chromatin and transcription regulation, post-transcriptional, translational and post-translational regulation. Due to the significant impact of leaf senescence on photosynthesis, nutrient remobilization, stress responses, and productivity, much effort has been made in devising strategies based on known senescence regulatory mechanisms to manipulate the initiation and progression of leaf senescence, aiming for higher yield, better quality, or improved horticultural performance in crop plants. This review aims to provide an overview of leaf senescence and discuss recent advances in multi-dimensional regulation of leaf senescence from genetic and molecular network perspectives. We also put forward the key issues that need to be addressed, including the nature of leaf age, functional stay-green trait, coordination between different regulatory pathways, source-sink relationship and nutrient remobilization, as well as translational researches on leaf senescence.

Leaf senescence is a highly coordinated, complicated process involving the integration of numerous internal and environmental signals. Salicylic acid (SA) and reactive oxygen species (ROS) are two well-defined inducers of leaf senescence whose contents progressively and interdependently increase during leaf senescence via an unknown mechanism. Here, we characterized the transcription factor WRKY75 as a positive regulator of leaf senescence in Arabidopsis thaliana. Knockdown or knockout of WRKY75 delayed age-dependent leaf senescence, while overexpression of WRKY75 accelerated this process. WRKY75 transcription is induced by age, SA, H₂O₂, and multiple plant hormones. Meanwhile, WRKY75 promotes SA production by inducing the transcription of SA INDUCTION-DEFICIENT2 (SID2) and suppresses H₂O₂ scavenging, partly by repressing the transcription of CATALASE2 (CAT2). Genetic analysis revealed that the mutation of SID2 or an increase in catalase activity rescued the precocious leaf senescence phenotype evoked by WRKY75 overexpression. Based on these results, we propose a tripartite amplification loop model in which WRKY75, SA, and ROS undergo a gradual but self-sustained rise driven by three interlinking positive feedback loops. This tripartite amplification loop provides a molecular framework connecting upstream signals, such as age and plant hormones, to the downstream regulatory network executed by SA- and H₂O₂-responsive transcription factors during leaf senescence.

Root hairs are an extensive structure of root epidermal cells and are critical for nutrient acquisition, soil anchorage, and environmental interactions in sessile plants. The phytohormone ethylene (ET) promotes root hair growth and also mediates the effects of different signals that stimulate hair cell development. However, the molecular basis of ET-induced root hair growth remains poorly understood. Here, we show that ET-activated transcription factor ETHYLENE-INSENSITIVE 3 (EIN3) physically interacts with ROOT HAIR DEFECTIVE 6 (RHD6), a well-documented positive regulator of hair cells, and that the two factors directly coactivate the hair length-determining gene RHD6-LIKE 4 (RSL4) to promote root hair elongation. Transcriptome analysis further revealed the parallel roles of the regulator pairs EIN3/EIL1 (EIN3-LIKE 1) and RHD6/RSL1 (RHD6-LIKE 1). EIN3/EIL1 and RHD6/RSL1 coordinately enhance root hair initiation by selectively regulating a subset of core root hair genes. Thus, our work reveals a key transcriptional complex consisting of EIN3/EIL1 and RHD6/RSL1 in the control of root hair initiation and elongation, and provides a molecular framework for the integration of environmental signals and intrinsic regulators in modulating plant organ development. Significance Root hairs are unicellular extensions of root epidermal cells that help plants increase water and nutrient uptake and improve soil anchorage, both of which are crucial for the globally recognized goal of yield improvement with reduced fertilizer use. Previous studies have implicated numerous genes and phytohormones in the control of root hair development. This work uncovers the molecular mechanism of ethylene (ET)-promoted root hair growth and identifies a transcriptional complex consisting of EIN3/EIL1 and RHD6/RSL1 as the key regulator of root hair initiation and elongation. As ET mediates the effects of various root hair stimuli, this work also elucidates a convergent signaling network that integrates diverse environmental cues and intrinsic signals to modulate plant organ development.

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 PtRD26^IR, 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 PtRD26^IR. Correspondingly, silencing of these factors decreased PtRD26^IR 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.