Physiological Mechanisms Of Potassium Nutrition Enhancing The Rice(Oryza Sativa L.) Resistance To Sheath Rot Disease

Rice is one of the most important grain crops in China,which plays an important role in ensuring food security.The wide variety of rice diseases and its frequent outbreak are the main factors threatening high and stable production of rice.Sheath rot disease(Sh R)caused by Sarocladium oryzae(S.oryzae)infection is a serious threat in rice production.The typical symptoms of Sh R are the dark brown lesions occur on flag leaf sheath(FLS)and the young panicles are partly be enclosed in the FLS at the booting stage,which makes the control of Sh R by using pesticides more difficult.Potassium(K)is an essential nutrient for rice growth,as well as a stress tolerance element.The previous studies have shown that K application obviously decreased the Sh R disease incidence.However,due to the special infection site and time of Sh R,K nutrition may play a critical role in regulating the physiological process of rice resistance to Sh R.In the present study,we combined field experiment and simulated inoculation experiment to explore the effects of K application on carbon metabolism and grains sink activity of rice under Sh R infection conditions;to reveal the effects of K application on lipid metabolism and lipid homeostasis of leaf sheath after Sh R infection.Subsequently,we selected the major lipid metabolism pathways regulated by K nutrition during S.oryzae infection,followed by using a mutant material to further explore the photosynthetic characteristics of host plant mediated by K.Finally,carbon metabolism and carbohydrate allocation mechanism,lipid homeostasis and photosynthesis regulation mechanism of rice resistance to Sh R infection were elucidated.The main results are summarized as follows:(1)K application significantly promoted the translocation of non-structural carbohydrates from stems to grains,decreased the carbon to nitrogen ratio of diseased leaf sheath and enhanced the carbon metabolism of the host plant.Compared with zero-K supply treatment,K application significantly increased the apparent transferred mass of NSC from stems to grains(ATM_NSC)by 53.3%-57.0%.Meanwhile,a linear plus plateau regression model revealed the relationship between stem ATM_NSC and the seed-setting rate.So,K application profoundly increased the seed-setting rate of diseased rice by increasing ATM_NSC.Compared with zero-K supply treatment,K application increased the ratio of carbon to nitrogen of leaf sheath by 7.5%-11.3%under the non-infection conditions,however,under the condition of Sh R infection,the corresponding values were increased by 18.4%-21.3%.The ratio of carbon to nitrogen of leaf sheath was negatively correlated with the disease index,which suggested that K application alleviated the severity of Sh R infection by enhancing the carbon metabolism of diseased plants.(2)K application profoundly increased the milling and appearance quality of diseased grains by improving grains sink activity.Compared with zero-K supply treatment,K application increased the grains sink activity by 6.5%-10.0%under the non-infection conditions,however,the corresponding values increased by 17.6%-22.4%under the condition of Sh R infection.It noteworthy that,the grains sink activity was negatively correlated with the broken rice rate and the chalkiness rate,which indicated that K application significantly decreased the broken rice rate and the chalkiness rate by improving grains sink activity.(3)K application increased the abundance and Alpha diversity of endophytes after S.oryzae infection.After S.oryzae inoculation,compared with zero-K supply treatment,K application decreased the abundance of Ascomycota by 12.3%.Compared with K sufficient supply treatment,K starvation profoundly decreased the Chao1 index,Shannon index and Simpson index by 19.4%,27.7%and 25.0%,respectively.(4)K application ensured the translocation of sucrose in diseased flag leaf sheath by decreasing the damage of cell membranes caused by S.oryzae infection.Under the condition of S.oryzae infection,K supply significantly decreased the accumulation of soluble sugar in the flag leaf and leaf sheath,which promoted the translocation of sugars from sheath to grains.In absence of Sh R,compared with zero-K supply treatment,K application decreased the malondialdehyde content and the relative electrical conductivity by 28.2%and 20.8%,respectively,however,after S.oryzae infection,the corresponding values decreased by 39.5%and 33.7%.These results suggested that K application alleviated the damage of cell membrane caused by S.oryzae infection and promoted the translocation of sucrose.(5)K application alleviated the rotting of diseased sheath by maintaining lipid homeostasis at the infection site.Compared with K supply treatment,after S.oryzae infection,K-starvation increased the rotting rate of leaf sheath approximately by 50%,and the content of 1-alkyl-2-acylglycerophosphoethanolamine and linoleic acid were increased approximately by 18 times.During the S.oryzae infection,the accumulation of hydrogen peroxide(H₂O₂)in the K-starved leaf sheath resulted in lipid peroxidation.The hyperaccumulation of linoleic acid in the infection site altered the lipids homeostasis.By contrast,sufficient K supply increased antioxidant-related genes expression,which maintained the lipid homeostasis of leaf sheath and alleviated the sheath rot caused by S.oryzae infection.(6)K application increased jasmonic acid(JA)content of diseased organs and enhanced the photosynthetic rate of host plant.Compared with the healthy plant,S.oryzae infection decreased the JA content of the flag leaf,leaf sheath and roots by 10.5%,17.1%and 18.7%,respectively.Sufficient K supply increased the integrity of chloroplast.Compared with the wild type(WT),the chloroplasts exposed to intercellular air space per leaf area(S_c/S)of aos mutants(with lower JA content)was decreased by 19.2%under K-deficiency conditions,however,the corresponding value was decreased by 13.5%under the condition of K supply.Additionally,compared with the K-starved plant,K application profoundly increased the content of glycolipids including digalactosyldiacylglycerol(DGDG)and monogalactosyldiacylglycerol(MGDG).The DGDG and MGDG content were positively correlated with the maximum rate of electron transport(J_max),which indicated that K application increased the photosynthetic rate of host plant by increasing the glycolipids content.Overall,sufficient K supply enhances the carbon metabolism of host plants,promotes the translocation of NSC from stems to grains,promotes the synthesis of glutathione and other antioxidants,and increases the leaf glycolipids content to enhance the local and systemic resistance to rice sheath rot disease.