Primary Resistance Mechanism Analysis Of Cassava Germplasma To Cassava Bacterial Blight

Cassava bacterial blight (CBB) is a kind of devastating cassava disease caused by Xanthomonas axonopodis pv. manihotis (Xam) and distributing throughout the whole cassava-growing countries. CBB is the most serious disease in the domestic cassava of China, which is a huge potential threat and very difficult to prevention. In this study, we chose the leaf of resistant and susceptible germplasm selected from our previous study as material to research the leaf tissue surface structural features, enzyme changes after pathogen infection, clone resistance gene homologous sequences and analysis, we expected to reveal the resistance mechanism of cassava by comparing the subtle differences between esistant and susceptible germplasm. The following findings:The results of microscopic observation showed that stomatal frequency and waxy material content had a certain correlation with resistant reaction. The stomatal density and waxy matter content of resistant germplasm were higher than medium-resistant germplasm, and susceptible germplasm were minimum. The numbers of stomata per unit area (328×246μm) of resistant and susceptible germplasms were higher than50and less than40respectively. The waxy material content of resistant germplasms was up to8.5mg/g, while the content of susceptible germplasms was below6.5mg/g.The relations of activity of peroxidase (POD), polyphenol oxidase (PPO), superoxide dismutase (SOD) and catalase (CAT) with resistant reaction were studied. The activity of POD was increasing and decreasing followed, the highest activity was appeared at the sixth day, and the resistant germplasms were significantly higher than susceptible germplasms. The PPO activity was obviously enhanced at the inoculative time and subsequently weakening, resistant germplasms were higher than susceptible germplasms. The CAT activity changes weren’t related with resistant reaction. The similar downtrend was emerged on the SOD activity of all the germplasms. These results showed strong correlations between resistance reaction and POD.Based on the unique conserved domain of NBS, STK and LRR reported plant resistant genes, three pairs of primers were designed and the genome DNA of E1340and GR911were used as template, amplification products of0.7kb,0.5kb and0.9kb were generated by PCR reaction, which were designated as NB1, SKI and LR1. Each fragment sequences. Three genes were found near the sites of NB1, SKI and LR1be encoded protein structure analysis, which were tentatively named SNB1, SSK1and SLR1. The SNB] had two exons. The ORF had510nucleotides and encoding170amino acids. The structural analysis showed SNBI had the feature similar with the resistant genes of NBS varieties. The SSK1had three exons of CDSil, CDSi2and CDS1. The ORF had726nucleotides and encoding242amino acids. The structural analysis showed SSKI had the feature similar with the resistant genes of STK varieties. The SLR1had no similar conserved domain with resistant genes of LRR varieties, while the homology of SLR1to the senescence-associated protein came from Norway spruce, Pea, Cypress and Fragrant lily were higher than60%. The results showed the NBS and STK varieties of resistant genes may play an important role, while the susceptible germplasms might lack relevant regulatory factors in downstream.