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D-pinitol, a key role in the regulation of osmotic pressure of Lotus japonicus nodule infected cells

News from our newspaper (Reporter Zhu Hanbin, Correspondent Zhou Fei) Tian Lu and Liu Leru, Ph.D. students of the Energy Plant Research Group of the South China Botanical Garden, Chinese Academy of Sciences, under the guidance of their supervisors, Associate Researcher Chen Yaping and Researcher Wu Guojiang, discovered that D-pinitol transporter can regulate the development of nodules by affecting the osmotic pressure of nodule infected cells. The related research was recently published in Journal of Experimental Botany. At the same time, the journal published the insights of this research from Oxford University professor Philip. S. Poole, saying that D-pinitol was confirmed to be the key role in maintaining the balance of osmotic pressure in nodule infected cells.

Plant-microbe symbiotic nitrogen fixation can provide a directly available nitrogen source for plant growth and play an important role in maintaining global food and ecological security. The formation of the symbiosis system is a complex process that requires the coordination of the metabolic systems of the host plant and the rhizobia. In this process, the peribacteroid membrane (PBS) acts as a physical barrier to maintain the stability of nitrogenase activity and is responsible for the exchange of substances and signal molecules between the host plant and the rhizobia.

After the 14 polyol transporters (LjPLT1-LjPLT14) of Lotus japonicus were isolated and characterized, this research focused further functional studies on LjPLT11, which is specifically expressed in root nodules. It is reported that the LjPLT11 located on the peribacteroid membrane can mediate the transport of D-pinitol in the infected cells of nodules to affect the growth and nodule development of Lotus japonicus.

Functional analyses of LjPLT11 in yeasts mutant EBY.VW4000 demonstrated that LjPLT11 was energy-independent and mediated the transportation of xylitol, two types of O-methyl inositol (pinitol and methyl inositol), xylose and galactose. Under the condition of symbiotic nitrogen fixation, reducing the expression of LjPLT11 in Lotus japonicus inhibits plant growth, resulting in changes in pinitol concentration in the cytoplasm of infected cells, abnormal bacteroid formation, and reduced nitrogenase activity.

Strangely, under the condition of sufficient nitrogen, reducing the expression of LjPLT11 can accelerate plant growth. The reason may be that under conditions of non-nodulation and sufficient nitrogen, pinitol requires energy for synthesis and transportation. Immunogold electron microscopy results show that LjPLT11 is located on the bacteroid membrane, which facilitates the transport of sugar alcohols between the bacteroid and plant cells.

Because rhizobia cannot directly use pinitol, and the symbiotic space between bacteria and the bacteroid membrane needs to maintain osmotic balance to maintain the normal exchange of nutrients inside and outside PBS, the researchers isolated the bacteroid and measured the internal and the external osmotic pressure. The results show that reducing the expression of LjPLT11 can increase the osmotic pressure of the intracellular cytoplasm of the four-week-old nodules while changing the pinitol content, while the osmotic pressure of the bacteroid decreases. Whether the bacteroid can accumulate pinitol on its own is still unclear, but the destruction of the osmotic balance in the symbiont space has formed a series of chain reactions, which leads to the malformation of the bacteroid membrane of the LjPLT11i strain and triggers the invasion. The accumulation and distribution of ROS in the stained cells are abnormal.

Overall, it is shown that LjPLT11 located in the peribacteroid membrane regulates the osmotic balance of the symbiont space by mediating the transport of D-pinitol in the infected cells by maintaining the stability of the bacteroid membrane.

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