Editing Isoetes (section)

==Biochemistry and genetics==
Quillworts use [[crassulacean acid metabolism]] (CAM) for carbon fixation. Some aquatic species do not have [[stoma]]ta and the leaves have a thick cuticle which prevents CO<sub>2</sub> uptake, a task that is performed by their hollow roots instead, which absorb CO<sub>2</sub> from the sediment.<ref>{{cite book |url=https://books.google.com/books?id=KFExDwAAQBAJ&q=%22Isoetes+leaves+lack+stomata+pores%2C+as+do+most+submerged+plants%22&pg=PA108 |title=Ecology of High Altitude Waters]|isbn=9780191056666|last1=Jacobsen|first1=Dean|last2=Dangles|first2=Olivier|date=18 August 2017|publisher=Oxford University Press }}</ref> This has been studied extensively in ''Isoetes andicola''.<ref name="book - form&function" /> [[Crassulacean acid metabolism|CAM]] is normally considered an adaptation to life in arid environments to prevent water loss with the plants opening their stomata at night rather than in the heat of the day. This allows CO<sub>2</sub> to enter and minimises water loss. As mostly submerged aquatic plants, quillworts do not lack water and the use of CAM is considered to avoid competition with other aquatic plants for CO<sub>2</sub> during daytime.<ref name="Haas2021-12">{{cite web |last1=Haas |first1=Michael J. |title=Quillwort genome highlights divergences in aquatic CAM photosynthesis |url=https://globalplantcouncil.org/quillwort-genome-highlights-divergences-in-aquatic-cam-photosynthesis/ |website=The Global Plant Council |date=2 December 2021 |access-date=29 December 2021}}</ref>

The first detailed quillwort genome sequence, of ''[[Isoetes taiwanensis|I. taiwanensis]]'',<ref name="Wickell et al 2021">{{cite journal |last1=Wickell |first1=David |last2=Kuo |first2=Li-Yaung |last3=Yang |first3=Hsiao-Pei |last4=others |first4=and 11 |title=Underwater CAM photosynthesis elucidated by Isoetes genome |journal=Nature Communications |date=2021 |volume=12 |issue=1 |page=6348 |doi=10.1038/s41467-021-26644-7 |pmid=34732722 |pmc=8566536 |bibcode=2021NatCo..12.6348W }}</ref> showed that there were differences from CAM in terrestrial plants. CAM involves the enzyme [[phosphoenolpyruvate carboxylase]] (PEPC) and plants have two forms of the enzyme. One is normally involved in photosynthesis and the other in central metabolism. From the genome sequence, it appears that in quillworts, both forms are involved in photosynthesis. In addition, [[Circadian rhythm|circadian]] expression of key CAM pathway genes peaked at different times of day than in angiosperms.<ref>[https://btiscience.org/explore-bti/news/post/secrets-of-quillwort-photosynthesis-could-boost-crop-efficiency/ Secrets of quillwort photosynthesis could boost crop efficiency]</ref> These fundamental differences in biochemistry suggest that CAM in quillworts is probably another example of [[convergent evolution]] of CAM during the more than 300 million years since the genus diverged from other plants. However, they may also be because of differences between life in water and in the air.<ref name="Wickell et al 2021" /> The genome sequence also provided two insights into its structure. First, genes and [[non-coding DNA|repeated non-coding regions]] were fairly evenly distributed across all the [[chromosomes]]. This is similar to genomes of other non-seed plants, but different from the seed plants ([[angiosperms]]) where there are distinctly more genes at the ends of chromosomes. Secondly, there was also evidence that the whole genome had been duplicated in the ancient past.<ref name="Wickell et al 2021" />

There are species that switch from CAM to C3 photosynthesis when they go from being submerged in water to living terrestrially, and develop stomata on their leaves. Some species (''I.&nbsp;palmeri'', ''I.&nbsp;lechleri'' and ''I.&nbsp;karsteni''), even under aerial conditions, rarely form stomata, and in some cases (''I.&nbsp;triquetra'' and ''I.&nbsp;andina'') appear to have completely lost the ability to produce stomata.<ref>[https://books.google.no/books?id=iv1MEAAAQBAJ&pg=PA192&dq=macrophytes+CAM+C3+stomata+palmeri+andina&hl=no&newbks=1&newbks_redir=0&sa=X&ved=2ahUKEwjG3KXtsKuLAxWIFxAIHc5lAKUQ6AF6BAgFEAM#v=onepage&q=macrophytes%20CAM%20C3%20stomata%20palmeri%20andina&f=false Permian Extinctions]</ref>