An explosive mix: C₄, C₃, _C2 and CCM

As if the task of explaining the details of the ‘normal’ C₃ Calvin Cycle of photosynthesis (P/S) to our students isn’t hard enough, we also need to appraise them of C₄ P/S  – with its spatial separation of initial CO₂ fixation into organic acids in mesophyll cells and its subsequent release and re-fixation via the enzyme Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase)  into the photosynthetic Calvin Cycle proper within bundle sheath cells*. As testing and trying as that is, nature always has to go one ‘better’, and ‘spoil’ things. So, the fin-de-millennial recognition of a variant of this C₄ P/S in which initial CO₂ fixation into 4-carbon acids and its subsequent release and re-fixation into the Calvin Cycle of C₃ P/S takes place within a single cell is kind of unwelcome (no matter how fascinating it is!). Well, anyway, it exists – in such higher plants as Suaeda (Borszczowia) aralocaspica, Bienertia cycloptera, B. sinuspersici and B. kavirense, all in the Chenopodiaceae (now within the Amaranthaceae) – so we need to get over it, and try and understand it. And that’s what Samantha Stutz et al. have been doing. Although these plants perform spatial separation of the two CO₂ fixation events within a single mesophyll cell, they do so using two distinct – dimorphic – chloroplasts. Already known is that light is necessary for development of the dimorphic chloroplasts in cotyledons in B. aralocaspica. In the dark they only have a single structural plastid type (which expresses Rubisco): light induces formation of dimorphic chloroplasts from the single plastid pool, and structural polarization leads to the single-cell C₄ syndrome. The aim of Stutz et al.’s study was to determine how growth under limited light affects leaf structure, biochemistry and efficiency of the single-cell CO₂-concentrating mechanism. Overall, the team found that the fully developed single-cell C₄ system in B. sinuspersici is robust when grown under ‘moderate light’. Where might this sort of work be going? Well, whilst it is interesting for its own sake – the pure pursuit of knowledge – it has a more applied dimension too. Central to all of this single-cell photosynthetic biology and biochemistry is the concept of CCM, carbon-concentrating mechanisms, whereby levels of CO₂ are increased in the vicinity of Rubisco so that it favours photosynthesis – CO₂-fixation – over photorespiration (so-called C₂ photosynthesis) which uses O₂ as substrate and consequently reduces photosynthetic efficiency. Well, in bids to replicate some of the greater photosynthetic efficiency of C₄ plants (largely by virtue of their diverse CCMs…), an attractive notion is to engineer various forms of CCM into C₃ crop plants. This approach is exemplified in the work of Mitsue Miyao et al., where they attempted to exploit enzymes of the facultative C₄ aquatic plant Hydrilla verticillata (which engages in single-cell C₄ P/S) to convert rice from its typical C₃ P/S into a single-cell C₄ photosynthesiser. Although they didn’t achieve their goal (and it’s good to know that ‘negative’ results can still be published!), their article is an interesting and soul-bearing account of the lessons learned in this work. As we continue our quest for that elusive boost in photosynthetic yield, we’ll no doubt continue to exploit any biochemical variant on the photosynthetic theme that nature displays. Which begs the question: how many more variants exist amongst the 325,000 species of flowering plants (let alone all the algae and other members of the plant kingdom)? Seems like we need more plant anatomists, plant biochemists, plant physiologists – as well as plant taxonomists (see my last post on this blog) – after all!

 

* That’s C₄ P/S as opposed to CAM (Crassulacean acid metabolism), which is also a version of C₄ P/S but which involves temporal separation of the same two carbon-fixation events in plants such as pineapple, cacti and agave. However, CAM is hardly ever referred to as C₄ P/S because the all-powerful Zea Supremacy lobby has commandeered the term for that spatially separated C₄ version found in plants such as maize… but don’t get me started on that!

 

[Intriguingly, and in addition to its dimorphic chloroplasts, Suaeda aralocaspica has dimorphic seeds, which exhibit distinct differences in dormancy and germination characteristics. Now, they say that things come in threes, so what’s the third dimorphy about this iconic species…? – Ed.]