Study On Resistance Mechanism Of Bayberry To Twig Blight Disease

Bayberry(Myrica rubra)is an important fruit tree native to the subtropical regions of China.It produces fruits with a unique taste and pharmacological characteristics,making it widely cultivated for more than 86666.67 hectare and its output value ranked first among all kinds of fruit in Zhejiang province.The twig blight of bayberry was a systematic disease caused by Pestalotiopsis versicolor and P.microsporay resulting in the death of the whole tree with the shoots increased year by year.Chinese bayberry industry is restricted by the death of 'Dongkui' bayberry,which is the main variety and highly sensitive to the disease.The disease was controlled by the comprehensive prevention and control,but cost much time and laborious.So the best method is to promote disease-resistant varieties.The'Zaojia' bayberry cultivated by Institute of Horticulture,Zhejiang Academy of Agricultural Sciences,is a highly resistant variety to the twig blight disease.Therefore,to clear the disease resistance mechanism of ZJ variety is conducive to promote and improve variety.In this paper,the differences of infection process of Pestalotiopsis,gene expressions and defense enzymes' activities between ZJ and DK were studied to elucidate the disease resistance mechanism of ZJ.The process of twig blight pathogen infecting resistance and sensitive bayberry varieties had significant difference.The difference of infection process between resistance and sensitive bayberry varieties after inoculation observed by frozen scanning electron microscopy.The results revealed that the Pestalotiopsis conidia were shrunk,the mycelium was shriveled and did not extend into plant cells after 6 h inoculation to ZJ;On the contrary,the conidia were full and the top was extended,the mycelia was normal and continued to extend to plant cells after 6 h nioculaiton to DK.Paraffin-embedded sections were made of the bayberry twigs inoculated with Pestalotiopsis.Microscopic observation showed that,the xylem cells were intact in ZJ,but obviously damaged in DK after 48 and 72 h of inoculation.The transcriptome differences between resistance and sensitive bayberry variety after inoculation.The different genes between resistance and sensitive bayberry variety after inoculation were screened by transcriptome sequencing and validated by fluorescence quantitative PCR.The results showed that the expression levels of MKK4 and WRKY40 genes and the gibberellin pathway genes GA2ox7,KAOl,KA02,RGA and KS in resistance cultivars leaves were significantly higher than in those of susceptible cultivars,but the expression levels of GA2oxl and KS genes in the twigs of ZJ were significantly higher than those of DK,and the rest expression of GA2oxl and KS was not significant,which indicating the basis of disease resistance of ZJ was better than DK.The expression levels of MKK4,transcription factor TGA4 and other 18 genes and CPS,GA2oxl and other 15 gibberellin pathway genes were significantly higher than those compared to the control after inoculation,which indicated that MKK4 and transcription factor TGA4 and other 18 genes,CPS and GA2oxl and other 15 gibberellin signaling pathway genes participated in the induction of resistance to bayberry twig blight.In addition,the relative expression levels of WRKY40,GA2ox7 and other 7 genes in ZJ leaves were higher than those of DK leaves at most of the detection time,and the expression levels of MADS-box protein,GA2ox7 and other 15 genes first reached the peak in ZJ.In the branches,the expression levels of SBP,GA20oxl and other 24 genes first reached the peak in ZJ.These genes may determine the high resistance of ZJ to bayberry twig blight.The defense factors difference between resistance and sensitive bayberry variety after inoculation.The activity levels of catalase(CAT),superoxide dismutase(SOD),phenylalanine ammonia lyase(PAL),polyphenol oxidase(PPO),polyphenol oxidase(POD),proline(PRO)and ascorbate peroxidase(APX)showing different trends at different time points and were higher than the initial levels in most time in leaves and twigs of ZJ and DK,showing ZJ and DK were able to induce defense reaction after inoculation.We found that the activity levels of active oxygen scavenging enzyme SOD,CAT,POD and PRO were higher than those of DK in most time points,which indicating the ability to scavenge active oxygen of ZJ was stronger than DK.This may be the reason for the ZJ highly resistance to twig blight disease.At the same time,it was found that the activities of CAT,POD and PAL in leaves of ZJ were higher than those of DK,this indicated that CAT,POD and PAL were also involved in the defense response of ZJ leaves.