Preliminary Study On The Mechanism Of Litchi Fruit Resistance To Phythothora Litchii Infection

Litchi downy blight, caused by Phythothora litchii, is one of the most severe disease of litchi. It results in a reduction of fruit production and economic value due to flower-and fruit-drop in pre-harvest and boosting the fruit-browning in harvest. The work about the mechanism of resistance against downy blight will provide foundation for disease control and breeding disease-resistance cultivars. In this study, we firstly established the system of resistance evaluation to downy blight. Following the evaluation disease-resistance of cultivars, the stable resistant and susceptible cultivars were identified through traced investigation for five years. The mechanism of resistance against downy blight were studied from the epidermal structure, physiological activity and profile of resistance-related genes between resistant and susceptible litchi cultivars. The main results were as followings:1. The disease symptom of 'Feizixiao' and 'Dadingxiang' were inoculated with 4×104 mL-1 sporangia occurred at moderate speed, and the disease index of 72 h were relatively stable, which was the proper time to symptom survey for resistance to downy blight. During 2012-2016, the system was applied to evaluate the resistance aganist downy blight among fifteen litchi cultivars. The resistance grades of these cultivars were almost same in different years, these confirmed that the resistance evaluation system of litchi fruit to downy blight is stable, and 'Heiye' and 'Guiwei' were the most stable resistant and susceptible cultivars, respectively.2. The ripen fruit of 'Heiye' and 'Guiwei',the resistant and susceptible cultivars respectively, inoculated with 4 × 104 mL-1 sporangium suspension. The change of litchi pericarp was investigated through scanning electron microscopy technique at 1 h, 6 h, 12 h, 24 h, 48 h, 72 h and 96 h after the inoculation. The results showed that turtle lobes were flat and honeycombs were convex and compact in 'Heiye',while the valleys of lobe were deep and the honeycombs were thin and erect in 'Guiwei',which is more easily attached by sporangia and spores of P. litchii. The pericarp structure of 'Heiye' and' Guiwei ' fruit were destroyed gradually as time goes on. However, 'Heiye' showed smaller injury than 'Guiwei' at the same time and need a longer time to be fully destroyed. The infection with P. litchii accelerated the breakdown of exocarp. The span of life cycle of P. litchii was longer on the 'Heiye' pericarp (96 h) than that on 'Guiwei'pericarp (72 h). 'Heiye', maybe due to dense structure of honeycomb, is more disease-resistant than 'Guiwei'.3. The physiological activity of litchi pericarps was determined at 0 h, 1 h, 6 h, 12 h,24 h,48 h,72 h and 96 h after the inoculation. Compared to 'Guiwei',the total phenolics,flavonoids,anthocyanin,MDA,PPO,PAL,POD,CAT,APX,GR,CHT,Cx,GLU and SOD of fresh pericarps in 'Heiye' were more higher than these in 'Guiwei'. The MDA,PPO, PAL, POD, CAT, APX, GR, CHT, Cx and GLU of fresh pericarps were increased during storage in two cultivars, while the total phenolics, flavonoids, anthocyanin and SOD were decreased. After infected by P. litchii, the total phenolics, flavonoids,anthocyanin, PAL, POD, APX, GR and GLU of 'Guiwei' were more obviously change than these of 'Heiye'. In gerenal, the physiological response to infection of P. litchii were more significant in 'Guiwei' than in 'Heiye'. It maye the important reason that 'Guiwei'is suspectible to downy blight.4. Based on the conserved domain of POD and CHT genes from public databases,169 of POD and 75 of CHT were retrieved from litchi genome respectively. The PODs of litchi were clustered into three groups by phylogenetic analysis. The same results were obtained for CHT. These groups were named as: LcCHTi, LcCHTii, LcCHTiii, LcPODi,LcPODii and LcPODiii respectively. The expression of nine genes selected randomly from each class were analyzed. The RT-PCR results were consistent with the former transcriptome analysis. Three CHTs of litchi were induced after the inoculation of P.litchii. The expression of LcCHTi1 and LcCHTiil changed more in 'Guiwei', while expression of LcCHTi2 changed more in 'Heiye'. Four PODs of litchi, LcPODi1,LcPODi2, LcPODi3 and LcPODiii2, were induced differentially in two cultivars. The LcPODii1 and LcPODiii1 were not induced, but the expression levels of two genes were higher in 'Heiye' than in 'Guiwei'. After infection with P. litchii, some CHTs and PODs of litchi were induced. Due to function differentiation among members of gene families,the gene expression was not positively related to the change of enzyme activity. The differential expression of some CHTs and PODs was one possible reason for their different resistance against P.litchiii.