Drought Tolerance Of Nostoc Flagelliforme And Its Physiological Recovery Upon Rehydration

Nostoc flagelliforme (Berk&Curtis) is a terrestrial filamentous nitrogen-fixation cyanobacterium. In China, it mainly distributes in the arid or semiarid Northern and Northwestern. N. flagelliforme had good drought tolerance, radiation tolerance and salt tolerance abilities, so it is a good material for investigating the mechanism of stress tolerance of cyanobacteria. As far as, the investigation mainly concentrated on the relationships between the fatty acids, compatible solutes or the enzyme for cleaning out the ROS and the drought tolerance ability of N. flagelliforme. But there is no report about the genes which participate in resisting the drought stress. The EPS can become a barrier between the cyanobacteria cell and the near environment, it can protect the cells from drought stress, UV radiation and heavy metals toxicity and etc. It does not know whether the different drought tolerance abilities of cyanobacteria which live in different niches have relationship with EPS or not. Potassium plays an important role in regulating the cell volume, protein synthesis, intracellular pH and preventing the Na+entrance and etc. It does not know whether potassium can alleviate the ammonium toxity and impact the nitrogen-fixation recovery of N. flagelliforme or not. If these problems are solved, it will help us know the mechanisms of stress tolerance of N. flagelliforme, and it will enrich the knowledge about the functions of potassium on cyanobacteria.After dry storage for3years, the photosynthetic recovery of N. flagelliforme was potassium limited. When samples were rehydrated in high K+medium, the K+absorption rate and content was higher than that in low K+medium. The relative Fv/Fm24h EC50value of NH4+in low and high K+medium was respectively22.17mM and64.35mM. These suggested that high concentrations of K+could alleviate the inhibitory effect of NH4+upon the photosynthetic recovery of samples. After adding10mM TEACl into high K+medium and low K+medium, the relative Fv/Fm24h EC50value of NH4+was respectively increased to70.78mM and46.34mM. This indicated that NH4+might enter into N. flagelliforme cells via the K+channel. Furthermore, NH4+could decrease K+absorption in high-K+medium.When N. flagelliforme were rehydrated in-K+medium or the time of samples rehydrated in BG11medium for short time, the photosynthetic oxygen evolution could not recover the level of normal and it impacted the electron transport from PS Ⅱ to PS I while it did not influence the electron transport from the QA to QB. The nitrogenase activity could not be detected when N. flagelliforme were rehydrated in-K+medium. But once the samples were firstly rehydrated in BG11medium for different time, then were transferred into BG11medium or-K+medium for different time, the nitrogenase activity were recovered gradually along with the time of sample rehydrated in BG11medium. Furthermore, the recovery of transcripts of nitrogenase related genes (nifD, nifH and nifK) and the photosynthesis related genes (psaB, petB and psbD) had relationship with potassium. In addition, the recovery of nitrogenase activity had plus relation with photosynthetic oxygen evolution. All of these suggested that potassium might affect nitrogen fixation by photosynthesis.When three Nostoc were rehydrated in BG11media supplemented with different concentrations of sorbitol, the drought tolerance ability sequence were N. flagelliforme, Nostoc commune and then Nostoc sphaeroides. Once the EPS were depleted from the three Nostoc, the drought tolerance abilities of them were the same. In addition, we had also analyzed the drought tolerance abilities of different sizes of artifical culture N. sphaeroide. The drought tolerance abilities might be better if the larger the size of N. sphaeroides. However, the EPS-depleted of different size of artifical culture N. sphaeroides drought tolerance abilities had no difference. All of these suggested that EPS might play an important role in drought tolerance abilities among the three Nostoc. The polysaccharide contents of three Nostoc had no difference, but the polysaccharide contents might be more if the larger the size of N. sphaeroides. These might indicated that the compositions of sheath (EPS) might have correlation with the drought tolerance abilities.We had cloned two genes which might function in stress tolerance of N. flagelliforme and investigated their biological functions. When N. flagelliforme was damaged by salt stress, the transcript and expression levels of drnfl and drnf8became higher. Furthermore, when N. flagelliforme were damaged by drought stress, the transcript and expression levels of drnf8became higher, but the transcript and expression levels of drnfl had no change. The DRNF1could hydrolate ATP, CTP and GTP. The DRNF8might be a DNA methyltransferase. drnfl/drnf8can enhance Synechocystis sp. PCC6803salt stress tolerance abilities, and drnf8can function better than drnfl. After comparing the transcript abundances of Synechocystis sp. PCC6803salt-related genes between wild type and the PpetE-drnf8and P4rbcL-drnfl Synechocystis sp. PCC6803, we found that in order to enhance the Synechocystis sp. PCC6803salt stress tolerance ability, drnf1and drnf8might enhance the glucosylglycerol synthesis and transport plus Na+extrusion, these were still needed more direct evidences to prove them.