Papaya Dieback in Malaysia: A StepTowards A New Insight of Disease Resistance

A recently published article describing the draft genome of Erwiniamallotivora BT-Mardi (1), the causal pathogen of papaya dieback infection in Peninsular Malaysia, hassignificant potential to overcome and reduce the effect of this vulnerable crop (2). The authors found that the draft genome sequenceis approximately 4824 kbp and the G+C content of the genomewas 52-54%, which is very similarto the reference genomes of other Erwinia species. They concluded that the draft genome would be useful for understanding host-pathogen interaction and determiningthe mechanism of infection of the disease.In addition to this, we believe that there are some furtherdirectionsthat should be considered in the future.     Firstly, the comprehensive analysis of pathogenicity genes in E. mallotivora, whichwould enable the comparison of genes important in pathogenicitybetween species in the genusErwinia, and highlight any differences. These include the most important and well-studied pathogenicity system in gram negative bacteria: the Type III secretion (TTSS).  Investigation of genes important in the pathogenesis of E. mallotivorawill decipher the function of these genes andunravel the complex interaction of the bacteria and host, and this will enable the understanding of their capabilities to enter plant cells,overcome host resistance and induce disease. This in-depth understanding will potentially allow the identification and application of new sources of biocontrolfor this disease.     Secondly, the elucidation and identification of the potential pathogen-inducible and defence-related genes in papaya. The papaya genome is available onlineto enable the identification of the putative immunity related genes (IRG) and it will be greatly useful for revealing the blueprint of host genes under pathogen stress. Wee at al. (3) reported the expression analysis for four possible defence related genes in the E. mallotivara host (zinc finger protein, leucine rich protein, aquaporin and peroxidase), in order to understand the defence mechanisms in papaya. This information will be valuable for strategies in generating host resistance and understanding host-pathogen interaction.   Thirdly,the discovery of papaya microRNAs (miRNAs) is also important. Previous studies have shown that miRNAs could act in wide range of biological processes as well as in the plant’s defence system towards biotic and abiotic stresses. Therefore, profiling and functional analysis of miRNAs would be important to generatenovel possibilities in improving resistance to dieback. The high potential application of miRNAs in the host defence mechanism indicates that they should be used or manipulated to confer pathogen resistance in papaya.     Finally, thedevelopment of resistant plants using a genetic engineering approach is also an important tool in the control of papaya dieback disease. Currently, there is no chemical controltreatment available and no resistant papaya varieties offeredin order to overcome this severe disease, and generating resistant plants using a biotechnology approach could be the solution. The strategy of disruption of bacterial virulence factors, improvement of plant defence responses and the expression of antimicrobial proteins of non-plant origincould be applied in papaya to confer resistance.     The draft genome of E. mallotivora BT Mardi is the first step and the initialkey to the understanding of the host-microbe interaction and mechanism of infection of this severedisease. This could be fundamental for plant pathologists, molecular biologists and botanists in Malaysia - it could help them to focus on the ultimate goal of control of papaya dieback disease.