The Regulatory Role Of G6PDH In Adaptation To Drought Stress In Soybean Seedlings

Glucose-6-phosphate dehydrogenase?G6PDH?is the pivotal and rate-limiting enzyme of the pentose phosphate pathway,which has been implicated in supplying reduced nicotine amide cofactors for biosynthesis and modulating the redox state in livingorganisms.G6 PDH is not only involved in regulating plant growth and development but also involved in responses to environmental stresses.However,the regulatory role and responsive mechanism of G6 PDH under drought stress need to be further studied.In this study,we investigatedthechangepattern and signal-regulation mechanism of G6 PDH activity in soybean seedlings under drought stress,and the regulatory role of G6 PDH in droughtadaptation was also investigated.The main results were summarized as follows:With the increase of PEG concentration,the relative water content declined gradually,while the relative electrolyte leakage and malondialdehy content increased significantly.Furthermore,the content of reactiveoxygenspecies?2O-·and H₂O₂?increased first and then decreased with the increasing PEG concentration.Osmotic adjustment substances such as proline and soluble sugar accumulated rapidly and the activities ofantioxidantenzymes?SOD,POD,APX and CAT?increased first and then declined as PEG concentration further rose.Under 15%PEG,the seedlings were severely wilted,and 20%PEG treatment led to death of seedlings,suggesting that the tolerance of soybean seedlings to drought was lower than 20%PEG concentration.Drought stress increased the total and cytosolic G6 PDH activity,butfailed to induce plastidic G6 PDH activity.Total and cytosolic G6 PDH activity increased gradually as the PEG concentration rose,and peaked at 10%PEG.Under 10%PEG,total and cytosolic G6 PDH activity increased first and then decreased with the time extension,and peaked at 24 h.Drought stress triggered a rapid ABA and H₂O₂ accumulation in soybean roots,which exhibited a similar change pattern with G6 PDH activity.Exogenous ABA or H₂O₂ enhanced the total and cytosolic G6 PDH activity,whereas Na2WO4?a ABA biosynthesis inhibitor?or KI?a H₂O₂ scavenger?significantly reduced the ABA or H₂O₂ content as well as the total and cytosolic G6 PDH activity,suggesting that drought-induced G6 PDH activity was positively regulated by ABA and H₂O₂.Moreover,DPI?a plasma membrane NADPH oxidase inhibitor?treatment showed the same results with KI that drastically inhibited the production of H₂O₂ as well as the total and cytosolic G6 PDH activity,indicating that H₂O₂ is derived from plasmamembraneNADPHoxidase-medicated pathway under drought stress.Further investigation showed that ABA induced H₂O₂ production by stimulating NADPH oxidase activity under drought stress.Additionally,exogenous SNP?a NO donor?reduced the total and cytosolic G6 PDH activity,while L-NNA?a NO biosynthesis inhibitor?inhibited the NO production and increased the total and cytosolic G6 PDH activity,suggesting that NOcould negatively regulate G6 PDH activity under droughtstress.Drought stress significantly increased the contents of GSH and ASA and the ratios of GSH/GSSG and ASA/DHA,while the effects resulted from drought stress were significantly inhibited by the addition of GN6P?a G6 PDH inhibitor?.Moreover,drought stress elevated the activity of the keyenzymes?GR,GPX,MDHAR and DHAR?in ASA-GSH cycle,and the above effect induced by drought could be also reversed by GN6 P.In summary,drought-induced increase in total G6 PDH activity results from enhanced cytosolic G6 PDH.ABA and H₂O₂ are required for drought-induced G6 PDH activity,and H₂O₂ seems to act downstream of ABA in this porcess.Enhanced cytosolic G6 PDH activity can maintain high levels of GSH and ASA as well as the ratios of GSH/GSSG and ASA/DHA through regulating the activity of the keyenzymes in ASA-GSH cycle?GR,GPX,MDHAR and DHAR?and then improve the ability of scavenging ROS.Thus,redoxstatebalance is reestablished so as to enhance tolerance of soybean seedlings to drought stress.