Abstract
In eukaryotes, regulated protein turnover is required during many cellular processes, including defense against pathogens. Ubiquitination and degradation of ubiquitinated proteins via the ubiquitin–proteasome system (UPS) is the main pathway for the turnover of intracellular proteins in eukaryotes. The extensive utilization of the UPS in host cells makes it an ideal pivot for the manipulation of cellular processes by pathogens. Like many other Gram-negative bacteria, Xanthomonas species secrete a suite of type-III effector proteins (T3Es) into their host cells to promote virulence. Some of these T3Es exploit the plant UPS to interfere with immunity. This review summarizes T3E examples from the genus Xanthomonas with a proven or suggested interaction with the host UPS or UPS-like systems and also discusses the apparent paradox that arises from the presence of T3Es that inhibit the UPS in general while others rely on its activity for their function.
Highlights
The bacterial genus Xanthomonas consists of a large group of Gram-negative plant pathogenic bacteria comprising 27 species that infect a wide range of economically important crop plants, such as rice, citrus, banana, cabbage, tomato, pepper, and bean (Ryan et al, 2011)
This review summarizes T3E examples from the genus Xanthomonas with a proven or suggested interaction with the host ubiquitin–proteasome system (UPS) or UPS-like systems and discusses the apparent paradox that arises from the presence of T3Es that inhibit the UPS in general while others rely on its activity for their function
Several Xanthomonas species and pathovars appear to utilize T3E proteins from widespread families such as the YopJ-like superfamily or XopD-like family to interfere with the UPS
Summary
The bacterial genus Xanthomonas consists of a large group of Gram-negative plant pathogenic bacteria comprising 27 species that infect a wide range of economically important crop plants, such as rice, citrus, banana, cabbage, tomato, pepper, and bean (Ryan et al, 2011). To date, ∼40 T3Es of the genus Xanthomonas have been identified, which are divided into groups based on their sequence identities (White et al, 2009) These T3Es function as virulence and avirulence factors either by suppressing PAMPtriggered immunity (PTI) or through the recognition by host immune receptors (Resistance proteins) and subsequent elicitation of the so called effector-triggered immunity (ETI; Jones and Dangl, 2006). Beyond its role in marking target proteins for degradation via the 26S proteasome, ubiquitination can regulate cellular signaling processes. Several components of the UPS were identified as regulators of plant immunity during PTI and ETI, such as pepper E3 ligase CaRING1 that is induced upon Xanthomonas infection and is required for the activation of cell death (Lee et al, 2011). Considering the involvement of the UPS in plant defense mechanisms, co-evolution has selected for T3Es and toxins that can manipulate ubiquitin and ubiquitin-like pathways in order to www.frontiersin.org
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