Mechanisms Of AM Fungi Regulating Proline Metabolism In Rice Under Low Temperature Stress

The symbiont-mycorrhiza formed by Arbuscular mycorrhiza(AM)fungi and terrestrial plant roots not only promotes host nutrient absorption but enhances its low temperature resistance.Proline(Pro)is an important stress protective substance.Proline accumulation in mycorrhiza under low temperature stress is one of the main mechanisms of AM fungi to improve host resistance.However,the underlying mechanism of proline accumulation in mycorrhizal symbionts under low temperature stress is still unclear.In this study,a pot experiment was conducted with rice as the host plant and Glomus intraradices as the AM fungal species.Nitric oxide(NO)was added to the rice inoculated with and without AM fungi(0 mmol·L-1 and 0.1 mmol·L-1)and nitrogen application(30 mg·L-1 and 80 mg·L-1),cultured to the critical point of effective tillering,temperature(15°C±1.0°C and 22°C±1.0°C),samples were taken after 7 days.The changes of key enzyme activities of nitrogen metabolism,proline metabolism substrate and key gene expression,and key enzyme activities of NO metabolism were investigated to investigate the AM symbiotic proline under low temperature stress.Metabolic characteristics,the role of NO in the process of AM probiotics promoting proline accumulation in rice,and the effects of AM fungi on proline metabolism in rice under different nitrogen supply levels,in order to explore the proline metabolism mechanism of AM fungi in response to low temperature stress.The main findings are as follows:Low temperature stress inhibits the growth and development of rice,and AM fungi promotes the accumulation of proline in rice,which in turn increases the low temperature resistance of rice.Low temperature reduced rice plant height,root length,fresh weight and dry weight,significantly inhibited the growth of rice.AM fungi had a significant positive effect on rice growth,and the root length could be restored to normal temperature.The infection of AM fungi increased the proline content of rice by 6.77%(P<0.05),the glutamate and ornithine content by 6.99% and 9.42%,respectively.The key genes of proline metabolism were Os P5CS1,Os P5CS2,Os OAT and Os Pro DH.The volume increased by 22.39%,41.97%,21.91% and 55.84%,respectively,both reaching a significant level of 5%.At the same time,AM fungi enhanced the key enzyme activities of nitrogen metabolism,and increased Nitric oxide synthase(NOS)activity and NO concentration by 12.37%(P<0.05)and 34.15%(P<0.05),respectively.Exogenous addition of NO promoted the formation of mycorrhizal symbionts,and the mycorrhizal infection rate increased by 25.93%(P<0.05)and 29.17%(P<0.05),respectively,under normal temperature and low temperature conditions.Exogenous NO increased the proline content of rice by 3.70%-6.49%,and the content of glutamic acid(Glutamate,Glu)decreased by 36.00%(P<0.05),but to ornithine(Ornithine,Orn).The content has little effect.At the same time,compared with the control,the addition of NO increased the expression levels of Os P5CS1,Os P5CS2,Os OAT and Os Pro DH in rice without inoculation with AM fungi by 1.48 times(P<0.05),1.14 times,2.13 times(P<0.05),respectively.1.32 times,the rice inoculated with AM fungi increased significantly by 1.60 times,1.58 times,2.13 times and 1.33 times,respectively.This indicates that under low temperature conditions,AM fungi enhance the metabolism of proline in rice,promote the accumulation of proline,and alleviate the harm of low temperature to rice.In this process,NO can act as an AM symbiotic signal molecule,regulate the proline metabolism of rice by activating glutamate and ornithine pathways synthesized by proline,and promote proline accumulation in rice.Under low temperature stress,the promotion of proline metabolism by AM fungi was closely related to the level of nitrogen supply.Under low temperature conditions,AM fungi promoted the growth and development of rice.The low nitrogen treatment of rice aboveground growth(MSR)and root growth(MRR)were higher by 89.87%(P<0.05)and 51.49%,respectively.The height was 45.36%(P<0.05).Nitrate reductase(NR)activity,Glutamine synthetase(GS)activity,NOS activity and NO concentration in the inoculated rice were lower than those in the non-inoculated rice.The height was 22.42%(P<0.05),23.01%(P<0.05),20.74% and 25.26%(P<0.05).At high nitrogen level,it was 4.49%,9.27%,13.43% and 6.46%,respectively(P<0.05).The infestation of AM fungi increased the proline content of rice by 14.61%(P<0.05)and 9.91%,and the glutamate content decreased by 16.63%(P<0.05)and 6.49%,respectively,under low nitrogen and high nitrogen conditions.Therefore,AM fungi promoted the metabolism of host nitrogen and proline metabolism at a lower nitrogen level,which is more conducive to the accumulation of proline and enhance the low temperature resistance of the host.In summary,low temperature inhibits the growth of rice,and the infection of AM fungi promotes the accumulation of proline in rice,thereby increasing the low temperature resistance of rice;in this process,NO acts as a signaling molecule and regulates the valley of proline synthesis.The alanine and ornithine pathways promote proline accumulation of symbiotics.In addition,under low temperature conditions,the promotion of AM fungi to rice is closely related to the level of nitrogen supply,especially at low nitrogen levels.