Mechanism Of Arbuscular Mycorrhizal(AM) Fungi Rhizophagus Irregularis Regulating Drought Tolerance And Potassium Metabolism Of Lycium Barbarum

Arbuscular mycorrhizal(AM)fungi can symbiosis with plants,promote potassium up-take and transport by the root system of host plant,and AM fungi also can uptake and transport potassium directly,which is participate in potassium metabolism,and improve the drought tolerance of host plants.This study was researched the effects of AM fungi-Rhizophagus ir-regularis inoculation on the growth,photosynthesis,antioxidant system,and water and nutri-ent uptake of Lycium barbarum with different potassium conditions under drought stress;the three compartments culture system was used to investigate the potassium in the L.barbarum of potassium-additional compartment,which transport by AM fungal hyphae network,to af-fect the growth,photosynthesis,and nutrient uptake of L.barbarum in the potassiumfree-compartment;the yeast potassium uptake-defective mutant strain CY162 was used to study the potassium uptake capacity of the high affinity K~+transporter gene(Lb HAK),which is in-duced by AM fungi,and the K~+channel gene 3(Lb KAT3),which is highly upregulated by AM fungi;the split-root system was used to test whether the Lb HAK and Lb KAT3 expression were systematically regulated by AM fungi;overexpression of Lb HAK and Lb KAT3 in tobacco(Ni-cotiana tabacum)to explore the effects of AM fungi on the growth,nutrient and water uptake of tobacco ovexpressing Lb HAK and Lb KAT3 under different potassium conditions.The mechanism of AM fungi regulating the drought tolerance and potassium metabolism of L.barbarum were revealed.The main results are as follows:1.AM fungi increased the growth,photosynthesis,and antioxidant capacity of L.barba-rum under drought stressUnder drought stress,inoculation of AM fungi significantly increased the colonization rate and arbuscular rate,root growth and leaf relative water content,significantly improved the leaf gas exchange parameters and chlorophyll fluorescence parameters,up-regulated the expression of ribulose-1,5-bisphosphate carboxylase/oxygenase large gene-Lbrbc L and the ribulose-1,5-bisphosphate carboxylase/oxygenase small gene-Lbrbc S,increased the activities of peroxidase and catalase,the concentrations of ascorbic acid and glutathione,up-regulated the expression of Lb Cu/Zn-SOD,Lb Mn-SOD and Lb Fe-SOD,and decreased the concentra-tions of hydrogen peroxide and malondialdehyde of L.barbarum in different potassium con-ditions.Moreover,inoculation of AM fungi and application of potassium reduced the concen-tration of proline and the expression of pyrroline-5-carboxylate synthase gene-Lb P5CS.The results suggested that inoculation of AM fungi promote the antioxidant defense and osmotic regulation of L.barbarum in different potassium conditions,reduce the oxidative damage caused by reactive oxygen species,and improve the drought tolerance and promote the growth of L.barbarum.2.AM fungi promoted the water and nutrient uptake of L.barbarum under drought stressThe Lb HAK and phosphorus transporter gene 4(Lb PT4)expression of L.barbarum were induced by AM fungi;application of extra potassium up-regulated the expression of Lb HAK and Lb PT4,and the aquaporin genes(Rir-AQP1 and Rir-AQP2)expression of AM fungi in mycorrhizal L.barbarum under drought stress.Under drought stress,inoculation of AM fungi significantly increased the potassium and phosphorus concentrations,up-regulated the Lb KAT3,K~+channel gene 1(Lb KT1),stelar K~+outward rectifier gene(Lb SKOR)expression on the roots of L.barbarum,regulated the aquaporin genes(Lb PIP1-1 and Lb PIP2-1;Lb TIP1-1,Lb TIP2-1,Lb TIP3-1,and Lb TIP4-1)expression on the roots of L.barbarum in different potassium conditions.The results suggested that inoculation of AM fungi can promote potas-sium,phosphorus and water uptake,improve the osmotic regulation(potassium participates in osmotic regulation),and the drought tolerance and the growth of L.barbarum under drought stress.3.Inoculation of AM fungi and foliar-applied potassium on the L.barbarum of the po-tassium additional compartment promoted L.barbarum growth of the potassiumfree-com-partment in a three compartments culture systemFoliar-applied potassium on the L.barbarum significantly increased the biomass,im-proved the photosynthesis,increased the potassium contents,and up-regulated the expression of Lb HAK,Lb KAT3,Lb SKOR,and Lb KT1 of neighboring mycorrhizal L.barbarum,and up-regulated the K~+channel gene 2(Lb AKT2)expression of itself.The results suggested that potassium,which is first transfer from shoots to roots of L.barbarum in potassium additional compartment by Lb AKT2,can transport to L.barbarum in potassiumfree-compartment by AM fungal hyphae network subsequently,and promote the potassium uptake,improve photo-synthesis,and ultimately promote the growth of potassium deficient L.barbarum in potassi-umfree-compartment.4.Overexpression of Lb HAK promote potassium,phosphorus and water transport from AM fungi to host plantsThe result of yeast complementary analysis showed that the potassium uptake capacity of Lb HAK was similar to the potassium uptake capacity of the potassium transporter TRKs in the wild-type yeast(BY4741),and all of them have strong potassium uptake capacity.The result of split-root system test showed that AM fungi locally induce Lb HAK expression.Over-expression of Lb HAK significantly increased the biomass,colonization rate and arbuscular rate,potassium and phosphorus contents,up-regulated the expression of Nt PT4 and Rir-AQPs of mycorrhizal tobacco,suggesting that overexpression of Lb HAK promote the potassium,phosphorus and water transport from AM fungi to host plants,and promote host plants growth.Overexpression of Lb HAK and application of 2 m M potassium increased the colonization rate and arbuscular rate,suggesting that increase the potassium accumulation or availability can promote AM colonization.5.Overexpression of Lb KAT3 promote potassium,phosphorus and water transport from AM fungi to host plantsThe result of yeast complementary analysis showed that the potassium uptake capacity of Lb KAT3 was significantly lower than the potassium uptake capacity of the potassium trans-porter TRKs of BY4741,which means Lb KAT3 has weak potassium uptake capacity.The result of split-root system test showed that AM fungi locally regulate Lb KAT3 expression.Overexpression of Lb KAT3 significantly increased the biomass,potassium and phosphorus contents of mycorrhizal tobacco,and up-regulated the expression of Nt PT4 and Rir-AQP1 of mycorrhizal tobacco,suggesting that overexpression of Lb KAT3 promote the potassium,phos-phorus and water transport from AM fungi to host plants and promote the tobacco growth.Application of 2 m M potassium and overexpression of Lb KAT3 increased the colonization rate,indicating that the increase of the potassium accumulation or availability can promote the AM colonization.In conclusion,the research discorvered that AM fungi promote nutrients and water uptake,photosynthesis,ability of ROS scavenging,and osmotic regulation of L.barbarum in different potassium conditions,and revealed the mechanism of mycorrhizal symbiosis inducing potas-sium transport to improve the drought tolerance of L.barbarum;clarified that AM fungi pro-mote potassium uptake and transport of host plant under drought stress and potassium defi-ciency condition,Lb HAK and Lb KAT3 may be key genes in mycorrhizal potassium transport,and overexpression of Lb HAK and Lb KAT3 promote nutrients and water transport from AM fungi to host plant.