The Key Role of Cyclic Electron Flow in the Recovery of Photosynthesis.pdf

Citation:Wang,S.;Li,W.;Wufuer,R.;Duo,J.;Pei,L.;Pan,X.The Key Roleof Cyclic Electron Flow in theRecovery of Photosynthesis in thePhotobiont during Rehydration of theLichen Cladonia stellaris.Plants 2023,12,4011.https:/doi.org/10.3390/plants12234011Academic Editor:Anja SchneiderReceived:20 October 2023Revised:24 November 2023Accepted:27 November 2023Published:29 November 2023Copyright:2023 by the authors.Licensee MDPI,Basel,Switzerland.This article is an open access articledistributedunderthetermsandconditions of the Creative CommonsAttribution(CC BY)license(https:/creativecommons.org/licenses/by/4.0/).plantsArticleThe Key Role of Cyclic Electron Flow in the Recovery ofPhotosynthesis in the Photobiont during Rehydration of theLichen Cladonia stellarisShuzhi Wang1,2,*,Wenfeng Li1,2,Rehemanjiang Wufuer1,2,Jia Duo1,2,Liang Pei1,2and Xiangliang Pan2,3,*1National Engineering Technology Research Center for Desert-Oasis Ecological Construction,Xinjiang Instituteof Ecology and Geography,Chinese Academy of Sciences,818 South Beijing Road,Urumqi 830011,China;(W.L.);(R.W.);(J.D.);(L.P.)2Xinjiang Key Laboratory of Environmental Pollution and Bioremediation,Xinjiang Institute of Ecology andGeography,Chinese Academy of Sciences,Urumqi 830011,China3Key Laboratory of Microbial Technology for Industrial Pollution Control of Zhejiang Province,College ofEnvironment,Zhejiang University of Technology,Hangzhou 310014,China*Correspondence:(S.W.);(X.P.)These authors contributed equally to this work.Abstract:Lichens are poikilohydric organisms and an important part of the ecosystem.They showhigh desiccation tolerance,but the mechanism of dehydration resistance still needs to be studied.The photosynthesis recovery of the photobiont in rehydrated lichen Cladonia stellaris after 11-yeardesiccation was investigated by simultaneously monitoring both photosystem I and II(PSI and PSII)activities.The responses of the photochemical efficiency and relative electron transport rate(rETR)ofPSI and PSII,and the quantum yield of the cyclic electron flow(CEF)were measured using a Dual-PAM-100 system.PSI recovered rapidly,but PSII hardly recovered in C.stellaris during rehydration.The maximal photochemical efficiency of PSII(Fv/Fm)was generally very low and reached aboutjust 0.4 during the rehydration.These results indicated that PSII had restored little and was largelyinactivated during rehydration.The quantum yield of PSI recovered quickly to almost 0.9 within 4 hand remained constant at nearly 1 thereafter.The results showed that the activation of the CEF in theearly stages of rehydration helped the rapid recovery of PSI.The quantum yield of the CEF made upa considerable fraction of the quantum yield of PSI during rehydration.A regulated excess energydissipation mechanism and non-photochemical quenching(NPQ)also recovered.However,the smallextent of the recovery of the NPQ was not enough to dissipate the excess energy during rehydration,which may be responsible for the weak activity of PSII during rehydration.The results indicatedthat both CEF and NPQ were essential during the rehydration of the photobiont in C.stellaris.Themethods used in the measurements of chlorophyll a fluorescence and P700+absorbance changesin this study provided a speedy and simple way to detect the physiological characteristics of thephotobionts of lichen during rehydration.This work improves our understanding of the mechanismbehind lichens desiccation tolerance.Keywords:P700+absorbance;chlorophyll a fluorescence;photosystem I;photosystem II;cyclicelectron flow;non-photochemical quenching;rehydration;lichen1.IntroductionLichens are symbiotic organisms composed of lichen-forming fungi(mycobionts)andcertain groups of cyanobacteria or green algae(photobionts)1,2.Lichens are an importantcomponent of the biological soil crusts of the soil surface in most arid and semi-arid regionsaround the world and are among the most important biotic components of the ecosystemsin these areas 3,4.As the number of climate change-induced drought events increases,agricultural land and food production are vulnerable to future water scarcity 5.In thisPlants 2023,12,4011.https:/doi.org/10.3390/plants12234011https:/ 2023,12,40112 of 14context,lichens can still be the guarantee of the maintenance and restoration of ecosystemproductivity under extreme climate changes.Photosynthetic organisms,including lichensand their photobionts,which are tolerant to desiccation,have been widely investigated toimprove drought tolerance in crop plants 6.It has been reported that lichens are desiccation-tolerant organisms that can survivein harsh environments where higher plants cannot survive 7,8.Lichens are often foundin a variety of extremely dry habitats such as rock surfaces,the Antarctic cold desert andhot deserts 9,10.In these habitats,the water supply is not delivered by rain but ratherin the form of fog,dew or humidity 8,11.Lichens are poikilohydrous and exposed torepeated desiccation/rehydration cycles 9.Consequently,lichens may be exposed tohigher levels of solar radiation and desiccation,and most lichens can tolerate desiccationand remain viable for months 12.During the desiccation/rehydration cycles,lichensand their photobionts use a variety of ways to cope with various stresses.These stressesinclude osmotic stress and oxidative stress,which are related to sudden changes in wateravailability 13.Dehydrated lichens can restore their photosynthetic activity afterwetting 14.The microalgae isolated from a Mediterranean fruticose epiphytic lichenthat adapted to xeric habitats,i.e.,Trebouxia sp.