The role of the ubiquitin-proteasome system in Agrobacterium tumefaciens-mediated genetic transformation of plants.

Agrobacterium tumefaciens-mediated genetic transformation of plants is the first example of transkingdom gene transfer and had been considered the only known natural example of such a case until the recent discovery of Bartonella henselae-mediated transformation of human cells under laboratory conditions (Schröder et al., 2011). In nature, the pathogenic soil bacterium A. tumefaciens induces neoplastic growths (crown gall tumors) on various plant species, including many agronomically important crops. During its infection, A. tumefaciensmobilizes a single-stranded copy of the bacterial transferred DNA (T-DNA) into the host cell and subsequently integrates it into the host genome (Gelvin, 2000, 2010; Tzfira and Citovsky, 2002; Pitzschke and Hirt, 2010). The wild-type T-DNA encodes several genes involved in auxin and cytokinin biosynthesis, and their expression in the infected plant cells leads to abnormal cell proliferation and the formation of tumors. With the help of other genes encoded by the T-DNA, the tumors then synthesize and secrete opines, amino acid derivatives that can be metabolized mainly by A. tumefaciens. This unique infection strategy allows A. tumefaciens to hijack the host cell machinery and turn it into its own “food factory.” Although A. tumefaciens mainly infects dicotyledonous plants in nature (De Cleene and De Ley, 1976), it can genetically transform virtually any eukaryotic species under laboratory conditions (Lacroix et al., 2006). Because of this broad host range, A. tumefaciens serves as a transformation vehicle of choice for the genetic manipulation of most plant species as well as numerous fungal species (Lacroix et al., 2006). Thus, understanding the molecular mechanism of A. tumefaciens infection is important not only to protect crops from the crown gall disease and to improve the efficiency of A. tumefaciens-mediated genetic engineering, but it also substantially advances our knowledge of fundamental aspects of genetic transformation and bacterial pathogen-host interactions. Recently, the ubiquitin-proteasome system (UPS) has emerged as a critical player in plant-pathogen interactions (Citovsky et al., 2009; Dielen et al., 2010; Trujillo and Shirasu, 2010). Numerous studies have shown that the plant UPS regulates the host defense responses, presumably by controlling the stability of the host and/or pathogen proteins. Moreover, increasing evidence suggests that several plant pathogens exploit the host UPS for efficient infection, further emphasizing the importance of the UPS in plant-pathogen interactions (Magori and Citovsky, 2011b). Consistent with this notion, the host UPS plays a critical role in the A. tumefaciens-plant interaction. Recent studies have shown that, upon A. tumefaciens infection, the host plants up-regulate or down-regulate several UPS-associated genes and proteins (Ditt et al., 2006; Anand et al., 2007, 2012; Zhao et al., 2011; Tie et al., 2012), some of which likely affect the efficiency of the A. tumefaciens infection (Zaltsman et al., 2010; Anand et al., 2012). In addition, A. tumefaciens is known to export into the host cell an F-box protein, a component of the SCF (for S-PHASE KINASE-ASSOCIATED PROTEIN1 (SKP1)-CULLIN1 (CUL1)-F-box protein) ubiquitin ligase complex, and facilitate infection via the UPSmediated protein degradation (Tzfira et al., 2004). Thus, A. tumefaciens represents a powerful model system to study how plants defend against invading pathogens via their UPS and how pathogens exploit the host UPS during infection. In this review, we focus on recent advances in understanding the role of the UPS in A. tumefaciens-mediated genetic transformation (summarized in Fig. 1).