Something new in NPQ

Here’s something new under the sun related to plants’ light problem: a new protein and NPQ component (and maybe a pathway)

NPQ pathways (listed on the right) siphon excess light energy away from photosynthesis.

NPQ pathways (listed on the right) siphon excess light energy away from photosynthesis. Brooks et al describe a new component SOQ1 that doesn’t exactly fit within known NPQ pathways.

Non-photochemical quenching (NPQ) is the term for the collection of strategies plants use to quench excess light energy by diverting it away from the photosystems. If you didn’t know this, check out yesterday’s post. NPQ can be divided into several categories: qE, qZ, qT, and qI. These are the known pathways that act on distinct timescales and require certain protein and chemical factors. Why is it important? Slow relaxation of NPQ is safe for plants, but reduces their overall productivity. If we knew more about these systems especially the slowest recovering ones, then plants could finally tap into productivity gains of ~13 – 32%. So we need to learn more about all of these components.

Brooks and coworkers describe the identification and characterization of a novel NPQ component SOQ1 (Suppressor of quenching1). Without a functional SOQ1, plants have greater NPQ under high light conditions that is very slow to relax (i.e. the NPQ condition sticks around for longer meaning a slower return to photosynthesis). The absence of SOQ1 affects only NPQ with no ill effects for photosynthesis. Using different treatments and mutants of other known NPQ pathway components, the authors conclude that SOQ1 isn’t part of any previously characterized NPQ strategy. SOQ1 is a large protein found in the thylakoid membrane with multiple functional domains. The thioredoxin-like domain is locate on the lumenal side of the membrane and is required for function. It is likely that SOQ1 has lumenal target proteins to which it donates electrons in order to prevent the induction of the slowly-reversible NPQ.

wild type: SOQ1 (blue) normally donates electrons to unknown targets in the lumen (purple) allowing for maximal photosynthesis.  soq1: When SOQ is absent, lumenal targets become oxidized causing some light energy to be dissipated (red arrow) instead of funneling into photosynthesis.

wild type: SOQ1 (blue) normally donates electrons to unknown targets in the lumen (purple) allowing for maximal photosynthesis.
soq1: When SOQ is absent, lumenal targets become oxidized causing some light energy to be dissipated (red arrow) instead of funneling into photosynthesis. This state persists for longer than normal.

Here’s how the method breaks down:

Hypothesis: A genetic suppressor screen will identify new components of the NPQ pathway.

Experiment: Brooks and coauthors use a genetic screen designed to identify a mutant of a new NPQ component. They treat the resulting mutant with additional chemicals and combine the new mutant with previously characterized NPQ mutants to place it within a known NPQ mechanism.

Results: The mutant suppressor of quenching1 (soq1) was identified and has increased NPQ relative to wild-type plants. This NPQ is not at the expense of reduced photosynthetic ability. The NPQ increase is still observed when soq1 is combined with other NPQ mutants. The extra NPQ induced in the soq1 mutant recovers very slowly. The SOQ1 protein is a thylakoid membrane protein with a thioredoxin-like domain and both of these domains are required for its function.

Conclusion: The mechanism of NPQ that is normally suppressed by the protein SOQ1 is unrelated to known pathways. It represents a form of NPQ which recovers very slowly (i.e. can’t switch back to photosynthesis as quickly).

Think Ahead: Future experiments will determine what other components are involved in SOQ1-mediated NPQ. Specifically, researchers want to know what the lumenal targets for SOQ1 reduction are and how these targets result in quenching.

A final note on NPQ… NPQ is something measureable using chlorophyll fluorescence in the lab, but so far mutants without NPQ do not show significant differences at the whole plant level in the lab. Because the NPQ strategies help plants deal with fluctuating light conditions, NPQ mutants would likely suffer when subjected to inconsistent light patterns because they cannot adjust as quickly as normal plants.

Johnna

Brooks, M. D., Sylak-Glassman, E. J., Fleming, G. R., and Niyogi, K. K. (2013) A thioredoxin-like/beta-propeller protein maintains the efficiency of light harvesting in Arabidopsis. Proc Natl Acad Sci U S A 110, E2733-2740

Sorry guys, this one’s behind a paywall. If you have access, I do recommend reading it if you are in the plant biology or photosynthesis field because it is well-written and many NPQ papers can be very confusing.

UPDATE FYI: It is PNAS policy to make all papers freely available after 6 months. So mark your calendar for next January or February and it should be available to you.

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