Mini-review: oomycete RXLR genes as effector-triggered immunity

Abstract

Oomycetes are known to secrete a vast arsenal of effectors that modulate the host defense system as well as facilitate establishing a parasitic infection in plants. In recent years, tremendous progress has been made in the field of effectromics based on studies of oomycetes, especially the cytoplasmic family of RXLR effectors. Yet, the biology of the RXLR effector family is still poorly understood. There has been a consensus regarding the structure of the RXLR motif in the mycologist community. However, the function of the RXLR motif is still unclear. First, different models have suggested that the role of the RXLR motif is either in translocation to a target destination inside a host cell or in the cleavage of itself followed by secretion. Second, recent studies have suggested different functional models for the RXLR motif. According to a widely accepted model, the RXLR motif is directly involved in the translocation of effectors to target sites. In contrast, a new study has proposed that the RXLR motif is involved in secretion rather than translocation. Thus, this review is an attempt to summarize the recent advances made in the functional analysis of the N-terminal domain of RXLR effectors.

Fig. 1. Domain organization of cytoplasmic RXLR effectors. Schematic drawings of PiAVR3a of Phytophthora infestans (Bos et al., 2006), PiAvrblb1 and PiAvrblb2 of P. infestans (Vleeshouwers et al., 2008; Oh et al., 2009; Oh et al., 2010), Avr1b-1 of P. sojae (Shan et al., 2004), and HpATR13 of Hyaloperonsopora parasitica (Allen et al., 2004). The numbers under the sequences indicate the amino-acid positions. The highlighted RXLR domain includes the RXLR sequence itself and the downstream dEER sequence. The gray colors distinguish the regions of the effector proteins that are involved in secretion and targeting from those involved in effector activity (Modified from Morgan and Kamoun, 2007; Oh et al., 2010).

Fig. 2. The 34-amino acid C-terminal region of AVRblb2 (Avrblb247-81) is sufficient for activation of Rpi-blb2–mediated hypersensitive cell death (HCD). Various AVRblb2-deletion constructs were co‐expressed with Rpi-blb2 by agro‐infiltration in Nicotiana benthamiana to determine the AVRblb2 domains required for the induction of the Rpi-blb2‐mediated HCD. A schematic view of the various deletion constructs is shown on the left. Symptoms of infiltration sites co‐expressing the AVRblb2 constructs with Rpi-blb2 are shown on the right (Modified from Oh et al., 2009). Photographs of symptoms were taken 5 days post-infiltration. SP, signal peptide.