Abstract
In plants, the vegetative to reproductive phase transition (termed bolting in Arabidopsis) generally precedes age‐dependent leaf senescence (LS). Many studies describe a temporal link between bolting time and LS, as plants that bolt early, senesce early, and plants that bolt late, senesce late. The molecular mechanisms underlying this relationship are unknown and are potentially agriculturally important, as they may allow for the development of crops that can overcome early LS caused by stress‐related early‐phase transition. We hypothesized that leaf gene expression changes occurring in synchrony with bolting were regulating LS. ARABIDOPSIS TRITHORAX (ATX) enzymes are general methyltransferases that regulate the adult vegetative to reproductive phase transition. We generated an atx1, atx3, and atx4 (atx1,3,4) triple T‐DNA insertion mutant that displays both early bolting and early LS. This mutant was used in an RNA‐seq time‐series experiment to identify gene expression changes in rosette leaves that are likely associated with bolting. By comparing the early bolting mutant to vegetative WT plants of the same age, we were able to generate a list of differentially expressed genes (DEGs) that change expression with bolting as the plants age. We trimmed the list by intersection with publicly available WT datasets, which removed genes from our DEG list that were atx1,3,4 specific. The resulting 398 bolting‐associated genes (BAGs) are differentially expressed in a mature rosette leaf at bolting. The BAG list contains many well‐characterized LS regulators (ORE1, WRKY45, NAP, WRKY28), and GO analysis revealed enrichment for LS and LS‐related processes. These bolting‐associated LS regulators may contribute to the temporal coupling of bolting time to LS.
Highlights
Leaf senescence (LS) is the sequential death of older leaves, one-by-one, as the plant matures, while whole plant senescence is the simultaneous death of all leaves at the end of the growing season in monocarpic species (Nooden et al, 1997)
We have identified 398 bolting time-associated genes (BAGs) expressed in mature leaf 6 that change expression at the time of bolting. 202 of these BAGs are known to be associated with leaf senescence (LS) (Zhonghai Li et al, 2020)
We identify a set of genes that change expression during bolting and dark-induced leaf senescence (DILS), but are not in the Leaf Senescence Database (LSD), meaning some may be novel early regulators of LS
Summary
Leaf senescence (LS) is the sequential death of older leaves, one-by-one, as the plant matures, while whole plant senescence is the simultaneous death of all leaves at the end of the growing season in monocarpic species (Nooden et al, 1997). A visual hallmark of LS is leaf yellowing, caused by chlorophyll degradation (Ougham et al, 2008; Tamary et al, 2019). During these processes, nitrogen (most commonly in the forms of nitrate, asparagine, and glutamine) and other macromolecules are recycled from dying leaves (sources) and relocated to growing tissues (sinks), including the reproductive organs (Havé et al, 2017). Histone Deacetyltransferase 6 (HDA6) regulates both flowering time and LS (Keqiang Wu, Lin Zhang, Changhe Zhou, Chun-Wei Yu, 2008). H3K9 acetylation via HAC1 (HISTONE ACETYLTRANSFERASE-1) promotes LS, and the transcription factor (TF) ERF022, a potential HAC1 target, positively regulates LS (Hinckley et al, 2019). Stress, defense, and LS signaling overlap and some TFs are known to bridge stress and LS signaling (SAG113, NAP, WRKY53) (Asad et al, 2019; Nir Sade, María del Mar Rubio-Wilhelmi, Kamolchanok Umnajkitikorn, 2018; Woo et al, 2013)
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