The base-excision repair process protects genomes by removing and replacing altered bases in DNA. Two analogous glycosylases, oxoguanine glycosylase (OGG) and formamidopyrimidine glycosylase (FPG), can start the process by removing oxidized guanine, the most common modification that leads to misreading of DNA. Plants possess genes for both types of glycosylases. We have tested the hypothesis that the two enzymes in plants have diverged in their specificities by inserting the genes for each enzyme from Arabidopsis thaliana L. into Escherichia coli strains designed to indicate the frequencies of the six possible single-base changes. Both enzymes retain the ability to reduce the rate of GC --> TA transversion mutations. Both enzymes also reduce the frequency of two other base-change mutations, GC --> AT and AT --> TA. We do not find a divergence in the repair capabilities of the two enzymes, as measured in E. coli, although surprisingly FPG appears to increase the rate of mutations in one particular strain. 

Citrus Huanglongbing (HLB) is associated with the fastidious bacterium,Candidatus Liberibacter, (Las) that is transmitted by a phloem-feeding insect (Citrus Psyllid). An ideal solution to combat citrus HLB is to completely eliminate the bacteria after a single course of the chemotherapy, either active directly on the bacteria or indirectly through induction of host defense compounds. Twenty-seven antimicrobial compounds were screened to test for in vivo activities against HLB bacterium while assessing their phytotoxicity to citrus using the optimized graft-based chemotherapy approach (Zhang et al., 2012). The Las bacterial titers were quantified by qPCR from the leaf samples that were taken at 4-months and 6-months after inoculation, respectively. The Las-infected plants were considered as Las positive with threshold cycle (Ct) values less than 32.0. The efficiency against the HLB bacterium of the tested compound was evaluated by Ct values in the inoculated plants (both scions and rootstocks), scion infected percentage and HLB bacterial transmission percentage. The phytotoxicity was determined by the survival and growth of scions treated by antimicrobial compounds. The clustering results indicated that 27 antimicrobial compounds were divided into 3 groups. The first group including 12 compounds, such as Zineb, was not effective in eliminating the HLB bacteria, with high scion infection (67.9% ± 14.4% on average)， Las transmission percentage (83.4% ± 13.85 on average) as well as the high bacterial titers. The second group of only two compounds was also not effective against the HLB bacterium, but had high phytotoxicity to citrus (less than 40% of the scion survival and 15% of the scion growth). The third group including 13 compounds, such as nicotine, was effective in eliminating the Las bacteria and had no phytotoxicity to citrus. The effective compounds will be further tested in the container-potted plants and in the field.

Citrus Huanglongbing (HLB) is one of the most serious diseases of citrus worldwide. The present study was undertaken to screen antibiotics against Candidatus Liberibacter asiaticus (Las) while simultaneously assessing phytotoxicity to citrus. Twenty-eight antibiotics from ten classes of medical-antibiotics and three agricultural-antibiotics were tested for in vivo activities against HLB bacterium using the previously optimized graft-based chemotherapy method (Zhang et al., 2012). First, samples for DNA extraction were taken at 4 months after inoculation; subsequent samplings were taken at 2 month intervals. The Las-infected plants were considered as Las positive by real-time qPCR with threshold cycle (Ct) values less than 32.0. The efficiency against the HLB bacterium of each compound was evaluated by Ct values in the inoculated plants (both scions and rootstocks), scion infected percentage and HLB bacterial transmission percentage. The phytotoxicity was determined by the survival and growth of scions treated by antibiotics. The results showed that beta-lactam antibiotics (Ampicillin, Pennicillin and Carbenicillin) were highly effective in eliminating the HLB bacteria, with undetectable Las titers in the inoculated plants by qPCR, and had no phytotoxicity to citrus, with more than 75% scion survival. Antibiotics sulfonamide and tetracycline suppressed the HLB bacterium with Ct values of 35.7 on average, less than 30% scion infection and 16.9% Las-transmission percentage. The effectiveness of some antibiotics, such as aminoglycoside and quinolone, depended on their absorptions and permeability throughout the citrus tree. Peptide antibiotics were not effective in eliminating or suppressing Las bacterium with less than 28.0 Ct values by qPCR and higher scion infection percentages and Las transmission rates. Three agric-antibiotics, Actidione, Validoxylamine A and Zhongsenmycin, were also effective in eliminating the HLB bacteria. Antibiotic combinations, such as beta-lactam and aminoglycoside, are suggested as future applications.