Towards Understanding How Mannitol Restores Cell Adhesion Defects In Qua2 Arabidopsis

Cell adhesion is crucial for plant growth and development.Perturbing pectin content can affect cell adhesion,however,our knowledge of the related mechanistic basis is limited.In this study,we have identified an Arabidopsis mutant dl1 that displays severe defects of cell adhesion.The mutant was named qua2-4 based on the gene cloned in the mutant.In this study,we focused on the effects of QUA2 gene and mannitol molecule on the methylesterification levels and the total contents of pectin which in turn impact cell adhesion in Arabidopsis.The main findings are summarized as follows:1.The dl1 seedlings show a vitrified appearance with severe defects of cell adhesion in various plant tissues and organs such as hypocotyls and roots,as well as a reduced length of both primary root and hypocotyl.Moreover,cell length of hypocotyls in dl1 was much shorter than that in wild type Arabidopsis.2.dl1 harbours a point mutation in QUA2 gene(At1g78240)thus as a consequence introducing a premature stop codon in the m RNA.The QUA2 gene encodes pectin methyltransferase protein.dl1 is allelic to qua2-1.We therefore renamed dl1 as qua2-4,and subjected it to further investigation.3.QUA2 gene is widely expressed in various Arabidopsis tissues as the QUA2 expression can be detected in leaves,veins,roots,root tips,stigmas,sepals,stamens and stalks.The number of QUA2 homologous proteins increases as plants move on up the evolutionary ladder,suggesting QUA2 proteins play essential roles in the life cycle of plants.4.Mannitol,often treated as a compatible solute that accumulates in the cytoplasm to balance external osmotic pressure,can restore the vitrification phenotype of qua2-4,promoting the elongations of the root,hypocotyl and root hair of rootstem transformation zone of qua2-4 mutant,whereas neither PEG nor Na Cl had such effects,suggesting that mannitol might serve as a signal molecule in the cell adhesion restoration in qua2.5.Mannitol application has no effects on the total pectin content in qua2,in marked contrast to WT in which mannitol stimulated significant increase of total pectin content,indicating the total pectin content is not the reason for the cell adhesion restoration in qua2.6.Mannitol can modify the ratio of low-and high-methylated pectin contents,and importantly,mannitol upregulated the expression of both QUA2 and PME genes,suggesting that related gene expression and pectin methylesteration state have a role to play in regulating the cell adhesion.Furthermore,I have demonstrated that exogenous mannitol could greatly suppress the accumulation of ROS caused by qua2 mutation which was likely also one of the reasons for restoring of the cell adhesion.In summary,mannitol can modulate the ratio of the low and high methyl esterificated pectins and rescue the cell adhesion defects caused by QUA2 deficiency.Our findings suggest mannitol has profound impacts on cell wall intergrity and plant physiology as a whole.Caution should be exercised when use mannitol as a compatible solute in plant cell research.