The Mechanism Of Manganese Ferrate Nanomaterials Enhancing Drought Resistance In Rice

As the global population continues to grow and climate change intensifies,the world food crisis is becoming more and more prominent,and the transformation of agriculture is imperative.In recent years,more and more research has shown that Nanomaterials（NMs）,with their unique physicochemical properties,have a wide range of potential applications in agricultural production.Compared with conventional fertilizers,NMs are more efficient and can significantly promote plant growth and enhance their resistance to biotic and abiotic stresses.Rice（Oryza sativa L.）,as one of the five major food crops in the world,has been of great concern for its yield and quality.Since drought is an important natural factor limiting rice yield,there is an increasing interest in yield maintenance and drought resistance enhancement in rice under drought stress.Currently,an increasing number of studies have found that different metal-based NMs can enhance the drought tolerance of crops;however,the specific regulatory mechanisms are not clear.Iron（Fe）is an important coenzyme factor for photosynthesis and respiration in plants;manganese（Mn）is an essential trace element for plants,and both of them not only promote photosynthesis in plants,but also enhance the activity of antioxidant enzymes in plants to protect them in drought stress.Therefore,in this study,manganese ferrate（Mn Fe₂O₄）NMs were synthesized and used for experimental exposure of rice under drought stress to investigate the regulatory mechanism of Mn Fe₂O₄ NMs on the enhancement of drought resistance of rice,especially on the alteration of root system conformation.The main findings were as follows:（1）Soil application of different concentrations of Mn Fe₂O₄ NMs under drought stress significantly promoted the growth of rice.Among them,10 mg·kg^-1 Mn Fe₂O₄ NMs treatment resulted in the highest accumulation of rice plant biomass,which was 66.7%,17.6%and 94.2%higher than the drought control and 1 and 50 mg·kg^-1 Mn Fe₂O₄ NMs treatments,respectively.In addition,10 mg·kg^-1 Mn Fe₂O₄ NMs significantly increased photosynthetic intensity（10.4%）,photosynthetic pigment content（12.8%）,and photosynthetic product accumulation（329.1%）of rice leaves;also,the plant uptake of Mn,Fe,P,and S was significantly higher than other treatment groups.Therefore,Mn Fe₂O₄ NMs alleviated the growth limitation of rice under drought stress by promoting the uptake of nutrients,photosynthetic intensity and photosynthetic product accumulation in rice.Because of its excellent drought tolerance,10 mg·kg^-1 was selected as the concentration to further explore the drought tolerance mechanism of Mn Fe₂O₄NMs.（2）The results of single particle inductively coupled plasma mass spectrometry showed that 10 mg·kg^-1 Mn Fe₂O₄ NMs were able to enter into rice plants,where the number of NMs was 4×10⁵·particles·g^-1 in roots and 2.8×10⁵·particles·g^-1 in leaves.The Mn Fe₂O₄ NMs entering into the roots significantly up-regulated the drought sensing gene CLE25 in rice roots（29.4%）,further promoting the synthesis of more drought-responsive signal substance cle25 small peptide and its translocation to the aboveground of the plant to be bound by the receptor gene NCED3.The expression of the receptor gene NCED3 was significantly up-regulated by 59.9%and 17.2%under NMs treatment compared to the drought control and ion control.This resulted in the activation of downstream abscisic acid（ABA）biosynthesis;the ABA content of rice leaves was significantly increased by 23.3%.Correspondingly,proline content in rice leaves increased by 38.9%,malondialdehyde content,an oxidative damage substance,decreased by7.2%,and leaf wax synthesis increased by 26.2%.Therefore,Mn Fe₂O₄ NMs can enhance drought resistance by enhancing the response of rice to drought stress.（3）Meanwhile,10 mg·kg^-1 Mn Fe₂O₄ NMs significantly increased the angle of the rice root system,and the root phenotype showed deeper landwardness and denseness,mainly by the following mechanisms:firstly,the root gene OR1 was further activated after Mn Fe₂O₄ NMs exposure,and the expression was significantly elevated by 14.8%;secondly,Mn Fe₂O₄ NMs up-regulated indoleacetic acid（IAA）biosynthesis.Compared with the drought control and ionic control,the IAA content was significantly increased by 23.3%and 14.2%under Mn Fe₂O₄NMs treatment.In addition,Mn Fe₂O₄ activated the expression of AUX and PIN,the gene families for polar transport of IAA,thus promoting the transport of IAA from the root tip to the meristematic and elongation zones and driving the development of root hairs.Meanwhile,AUX2,AUX3,and PIN1a and PIN2 were asymmetrically distributed and expressed in root cells to control the growth of different near-ground lateral root cells,thus changing the root growth angle and showing a stronger tendency of groundward growth,thus making rice better resistant to drought stress.（4）The results of the fertility experiments showed that 10 mg·kg^-1 Mn Fe₂O₄ NMs increased the yield of rice under drought stress.Compared with the drought control,the filling rate,thousand grain weight and spike length were significantly increased by 61.1%,22.5%and41.3%,respectively.In addition,Mn Fe₂O₄ NMs improved the nutritional quality of rice seeds to some extent and significantly enhanced the gluten content in rice.The content of gluten,an important indicator affecting the palatability of rice,was significantly enhanced by 15.7%compared with the drought control.The contents of Ca（135.1%）,P（21.2%）,Fe（43.6%）,Mn（31.1%）,and K（19.3%）elements in rice were also significantly enhanced.