Abstract
Leaf senescence is an important biological process that contributes to grain yield in crops. To study the molecular mechanisms underlying natural leaf senescence, we harvested three different developmental ear leaves of maize, mature leaves (ML), early senescent leaves (ESL), and later senescent leaves (LSL), and analyzed transcriptional changes using RNA-sequencing. Three sets of data, ESL vs. ML, LSL vs. ML, and LSL vs. ESL, were compared, respectively. In total, 4,552 genes were identified as differentially expressed. Functional classification placed these genes into 18 categories including protein metabolism, transporters, and signal transduction. At the early stage of leaf senescence, genes involved in aromatic amino acids (AAAs) biosynthetic process and transport, cellular polysaccharide biosynthetic process, and the cell wall macromolecule catabolic process, were up-regulated. Whereas, genes involved in amino acid metabolism, transport, apoptosis, and response to stimulus were up-regulated at the late stage of leaf senescence. Further analyses reveals that the transport-related genes at the early stage of leaf senescence potentially take part in enzyme and amino acid transport and the genes upregulated at the late stage are involved in sugar transport, indicating nutrient recycling mainly takes place at the late stage of leaf senescence. Comparison between the data of natural leaf senescence in this study and previously reported data for Arabidopsis implies that the mechanisms of leaf senescence in maize are basically similar to those in Arabidopsis. A comparison of natural and induced leaf senescence in maize was performed. Athough many basic biological processes involved in senescence occur in both types of leaf senescence, 78.07% of differentially expressed genes in natural leaf senescence were not identifiable in induced leaf senescence, suggesting that differences in gene regulatory network may exist between these two leaf senescence programs. Thus, this study provides important information for understanding the mechanism of leaf senescence in maize.
Highlights
During the life-cycle, a leaf undergoes at least three different development phases: a functional increasing phase at the early growth stage; a full functionality phase at the mature stage; and a functionality decreasing phase at the senescence stage [1]
According to the physiological change of the ear leaf, the developing ear leaf after pollination can be divided into three classes: mature leaves [ML, 0–14 days after pollination (DAP)]; early senescent leaves (ESL, 15–24 DAP); and later senescent leaves (LSL, 25–30 DAP)
In ML, the chlorophyll content in the ear leaf remains at the maximum level, and the photosynthesis rate reaches its peak value
Summary
During the life-cycle, a leaf undergoes at least three different development phases: a functional increasing phase at the early growth stage; a full functionality phase at the mature stage; and a functionality decreasing phase at the senescence stage [1]. Leaf senescence is regarded as a massive operation of programmed cell death, it contributes critically to plant fitness by controlling the remobilization of micro- and macro-nutrients to growing and reproductive organs. Previous studies have elucidated the molecular mechanisms underlying leaf senescence through identification and characterization of senescence-associated genes (SAGs) and senescence-related mutants in plants such as Arabidopsis [5,6,7,8,9,10,11,12,13,14,15]. Genetic and genome-wide analyses suggest that the regulation of gene expression is complex during leaf senescence, with a large number of genes exhibiting differential expression patterns during senescence. At the onset of leaf senescence, a subset of SAGs is up-regulated and the majority of genes expressed in non-senescent leaves are down-regulated, including photosynthesis-related genes
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