Evaluation of CLas resistance/tolerance and expression analysis of resistance-related genes in 14 Citrus resources

<bold>Objective</bold> Citrus is one of the most economically important fruits in China，ranking first globally in both cultivation area and yield.Huanglongbing（HLB），caused by <italic>Candidatus</italic> Liberibacter asiaticus（<italic>C</italic>Las），is a devastating disease.Currently，the lack of highly <italic>C</italic>Las-resistant citrus germplasms has significantly hindered progress in disease-resistant breeding programs.This study evaluates the resistance of 14 citrus germplasms to <italic>C</italic>Las and analyzes the expression of resistance-related genes（<italic>PR1</italic>，<italic>PR2</italic>，<italic>PR5，</italic>and <italic>CRK10</italic>）.The results will provide both theoretical bases and elite germplasms for HLB-resistant citrus breeding. <bold>Method</bold> Two-year-old plants of 14 citrus germplasms were inoculated with <italic>C</italic>Las via graft transmission using infected buds.After inoculation，plant growth was assessed periodically，and <italic>C</italic>Las was detected by conventional PCR and quantitative PCR.At 18 months after bud-grafting inoculation，the expression levels of disease resistance-related genes （<italic>PR1</italic>，<italic>PR2</italic>，<italic>PR5</italic>，and <italic>CRK10</italic>） were analyzed by qPCR. <bold>Result</bold> Based on integrated analyses of growth performance，<italic>C</italic>Las bacterial titers，and resistance-related gene expression patterns，the 14 citrus germplasms were classified into four categories of <italic>C</italic>Las resistance/tolerance：1 highly resistant germplasm：‘Zhilaimeng-1’ （<italic>C. australasica</italic>）.2 highly tolerant germplasms：‘Zhike’（<italic>P. trifoliata</italic>） and ‘Xiaoye Nianju’（<italic>C. reticulata</italic>）.3 moderately tolerant germplasms：‘Shatianyu’（<italic>C.maxima</italic>），‘Zhilaimeng-2’（<italic>C. australasica</italic>），and ‘Zhilaimeng-3’（<italic>C. australasica</italic>）.8 susceptible germplasms：‘Shatangju’（<italic>C. reticulata</italic>），‘Kuweisuancheng’（<italic>C. aurantium</italic>），‘Tangcheng’，‘Wogan’（<italic>C. reticulata</italic>×<italic>C. sinensis</italic>），‘Bianyizhicheng’（<italic>P. trifoliata</italic>），‘Daidai’（<italic>C. aurantium</italic>），‘Tetraploid Xiangcheng’（<italic>C. junos</italic>）and ‘Bianju’（<italic>C. reticulata</italic>）.Compared with the susceptible control ‘Tangcheng’（<italic>C. reticulata</italic>×<italic>C. sinensis</italic>），the expression levels of resistance-related genes <italic>PR1</italic>，<italic>PR2</italic>，and <italic>PR5</italic> were significantly upregulated in ‘Zhilaimeng-1’（<italic>C. australasica</italic>）.Notably，<italic>PR5</italic> showed extreme upregulation in ‘Xiaoye Nianju’（<italic>C. reticulata</italic>），while <italic>CRK10</italic> was highly expressed in ‘Zhike’（<italic>P. trifoliata</italic>）. <bold>Conclusion</bold> Significant variation in <italic>C</italic>Las resistance is observed among citrus germplasms.‘Zhilaimeng-1’（<italic>C. australasica</italic>） exhibits durable resistance，while ‘Zhike’（<italic>P. trifoliata</italic>） and ‘Xiaoye Nianju’（<italic>C. reticulata</italic>）showed high tolerance.This disease resistance is associated with specific upregulation of resistance-related genes，where immune responses mediated by the salicylic acid pathway（<italic>PR1</italic>，<italic>PR2</italic>，and <italic>PR5</italic>）and <italic>CRK10</italic> played pivotal roles against <italic>C</italic>Las infection.