Callose synthase family genes plays an important role in the Citrus defense response to Candidatus Liberibacter asiaticus

Abstract

Huanglongbing (HLB) is a destructive disease of citrus. The disease is caused by the phloem-limited fastidious proteobacterium Candidatus Liberibacter asiaticus, which is transmitted by the Asian citrus psyllid (Diaphorina citri). The symptoms of HLB have been related by callose accumulation in the phloem sieve plates. The key class of enzymes for callose synthesis is the Callose Synthases. The callose synthase genes (calS) expression is modulated for biotic and abiotic stresses. In this study, nine C. sinensis calS genes (CscalS) were identified and the expression patterns were analyzed in CaLas inoculated and healthy plants. At 120 days after inoculation (dpi), CscalS2 and CscalS7 were significantly up-regulated in the HLB positive plants. At 360 dpi CscalS7 and CscalS12 were significantly up-regulated in the HLB positive plants. Our results confirmed that CaLas infection is accompanied by the increased deposition of callose in the phloem sieve tubes and accumulation of starch in the leaves. It is suggested that the increased deposition of callose in the phloem sieve tubes is a hypersensitivity reaction, inhibiting phloem transport to consequently reducing the bacterial colonization via phloem, what possibly contribute to the starch accumulation in the leaves and the development of HLB symptoms. It is also proposed that CscalS2, CscalS7 and CscalS12 are involved in citrus defense against CaLas, forming a complex in the phloem.