Screening Of Molecular Marker Putatively Related With Citrus Canker Resistance

Citrus canker is a disease caused by the bacterium Xanthomonas axonopodis pv. citri. It can infect the major citrus cultivars commercially cultivated. The disease might lead to defoliation, dieback and fruit drop and infected fruits that remain are less valuable or entirely unmarketable. It will increase the producing cost of citrus fruits and trigger quarantines. Breeding and use of resistant cultivars, is an effective and thorough way against citrus canker. But it is still hindered by the limited research on citrus canker so far.The aim of this research is to find molecular markers related to citrus canker resistance. A segregation F1 population was prepaired with canker resistant Ichang Papeda NO. 2586 as seed parent and susceptible Linnan Shatian pummelo as pollen parent. Resistance evaluation in field was carried out with part of the F1 population. According to the requirement of BSA (Bulked segregant Analysis), resistant and susceptible pools were constructed with extreme resistant individuals and susceptible ones. SSR and SRAP molecular marker technology were used to screen the pools. Preliminary polymorphic markers were verified with both parents and individuals in each pool. The verified markers were further tested with all evaluated F1 population and generally recognized resistant or susceptible genotypes. The recombination frequency was calculated with maximum likelihood method and transformed into centimorgans(cM) according to the Kosambi function.The main results were as follows:(1) 80 plants from the F1 population were randomly selected and inoculated with citrus canker for resistance evaluation in field. The result revealed that resistance segregation in the F1 progenies was significant. Except for 2 individiuals died during evaluation, there are 42 resistant plants and 36 susceptible ones. The ratio of resistant and susceptible plants was close to1:1. By Chi-square Test analysis,χ²=0.16. far less than theχ_0.05²=3.84, it complies with Mendelian inheritance.(2) A steady and reproducible system for molecular marker test was constructed. 67 polymorphic primer combinations between the two pools were found from 360 SRAP primer combinations, 26 polymorphic primer combinations between the two pools were found from 285 SSR primer combinations.(3) A SSR marker putatively related to citrus canker resistance was discovered. The marker was found to be polymorphic between the eight resistant plants and the eight susceptible ones. According to its amplified product size, the marker was named as CR14-110. Test of 78 F1 individuals indicated that CR14-110 was found in 39 resistant plants and 6 susceptible ones, while the fragment was absent in 3 resistant plants and 30 susceptible plants. The recombination frequency was 9.38±3.5% as calculated with maximum likelihood method and the genetic distance was 9.11cM after transformed into centimorgans(cM) according to the Kosambi function. Co-segregate analysis of F1 population showed that correlation coefficient was 0.79 between the marker and citrus canker resistance.(4) In the test of generally recognized resistant/susceptible genotypes of citrus and related genera, the marker was not found in susceptible genotypes (C. sinensis Osbeck,C.grandis Osbeck,Poncirus trifoliata and its progenies), and was observed in most resistant genotypes (C. ichangensis, C. reticulata, Fortunella and C. junos).