Huanglongbing (HLB) is one of the main problems in citrus areas of many orange producing countries. It is caused by the bacterium Candidatus Liberibacter spp. and the symptoms of the disease have been related with the accumulation of callose and phloem proteins (PP2) in the tubes of sieve elements of the phloem of the infected plant. The mechanism of accumulation of PP2 proteins in sieve plates of the citrus phloem is possibly a defense reaction of the plant to reduce the bacterial colonization capacity via phloem. Triggering this response, the plant reduces the adequate transport of photoassimilates to the sink organs and generates an accumulation of photoassimilates in the source organs, which leads to the symptoms of HLB. Due to the serious damage caused, the HLB has mobilized the attention of research groups for the development of resistant cultivars. However, the genetic breeding of citrus is limited because this is a perennial plant, as well as for pollen incompatibility, polyembryony and parthenocarpy. The interfering RNA technique (RNAi) has been applied in other agricultural crops to obtain more disease tolerant plants and to identify the function of genes involved in abiotic and biotic stresses. However, works of silencing in citrus plants using RNAi are still incipient. Thus, our objectives are the knockdown of pp2 genes in sweet orange plants using the RNAi technology to confirm the function of the pp2 genes, in addition to obtaining citrus plants with greater tolerance to HLB.
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