COP9 Signalosome-Regulated Proteolysis: Turning Off Ascorbic Acid Synthesis When the Lights Go Out

Light provides both an essential source of energy and a major source of abiotic stress, as intense light can be a brutal onslaught of solar radiation, with its accompanying oxidative stress and heat. Good morning; better take your vitamins. Indeed, plants, which make rather than take their vitamins, use ascorbic acid (AsA), or vitamin C, as a key antioxidant in photoprotection (reviewed in Smirnoff, 2000). Plants synthesize AsA via multiple pathways and do seem to make their vitamins in the morning, as AsA contents increase during the day and decrease at night.

The mechanism of this change in AsA synthesis involves several levels of regulation. Light causes transcriptional induction of some AsA biosynthetic enzymes, but the decreased levels in the dark appear to be posttranscriptionally regulated. Wang et al. (pages 625–636) examine the mechanisms controlling these changes and find that in seedlings, AsA levels and levels of the AsA biosynthetic enzyme GDP-mannose pyrophosphorylase (VTC1) are higher during the day. To identify the factors regulating VTC1 levels, the authors used a yeast two-hybrid screen to find proteins that interact with VTC1. This screen identified the photomorphogenic factor COP9 signalosome (CSN) subunit CSN5B, and the authors confirm by gel filtration, bimolecular fluorescence complementation, and coimmunoprecipitation that VTC1 interacts with CSN5B and associates with the CSN complex. The csn5b mutants showed substantially increased AsA contents (see figure) in darkness, and double mutants with partial loss-of-function vtc1 alleles showed partially restored AsA levels, indicating that CSN5B acts to decrease AsA. Further examination showed that CSN5B affects AsA contents by targeting VTC1 for degradation via the 26S proteasome pathway in the dark.

CSN5B is required for light-regulated changes in AsA contents. AsA contents of mutant seedlings under regular dark-light cycling (left) and under darkness (center) or continuous light (right). Mean values are shown, and error bars represent sd. Asterisks represent significant differences from control by Student’s t test. (Reprinted from Wang et al. [2013], Figure 5.)

Why do plants go to all this trouble to reduce AsA? High AsA can be beneficial in stress responses; for example, the authors found that csn5b mutants, which have high AsA contents even in the dark, also had higher salt tolerance and reduced sensitivity to oxidative stress generated by treatment with methyl viologen. However, the existence of a mechanism to reduce AsA content in the dark indicates a possible metabolic cost to maintaining high levels of AsA. Indeed, accumulating examples, such as protein degradation limiting the jasmonate defense response, indicate that limiting a response or adaptation may be crucial for the plant to use its limited metabolic resources most efficiently. Therefore, these findings provide both an interesting avenue for future research and another cautionary tale in the search for a simple solution to engineering broad stress tolerance in plants.