Evaluation of the TA system in the inhibition of X. citri PAMPs in copper stress and PTI repression in sweet orange

• Grant number: 17/25520-8
• Support Opportunities: Scholarships in Brazil - Scientific Initiation
• Start date: March 01, 2018
• End date: February 29, 2020
• Field of knowledge: Agronomical Sciences - Agronomy - Plant Health
• Principal Investigator: Alessandra Alves de Souza
• Grantee: Giovana Betin Peruchi
• Host Institution: Instituto Agronômico (IAC). Agência Paulista de Tecnologia dos Agronegócios (APTA). Secretaria de Agricultura e Abastecimento (São Paulo - Estado). Campinas , SP, Brazil
• Associated research grant: 13/10957-0 - Xylella fastidiosa-vector-host plant interaction and approaches for citrus variegated chlorosis and citrus canker control, AP.TEM

Abstract

Citrus canker is caused by Xanthomonas citri subsp. citri, which affect all citrus species and cause huge losses to Brazilian and world agribusiness. However, the severity of the disease differs according to the citrus species and their varieties. The different levels of resistance may be associated to a better efficiency of these species in the recognition of pathogen molecules (PAMPs/MAMPs) resulting in activation of the innate immunity of the host (PTI). It has been proved that species more resistant to citrus cankers are able to recognize the pathogen molecules and activate a defense response. In general, this recognition occurs trough plants membrane receptors, called pathogen molecular recognition receptors (PRR). These receptors generally recognize molecules that are kept among microorganisms, allowing their functionality in a wide range of pathogens. Some studies of X. citri are aimed at exploring this mechanism of activation of PTI to look for more resistant varieties to citrus canker. Though, currently copper is used to control the bacteria dispersion in the citrus orchards. Previous works by our research group has shown that when copper is sprayed at sub-inhibitory concentrations, the opposite effect is observed, meaning the bacteria increase their population and promote more symptoms. Interestingly, one of the genetic mechanisms of bacterial survival activated under stress conditions is the Toxin-Antitoxin (TA) system. Under stress conditions toxins from the TA system suppress bacterial growth as a result of inactivation of genes associated with essential cellular processes, however, after the end of stress the cells recolonize the environment due to the inactivation of the toxin by the antitoxin. Our research group has been working with TA systems activated in copper stress and we have verified that some toxin targets are genes associated with cell division and movement. This way, the products of these genes are the same ones that are recognized by the PRRs in the host plants for PTI activation. The hypothesis of this work is that under stress conditions X. citri activates the TA systems by repressing genes which would be recognized by the host and in the absence of these molecules, occurs suppression of PTI. Consequently without the activated PTI system the host plant is infected efficiently by the bacteria increasing the symptoms of citrus canker (phenotype observed in our experiments).In case of our hypothesis is true, this will be the first evidence that the TA system can have a direct implication on plant-pathogen interaction, besides survival under stress conditions. (AU)