Understanding the molecular regulation of senescence in Glycine max
Senescence is the natural process of aging and is a crucial factor in the source-sink relationship. Following germination, the developing soybean seedling relies on the nutrient reserves stored in the cotyledons to sustain heterotrophic growth. A leaf undergoes several important changes during development. As the newly initiated leaf expands and gains photosynthetic competency, it undergoes a transition from nutrient sink to source. Later it undergoes senescence, where nutrients that remain in the leaf are mobilized and exported to new sinks such as the seed. The time and length of senescence is crucial to seed quality and overall plant yield. Plants that do not produce a sink, either by mechanically depodding or genetic sterility have been shown to have different physiological features including increased starch and nitrogen composition in the leaves and a delayed senescence (or a “stay green” phenotype). Thus understanding the molecular regulation of senescence and resource allocation in soybean can help optimize the growing/photosynthesizing period and achieve higher yields. Expression profiling by RNA sequencing (RNAseq) was used to compare levels of gene expression in leaves and cotyledons of normal soybean, as well as depodded and male sterile trifoliate leaves to identify transcriptional changes throughout development. Over 38,000 transcripts were found to be expressed during the course of both leaf and cotyledon development. Differentially expressed genes were placed into developmental profiles. Analysis of the enrichment of biological functions within genes sharing common expression profiles highlights the main processes occurring within these defined temporal windows of normal leaf and cotyledon development. Genes expressed during the later stages of both tissues shared common functions, suggesting that the mechanisms of senescence are conserved between tissues. Genes associated with photosynthetic processes and light responses decreased during the later stages of development, while genes associated with nutrient remobilization increased. Over 1,000 genes were identified with predicted regulatory functions that may have a role in control of leaf or cotyledon senescence. Transcription factor families such as the WRKY, NAC and GRAS were shown to be important regulators of plant senescence in both leaves and cotyledons. Gene expression profiles from normal leaf development were compared with those from sterile plants and leaves from plants where pods were continuously removed. In addition to observing a deferral of the expression of senescence associated genes, we identified genes that were not expressed during normal leaf development but highly expressed in the sterile and depodded plants. These changes highlighted several metabolic pathways that were involved in distinct modes of resource partitioning in the “stay green” leaves. A complete understanding of these pathways will enable the development of soybean lines that realize their maximum yield potential.
Ma, Purdue University.
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