Abstract
A wide range of miRNAs have been identified as phloem-mobile molecules that play important roles in coordinating plant development and physiology. Phytoplasmas are associated with hundreds of plant diseases, and the pathogenesis involved in the interactions between phytoplasmas and plants is still poorly understood. To analyse the molecular mechanisms of phytoplasma pathogenicity, the miRNAs profiles in mulberry phloem saps were examined in response to phytoplasma infection. A total of 86 conserved miRNAs and 19 novel miRNAs were identified, and 30 conserved miRNAs and 13 novel miRNAs were differentially expressed upon infection with phytoplasmas. The target genes of the differentially expressed miRNAs are involved in diverse signalling pathways showing the complex interactions between mulberry and phytoplasma. Interestingly, we found that mul-miR482a-5p was up-regulated in the infected phloem saps, and grafting experiments showed that it can be transported from scions to rootstock. Based on the results, the complexity and roles of the miRNAs in phloem sap and the potential molecular mechanisms of their changes were discussed. It is likely that the phytoplasma-responsive miRNAs in the phloem sap modulate multiple pathways and work cooperatively in response to phytoplasma infection, and their expression changes may be responsible for some symptoms in the infected plants.
Highlights
Mulberry trees that have long been cultivated for sericulture are susceptible to many diseases, among which yellow dwarf disease caused by phytoplasma is one of the most devastating[1]
To identify phloem-enriched sRNAs and ensure that the sRNAs identified in phloem sap did not result from contamination during sampling, the frequency of the ribulose bisphosphate carboxylase oxygenase (RuBisCo) large subunit gene that would be expected to in leaves, but not the sieve element–companion cell complex, was determined to assess the purity of the phloem sap sampled
The results detected no RuBisCo mRNA in the phloem sap samples, but this mRNA was clearly present in leaf tissue
Summary
Mulberry trees that have long been cultivated for sericulture are susceptible to many diseases, among which yellow dwarf disease caused by phytoplasma is one of the most devastating[1]. When subjected to pathogen infection, the host plant activates sophisticated response mechanisms to reprogramme the expression of genes, proteins and metabolites[5]. Different miRNAs, in addition to individual miRNAs in the same family, may be expressed differentially in various tissues and have different functions in response to the same pathogen infection[35]. Phloem is the major route for the translocation and distribution of organic metabolites and an important mediator of whole-plant communication involved in whole plant events, including stress responses and long-distance signalling[36]. Identification and characterization of phytoplasma-responsive miRNAs in the phloem sap promises to enhance our understanding of the molecular mechanisms involved in yellow dwarf disease symptom development. The results reported here may facilitate our understanding of phytoplasma pathogenicity
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