Functional analysis of TAL effectors from Xanthomonas citri and Xanthomonas aurantifolii pathotype 'C' and gene target activation in host plants

Author(s)

Valeria Yukari Abe

• Document type: Doctoral Thesis
• Press: Campinas, SP.
• Institution: Universidade Estadual de Campinas (UNICAMP). Instituto de Biologia
• Defense date: 2016-03-29

Examining board members

Celso Eduardo Benedetti; Cleverson Carlos Matiolli; Andrea Balan; Francisco de Assis Alves Mourão Filho; Henrique Ferreira

• Advisor: Celso Eduardo Benedetti

Abstract

Citrus canker, caused by Xanthomonas citri and Xanthomonas aurantifolii, is a serious disease that affects most commercial citrus plantations in Brazil. While X. citri causes canker in all commercial citrus varieties, X. aurantifolii pathotype C causes disease only in `Mexican¿ lime, triggering defense responses in sweet orange. Xanthomonas spp. usually translocate effector proteins into the host cell cytoplasm during the infection to suppress plant defenses and enhance bacterial growth. X. citri and X. aurantifolii C employ transcription activator-like (TAL) effectors of the PthA family to activate host disease susceptibility (S) genes. The X. citri strain 306 encodes four variants of PthA proteins, but only PthA4 was shown to act as pathogenic factor on citrus. On the other hand, the X. aurantifolii C strain ICMP 8435 encodes two variants of PthCs that could act as avirulence proteins (Avr) in sweet orange. This work aimed to assess the contribution of each PthA in citrus canker development and bacterial growth in planta; and verify the possible role of PthCs as Avr proteins in sweet orange. In addition, the expression of direct targets of these effectors in citrus were evaluated to identify candidate S genes associated with canker development. Thus, deletion mutants for the pthAs genes were obtained, which were complemented with pthCs genes. These mutants were used to challenge various commercial citrus varieties for canker symptoms evaluation and gene expression analysis. It was found that PthA4 was essential for citrus canker induction in all citrus varieties tested; however, PthA1 and PthA3 played an additive role in canker development induced by PthA4, acting in a host-dependent manner. Deletions in two or more pthA genes reduced bacterial growth in planta more pronouncedly than single deletions, suggesting an additive role of PthAs in pathogenicity. Furthermore, it was observed that PthC1 does not act as an Avr protein in sweet orange plants. Additionally, the contribution of PthAs 1 and 3 in canker formation in `Pera¿ plants did not correlate with the activation of the canker S gene LOB1 (LATERAL ORGAN BOUNDARIES 1), the only citrus canker S gene known to date, but with the induction of other PthA targets, including LOB2 and citrus dioxygenase (DIOX). LOB1, LOB2 and DIOX showed differential PthA-dependent expression between `Pera¿ and `Tahiti¿ plants that appears to be associated with nucleotide polymorphisms found at or near PthA-binding sites. The data shown here also indicates that LOB1 activation alone is not sufficient to elicit cankers on citrus, and that DIOX acts as a canker S gene in `Pera¿ but not `Tahiti¿ plants, suggesting that activation of multiple S genes, such as LOB1 and DIOX, is necessary for full canker development (AU)

• FAPESP's process: 11/19988-0 - Functional analysis of the TAL effectors from Xanthomonas citri and Xanthomonas aurantifolii patotipo C
• Grantee: Valeria Yukari Abe
• Support Opportunities: Scholarships in Brazil - Doctorate