Inhibition And Structural-activity Analysis Of Two Fatty Aldehydes And Their Structural Analogues Against Geotrichum Citri-aurantii

Sour rot is one of the most serious postharvest diseases of citrus and aroused destructive loss on postharvest citrus in recent years.Currently,most chemical fungicides commonly used in citrus industry did not have obvious inhibitory effects on sour rot.Therefore,safe and effective strategies for the control of citrus sour rot are urgently required.Glutaraldehyde(GA)and trans-2-hexenal are food additives in China.They have strong antibacterial activity,but their effects on citrus pathogens are rarely reported.This study aims to investigate the antifungal activity of GA and trans-2-hexenal against G.citri-aurantii and the possible mechanism involved.In addition,the relationship between the antifungal activity and structure of aliphatic compounds similar to their structure GA and trans-2-hexenal is also determined,in order to promote the synthesis of new structures of fungicides and provide a theoretical reference for the control of sour rot.The main results of this paper are as follows:(1)Inhibition of GA on G.citri-aurantiiGA was found to effectively inhibit the growth of G.citri-aurantii mycelia in vitro,with the MIC and MFC values being both 1.00 ?L/mL.In vivo assays demonstrated that the disease incidence rate in control fruits was 100%,those in GA-treated(1 × and 5 × MFC)fruits were only 40% and 20%,respectively(P < 0.05).Scanning electron microscopy(SEM)showed that GA caused irreversible damage to the morphology of G.citri-aurantii.CFW and PI staining experiments revealed that the membrane permeability rather than the cell wall integrity of G.citri-aurantii mycelia was severely disrupted after the addition of GA.In addition,the reactive oxygen species(ROS)level continued to rise,the treatment groups content at the time of 120 min was 5.87(1/2 MIC group)and 5.06 times(MIC group)of the control samples.The LOX content of the 1/2 MIC and MIC glutaraldehyde treatment group reached the highest level at 30 min,which was 15.83 and 11.53 times of the control group.The total MDA content was generally higher than the control group throughout the experimental period.The results showed that GA could induce the accumulation of ROS and lead to cell membrane damage and membrane peroxidation.(2)Inhibition of trans-2-hexenal on on G.citri-aurantiiTrans-2-hexenal was found to effectively inhibit the growth of G.citri-aurantiimycelia,with the MIC and MFC values being 0.50 and 1.00 ?L/mL,respectively.In vivo application of trans-2-hexenal(1 × and 10 × MFC)could significantly reduce the disease incidence and lesion diameter of sour rot in citrus fruit inoculated with G.citri-aurantii.The SEM showed that the treatment groups resulted in charges of surface morphology such as depression and irregular distortion of G.citri-aurantii mycelia.The results of CFW staining showed that blue fluorescence intensity of the mycelium became weaker,which indicated the integrity of cell wall was severely disrupted.However,PI staining showed that the cell membrane was not damaged.The ROS experiment indicated that trans-2-hexenal did not cause oxidative stress.Therefore,the cell wall was the target of trans-2-hexenal against G.citri-aurantii.(3)Structure-activity analysis of structural analogues of GA and trans-2-hexenalThe structural analogues of GA and trans-2-hexenal have different antifungal activity against G.citri-aurantii,and antifungal activity of ?,?-unsaturated olefin is generally strong.The bond length and frontier orbital energy of 22 aliphatic compounds were calculated by density functional analysis(DFT)of Gaussian09 software.Wherein bond length of the carbonyl group and carbon-carbon double bond is shorter than others,which contributes to enhancing its antifungal activity.The frontier orbital energy diagram shows that the hydroxyl,carboxyl,aldehyde,and carbon-carbon double bond are all reactive groups.Further calculation of the energy difference found that the aldehydes and carbon-carbon double bond had the smallest energy difference and the highest reactivity.The QSAR mathematical equation between energy difference and antifungal activity was established and the regression was significant.Furthermore,three new structure of aliphatic compounds were designed and predicted the MIC values against G.citri-aurantii were not higher than 0.50 ?L/mL by structure-activity analysis.Above all,this study indicated that GA and trans-2-hexenal exhibited strong antifungal activity against G.citri-aurantiiin and the possible antifungal targets were cell membrane and cell wall,respectively.The structure-activity analysis showed that the aldehyde group and carbon-carbon double bond were the molecular activity.This study will provide a theoretical reference for the control of sour rot and the development of new fungicides.