(TR9),could recover their photosyntheticactivity after desiccation for 1,2 and 3 months 15.The understanding of desiccation tolerance of lichens and lichen-forming algae wasreviewed in a previous study,which showed the constitutive mechanisms and the inductionofprotectionmechanismsinthedesiccationtoleranceoflichens2.However,thesummarizeddatarevealedthattheknowledgeaboutdesiccationtolerancemechanismsinlichenswasmuchscarcer than in other organisms such as bryophytes or vascular resurrection plants.Someprevious studies showed that lichens can become photosynthetically active after rehydrationwith water vapor alone or with liquid water 11,1618.Those studies mainly discussedthe recovery of the activity of photosystem II(II),which was often tested by measuringchlorophyll a fluorescence 19,20.Some dehydration and rehydration experiments werecarried out with algae isolated from lichens 6,21.The findings with Trebouxia,the mostabundant chlorophytic photobiont in lichen,indicated that the slow-dried and rapid-driedalgae showed significant differences in the recovery of photosynthetic activity.Slow-driedalgae restored PSII electron transport to a higher value after rehydration.Measurementof the absorbance change in P700 of Trebouxia showed that desiccation did not affect PSIfunctionality 21.However,only a small number of species of lichens and bryophytes haveactually been tested for desiccation tolerance,and the mechanisms for testing desiccationtolerance are still limited 11.Additionally,the response of photochemical efficiency andthe relative electron transport rate(rETR)of photosystem I(PSI)during the desiccation andrehydration of lichens are still unclear.Little is known about the differences between therecovery potential of PSII and PSI in the rehydration process and the underlying mechanisms.It was proposed that the cyclic electron flow(CEF)around PSI functioned in adap-tation to environmental stress in higher plants 22,23.Gao and Wang(2012)found thatthe CEF in Porphyra yezoensis(Bangiales,Rhodophyta)played a significant physiologicalrole during desiccation and rehydration 24.Beckett et al.(2023)suggested that dual PAMcould effectively measure alternative electron flows in lichen photobionts 25,and therebyit could be used to study how lichens coped with the desiccation and rehydration condi-tions.The results showed that the CEF appeared to be much higher in lichen photobiontsthan angiosperms 25.However,whether the CEF is activated during rehydration afterdesiccation and the physiological role of CEF in the photobiont of lichen remain unclear.Although the existing literature shows that most lichens can tolerate desiccation andremain viable for months 12,only a small number of species of lichens have actually beentested for desiccation tolerance.The time limit for lichens to withstand dehydration wasnot well studied.This work will explore whether lichens have the possibility to restoretheir photosynthetic activity after long-term dehydration for more than 10 years.In ourpreliminary experiments,we found that Cladonia species like Cladonia stellaris could toleratedesiccation for months.We postulate that symbiotic algae can react quickly in the processPlants 2023,12,40113 of 14of rehydration from drought to restore the physiological activity.The rapid recovery ofphotosynthetic organisms can promote the rehydration of lichens and provide their fungalenergy substances,which plays an important role.The objective of this study is to testthe recovery of the activities of the photosystems and the physiological role of CEF inthe photobiont of lichen during rehydration after long-term desiccation.The thalli ofCladonia stellaris(Opiz)Pouzar&V ezda,which had been kept dry for 11 years,were usedto perform the rehydration experiment.The response of the photochemical efficienciesand rETRs of PSI and PSII of the photobiont were measured during rehydration with thehelp of a Dual-PAM-100 system.The CEF was also measured to detect whether it wasactivated and to reveal its physiological role during rehydration.The results showed thatPSI recovered rapidly during rehydration,but PSII hardly recovered in the experiment.Thequick activation of the CEF played an important role in the recovery of the activity of PSI.2.Results2.1.Scanning Electron Microscopy(SEM)ObservationSEM images showed the microstructure of the surface and cross-section