Abstract
Leaves are the most critical portion of forage crops such as alfalfa (Medicago sativa). Leaf senescence caused by environmental stresses significantly impacts the biomass and quality of forages. To understand the molecular mechanisms and identify the key regulator of the salt stress-induced leaf senescence process, we conducted a simple and effective salt stress-induced leaf senescence assay in Medicago truncatula, which was followed by RNA-Seq analysis coupled with physiological and biochemical characterization. By comparing the observed expression data with that derived from dark-induced leaf senescence at different time points, we identified 3,001, 3,787, and 4,419 senescence-associated genes (SAGs) for salt stress-induced leaf senescence on day 2, 4, and 6, respectively. There were 1546 SAGs shared by dark and salt stress treatment across the three time points. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses showed that the 1546 SAGs were mainly related to protein and amino acids metabolism, photosynthesis, chlorophyll metabolism, and hormone signaling during leaf senescence. Strikingly, many different transcription factors (TFs) families out of the 1546 SAGs, including NAC, bHLH, MYB, and ERF, were associated with salt stress-induced leaf senescence processes. Using the transient expression system in Nicotiana benthamiana, we verified that three functional NAC TF genes from the 1546 SAGs were related to leaf senescence. These results clarify SAGs under salt stress in M. truncatula and provide new insights and additional genetic resources for further forage crop breeding.
Highlights
In nature, leaf senescence is an inevitable process by which plants reallocate nutrients, altering their metabolism and energy cycles to redirect more nutrients from senescent or dying leaves to new leaves, flower development, seed maturation, or other tissues and storage organs (Hörtensteiner and Feller, 2002; Yoshimoto et al, 2009; Bar-Dror et al, 2011)
We examined salt stress-induced senescence in Medicago truncatula leaves using a time-course analysis of the expression patterns of senescence-associated genes (SAGs)
As a positive control for our data analysis, we included a dark-induced senescence treatment. This experimental design enabled the superimposition of the data from both treatments and helped us to obtain time-course differential expression network data for those genes associated with salt-induced leaf senescence
Summary
Leaf senescence is an inevitable process by which plants reallocate nutrients, altering their metabolism and energy cycles to redirect more nutrients from senescent or dying leaves to new leaves, flower development, seed maturation, or other tissues and storage organs (Hörtensteiner and Feller, 2002; Yoshimoto et al, 2009; Bar-Dror et al, 2011). Premature senescence caused by stress can significantly result in yield loss and food quality reduction (Uauy et al, 2006; Yamada et al, 2009; Gregersen et al, 2013). Through delaying the senescence of functional leaves by 1 day, it has been calculated that yields can be increased by 2–10% in major crops such as maize, soybeans, Salt-Induced Leaf Senescence in M. truncatula cotton, rice, and wheat (Buchanan-Wollaston et al, 2003; Liu et al, 2015; Schippers et al, 2015). Leaf senescence has a more direct impact on the production of high-quality forage (Du et al, 2020). Many researchers focused on improving crop yields and agricultural productivity by delaying the leaf senescence process (Breeze, 2020; Bian et al, 2021; Li et al, 2021).
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