Abstract
Senescence is a genetically controlled mechanism that modifies leaf chemistry. This involves significant changes in the accumulation of carbon- and nitrogen-containing compounds, including asparagine through the activity of asparagine synthetases. These enzymes are required for nitrogen re-assimilation and remobilization in plants; however, their mechanisms are not fully understood. Here, we report how leaf curing—a senescence-induced process that allows tobacco leaves to dry out—modifies the asparagine metabolism. We show that leaf curing strongly alters the concentration of the four main amino acids, asparagine, glutamine, aspartate, and glutamate. We demonstrate that detached tobacco leaf or stalk curing has a different impact on the expression of asparagine synthetase genes and accumulation of asparagine. Additionally, we characterize the main asparagine synthetases involved in the production of asparagine during curing. The expression of ASN1 and ASN5 genes is upregulated during curing. The ASN1-RNAi and ASN5-RNAi tobacco plant lines display significant alterations in the accumulation of asparagine, glutamine, and aspartate relative to wild-type plants. These results support the idea that ASN1 and ASN5 are key regulators of asparagine metabolism during leaf curing.
Highlights
Senescence is the terminal phase of the leaf development process
By resolving the expression profile of asparagine synthetase genes (ASN) in tobacco during curing, we identified four genes ASN1-S, ASN1-T, ASN5-S, and ASN5-T that are upregulated during leaf curing
We eventually showed that Asn production during air curing results from the specific activity of asparagine synthetases encoded by ASN1 and ASN5
Summary
Plant cells initiate a range of responses that include a decline in photosynthetic capacity, reduction in chlorophyll and water content, transcriptional activation of the senescence-associated genes (SAGs), degradation of cellular structures, and, eventually, re-assimilation and remobilization of mineral nutrients and nitrogen-containing molecules from the leaves. Nitrogen is essential for growth and development. A major aspect of leaf senescence is the metabolic transition from anabolism to catabolism. During their growth and development, plants can primarily assimilate inorganic nitrogen present in soil nitrate, which is provided by fertilization or bacterial nitrification, for example [1]. The nitrogen from NO3 − and NH4 + can be incorporated into amino acids and stored in a range of proteins, including the Rubisco, chloroplastic glutamine synthetase (GS) GS2, and vegetative storage proteins (VSP) [3,4]
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