of the thalliof C.stellaris after rehydration for 21 h(Figure 1).The structure of the thalli was incom-pact,and the main part of the thalli was constructed by the mycobiont or fungal hyphae.Figure 1a displays the overall morphology of the surface of a lichen thallus(bar=10m).The distribution of fungal hyphae and photobionts is shown on the surface of the thalli(Figure 1a,b).A cross-section of the thalli is also shown,where many pores are located(Figure 1c).The structure of the pores can be seen more clearly in Figure 1d.The cortex andmedulla of the thalli are presented(Figure 1c,d).In a cross-section of the thalli,the internalsurfaces of the lumens of fungal hyphae are shown(Figure 1d,bar=2 m).Plants 2023,12,x FOR PEER REVIEW 3 of 15 was not well studied.This work will explore whether lichens have the possibility to re-store their photosynthetic activity after long-term dehydration for more than 10 years.In our preliminary experiments,we found that Cladonia species like Cladonia stellaris could tolerate desiccation for months.We postulate that symbiotic algae can react quickly in the process of rehydration from drought to restore the physiological activity.The rapid re-covery of photosynthetic organisms can promote the rehydration of lichens and provide their fungal energy substances,which plays an important role.The objective of this study is to test the recovery of the activities of the photosystems and the physiological role of CEF in the photobiont of lichen during rehydration after long-term desiccation.The thalli of Cladonia stellaris(Opiz)Pouzar&Vzda,which had been kept dry for 11 years,were used to perform the rehydration experiment.The response of the photochemical efficien-cies and rETRs of PSI and PSII of the photobiont were measured during rehydration with the help of a Dual-PAM-100 system.The CEF was also measured to detect whether it was activated and to reveal its physiological role during rehydration.The results showed that PSI recovered rapidly during rehydration,but PSII hardly recovered in the experiment.The quick activation of the CEF played an important role in the recovery of the activity of PSI.2.Results 2.1.Scanning Electron Microscopy(SEM)Observation SEM images showed the microstructure of the surface and cross-section of the thalli of C.stellaris after rehydration for 21 h(Figure 1).The structure of the thalli was incompact,and the main part of the thalli was constructed by the mycobiont or fungal hyphae.Figure 1a displays the overall morphology of the surface of a lichen thallus(bar=10 m).The distribution of fungal hyphae and photobionts is shown on the surface of the thalli(Figure 1a,b).A cross-section of the thalli is also shown,where many pores are located(Figure 1c).The structure of the pores can be seen more clearly in Figure 1d.The cortex and medulla of the thalli are presented(Figure 1c,d).In a cross-section of the thalli,the internal surfaces of the lumens of fungal hyphae are shown(Figure 1d,bar=2 m).(a)(b)Plants 2023,12,x FOR PEER REVIEW 4 of 15 (c)(d)Figure 1.SEM images of the microstructure of thalli of C.stellaris.(a)Image of the surface of thalli;(b)the mycobionts or fungal hyphae on the surface of the thalli;(c)the structure of the surface and a cross-section;(d)an enlarged image of the cross-section.2.2.Maximal Photochemical Efficiency of PSII(Fv/Fm)Pieces of thalli of C.stellaris were rehydrated to test the recovery of photosynthetic activity.Fv/Fm increased with the rehydration time(Figure 2).After rehydration for 2,4 and 6 h,Fv/Fm reached 0.04,0.18 and 0.33,respectively.Fv/Fm recovered rapidly during the first 6 h and increased slightly thereafter.Fv/Fm was generally very low(below 0.4)over the entire experimental time,which was much lower than that of other photosynthetic organisms.Figure 2.Maximal photochemical efficiency of PSII(Fv/Fm)during rehydration.Data were detected after 20 min dark-adaptation during each measurement(n=4).2.3.Quantum Yields of Two Photosystems and CEF during Rehydration The values of Y(I),Y(II)and Y(CEF)at the beginning of the rapid light curve mode(RLC mode)(where the PAR intensity of the actinic light was 30 mol m2 s1)changed differently during rehydration(Figure 3).The measurements in RLC mode were con-ducted with an actinic light with increasing intensity(30,37,46,77,119,150,240,363,555 and 849 mol m2 s1).A saturating pulse was applied after each period of actinic light to determine the maximum fluorescence and P700+signals.Parameters such as quantum yields and electron transport rates(ETRs)are calculated based on measurements of these signals.Y(I)recovered quickly to almost 0.9 within 4 h and remained constant at nearly 1 thereafter.Y(II)increased a little and reached about 0.4 at 21 h.Y(CEF)rapidly increased during the first 4 h of rehydration and then decreased a little with the rehydration time.Figure 1.SEM images of the microstructure of thalli of C.stellaris.(a)Image of the surface of thalli;(b)the mycobionts or fungal hyphae on the surface of t