Influences Of Arbuscular Mycorrhizal Fungi(AMF) Inoculation On Drought Resistance Mechanism Of The Potted Citrus Plantlets

Soil drought severely restricts the growth and yield of crops,and as the major limiting factor,water deficit has a significant impact on the reduction of acreage and yield,with its influence exceeding the sum of other factors‘.The average annual rainfall in citrus production areas of China is over 1000 millimeters.However,because of the uneven distribution of rainfall time,seasonal drought occurs often.Meanwhile,due to the fact that citrus has few and short root hairs,citrus crops are so sensitive to soil moisture and their root system suffers severely in the extreme environment,both wet and dry.Arbuscular mycorrhizal fungi(AMF)can form a soil mutual symbiosis with citrus crops and improve their drought resistance.How AMF improve the drought resistance of host plants is a complex biological process,which involves a number of physiological,molecular and other multiple pathways.Although a large number of studies have shown that AMF contributes greatly to the drought resistance process of plants,its mechanism is not yet clear.Some experiments even show the opposite results.Therefore,it is necessary to figure out the exact mechanism of AMF acting on citrus under drought stress.In the research,1-year-old citrus(Citrus reticulata Blanco cv.Ponkan/ Citrus sinensis(L.)Osbeck×Poncirus trifoliate(L.)Raf)plantlets were used to analyze the effects of AMF Claroideoglomus etunicatum(C.e)and Funneliformis mosseae(F.m),under different water conditions.Six treatments were carried out:(1)well watered and non-mycorrhizal(WW-NAM),(2)well watered and C.e(WW-C.e),(3)well watered and F.m(WW-F.m),(4)drought stressed and non-mycorrhizal(DS-NAM),(5)drought stressed and C.e(DS-C.e),(6)drought stressed and F.m(DS-F.m).There were 36 pots in each treatment and therefore 216 pots in total.12 pots were disposed randomly in each plot,and the experiment was repeated three times.The potted experiment is to study the impact of AMF on citrus growth,photosynthesis,osmoregulation substance content,antioxidant enzyme activities,endogenous hormone balance,and molecular mechanisms,under long-term drought stress conditions.The main results were as follows:1.Citrus roots can be effectively infected by AMF of C.e and F.m,and the infection rates were all over 50%,indicating that there was a solid symbiosis between AMF and citrus,and even under drought stress,the infection rate could reach 44.50% and 48.50%,respectively.Meanwhile,the mycorrhizal dependence of citrus plantlets was higher under drought stress.Generally speaking,F.m is better than C.e.2.Compared with the non-mycorrhizal treatment,both AMF inoculation significantly increased the plant height,leaf area,chlorophyll content,chlorophyll fluorescence inducing kinetic parameters,and net photosynthetic rate of citrus plantlets to different degrees,promoted plant growth,and alleviate drought damage to the photosynthetic system.The mycorrhizal symbiosis also promoted the root development of citrus plantlets,increased the root volume,new root amounts,maximum lateral root length,root activity,and root-shoot ratio,increased biomass accumulation and migrated to lateral roots,enhanced root plasticity,and improved drought resistance.Compared with the non-mycorrhizal control treatment,drought stress combined inoculated with F.m showed the most significant effect,increasing 40.03%,74.37%,19.89%,100.00% and 26.54%,respectively.3.Under drought stress,both AMF inoculation have promoted further the accumulation of soluble sugar and soluble protein in citrus leaves and roots,increased the activities of superoxide dismutase(SOD),peroxidase(POD)and catalase(CAT),and significantly decreased the content of proline(Pro)and malondialdehyde(MDA),with MDA reducing more with the extending of stress time.The results also indicated that on the one hand,AMF-inoculation could improve osmotic regulation,and the effect is more significant in the early stage.On the other hand,AMF-inoculation could increase the activities of antioxidant enzymes and reduce the oxidative damage,with a more significant effect in the later stage.They jointly enhanced the resistance of host plants to drought stress.4.Under drought stress,two kinds of AMF inoculated could significantly promoted the indoleacetic acid(IAA),gibberellin(GAs),ribosylzeatin(ZR),and abscisic acid(ABA)contents of citrus leaves and roots.They also increased IAA-ABA,GAs-ABA,and ZR-ABA ratio of citrus roots.Regulate the balance of endogenous hormones,and enhanced the positive adjustment effect in the process of plant growth.The mycorrhizal effect grows over time.At the same time,compared with the non-mycorrhizal treatment,the mycorrhizal plants maintain higher ABA accumulation under drought conditions,which will induc the stomatal closure in guard cells and reduc the water loss by transpiration.Therefore,mycorrhizal citrus plantlets survived better under the drought environment.5.With the help of qRT-PCR technology,the expression of ABA key enzyme biosynthetic genes CsNCED1 and CsNCED2 were significantly different in different treatment stages.CsNCED1 gene expression increased continuously,while CsNCED2 gene expression increased first and then decreased.The correlation analysis showed that the change of ABA content was basically consistent with the expression changes of these two genes.The tests also revealed that,both C.e and F.m symbiosis could induce the up-regulated expression of CsNCED1 and CsNCED2 gene in leaves and roots of citrus under drought conditions.In addition,the effect of AMF on roots was superior to that of leaves in the early stage of stress,which effectively enhanced the drought resistance ability of citrus.