Functional Identification Of Apple MdHB7 In Response To Glomerella Leaf Spot And Valsa Mali Infections

As the major fungal diseases affecting the healthy development of apple industry in recent years,Glomerella leaf spot and Valsa mali occur widely in the main apple producing areas in China,which seriously reduce the yield and quality of apples and cause huge economic losses to fruit tree growers.At present,the effective control of these two fungal diseases is an urgent problem for the apple industry.Therefore,the molecular level of exploration provides a new solution for it.HD-Zip transcription factor is a unique transcription factor in higher plants,which is widely involved in plant response to biological stress and plays an important role in plant immunity.However,it has been rarely reported in apples at present.Therefore,in this study,we inoculated Glomerella leaf spot and Valsa mali with HD-ZipⅠsubfamily members’MdHB7 overexpression and interference transgenic seedlings obtained in the laboratory.By comparing the expression levels of reactive oxygen burst,hormone level,phenolic substance content,antioxidant enzyme system,disease-resistance related proteins and disease-resistance signaling pathway of transgenic and wild-type plants infected by pathogenic bacteria,the mechanism of MdHB7 in response to biological stress was initially analyzed.The main research results are as follows:1.MdHB7 expression was induced by Glomerella leaf spot infection,and it negatively regulated apple GLS resistance.Compared with the wild type,ABA synthesis and accumulation were significantly increased in 35s::MdHB7 apple plants,while SA accumulation and the expression of SA pathway related genes were significantly decreased,leading to the decrease of disease resistance.At the same time,the content of phenolic substances and the decreased activity of antioxidant enzyme system in 35s::MdHB7 plants also led to the decrease of the antibacterial and antioxidant capacity,resulting in a large amount of H₂O₂ content accumulation.In addition,the activities of chitinase,β-1,3-glucanase and phenylalanine ammonia-lyase in MdHB7 were relatively lower,while the activities of polyphenol oxidase were significantly higher than that of the wild-type.These ultimately caused 35s::MdHB7 plants more susceptible to GLS.The MdHB7 RNAi transgenic plants showed opposite results,that is,the transgenic plants with MdHB7 knocked down showed significantly increased resistance to GLS,which was manifested as lower ABA content and polyphenol oxidase activity,higher SA content,phenolic substance content,antioxidant enzyme activity,chitinase andβ-1.3.Activities of glucanase and phenylalanine ammonia-lyase.2.MdHB7 expression was also induced by Valsa mali（Vm）infection and it negatively regulated apple Vm resistance.Compared with the wild type,the content of SA and the expression levels of SA pathway related genes in 35S::MdHB7 plants were inhibited,which were significantly lower than those in the wild type.At the same time,the content of phenolic substances,chitinase andβ-1,3-glucanase activity were relatively lower,and finally the higher accumulation of H₂O₂,which showed the reduction of resistance to Vm.On the contrary,the MdHB7 RNAi apple plants showed enhanced accumulation of SA and phenolic substances,increased activities of chitinase andβ-1,3 glucanase,decreased accumulation of H₂O₂,leading to an improved resistance of MdHB7 RNAi apple plants to Vm.In conclusion,MdHB7 plays negative roles in GLS and Vm resistance,and the increase of its expression level will reduce the resistance of apple plants to the two tested fungal diseases.