Effects Of Light And Darkness On The Photosynthetic System Development Of Cunninghamia Lanceolata Cotyledons

Light is one of the most important enviormental factors affecting plant growth and development.Angiosperm seedlings grown in complete darkness keep pale cotyledons because of no chlorophyll synthesis,while gymnosperm seedlings could develop green cotyledons because they have the ability to synthesize chlorophyll in complete darkness.With the materials of gymnosperm,the research on the process of gymnosperm seedlings development is able to understand the influence of light on seedlings morphygenesis、chloroplasts development as well as photosynthesis of gymnosprem.Meanwhile,research on the chloroplast,photosynthetic system development and seedlings morphogenesis of Chinese fir in light and dark conditions would provide reference for seedlings cultivation and timber production because Chinese fir is one of the most widely cultivated coniferous timber trees in southern China.The gymnosperm Cunninghamia lanceolata(Chinese fir)was studied in this research.We used laser confocal microscope(LSM)and transmission electron microscope(TEM)to observe the microstructure and ultrastructure of plastids.We compared the activeties of phosystems I(PSI)and II(PSII)between dark-and light-grown Chinese fir seedlings using Maxi-Imaging-PAM and DUAL-PAM-100 instruments.We also analyzed the accumulation of photosynthetic complexes in light-and dark grown Chinese fir cotyledons at the protein and mRNA levels by using Blue-native PAGE,SDS-PAGE,Western blot and Illumina sequencing technique.The results showed that Chinese could synthesize chlorophyll in darkness,which was completelly different from angiosperm.Compared with the light-grown seedlings,dark-grown plastids also developed grana and thylakoid membrane althougth not well with those in light.And prolamellar bodies(PLBS)also obsearved in this plastids,which was the typical character of etiochloroplast.Chlorophyll fluorescence parameters results indicated that etiochloroplasts had higher PSI-drived cyclic electron flow(CEF)rate than PSII-mediated photon harvesting.PSII recovery time of PSⅡ in Chinese fir was shorten than Arabidopsis,which benefit from the ATP produced in PSⅠ-driven CEF.Both light-and dark-grown seedlings could accumulate and assemble related protein complexes and super-complexes,and the express levels in dark were significantly lower than those in light.The quantities of of PsaC,D1 and CF1αproteins in dark-grown seedlings were about one-quarter of those in light-grown seedlings.The two proteins involved in CEF,NdhH and FNR,showed an obviously increased express in dark-grown seedlings to about half of them in light-grown seedlings.Furthermore,FNR located between free protein and compound with Cyt b6f.In light-grown seedlings,FNR were mostly seperated from the free protein portion,while FNR were seperated from super-complex with Cyt b6f in dark-grown seedlings.This was accordance with phtosynthetic results of higher activity of PSⅠ than PSⅡ suggesting the existence of FNR-and NDH-mediated CEF pathways in dark-grown seeedlings.Furthermore,it’s remarkable that there were different comformation on light harvesting protein complex(LHC)between light-and dark-grown seedlings.LHCⅡ had three forms in two samples:LHCⅡ monomers,LHCⅡ trimers and PSⅡ-LHCⅡ supercomplexes.In dark-grown thylakoid membranes,LHCⅡ was maintained mostly in free monomeric form.We infered that this situation was an adaption to long periods of darkness,which was accordance with the low Fv/Fm and high Fo of PSⅡ.And light conditions increased the assembling of LHCⅡ and PSⅡ,therefore the proportion of LHCⅡ within PSⅡ-LHCⅡ in light-grown thylakoid membranes were significantly more than in those of dark.The transcriptome sequencing data showed that light could promote the express levels of genes encoding LHC and others protein complexes related to photosynthesis.And light might affect nuclear genes more than it affects chloroplast genes.All in all,with the seedlings morphogenesis,chloroplast structure,photosynthetic system activity and the express level of related genes and protein complexes(super-complexes)were analyzed in this research,We revealed the phenomenon that light-independent photosynthetic appratus of Chinese fir had the ability to proceed photosynthesis.Meanwhile,we illuminated the mechanism of the light-denpendent and light-independent photosynthetic appratus development and electron transport of gymnosperm in early stage.This gave a basis for further study on the evolution of photosynthetic system of gymnosperm.