RACK1 Functions in Rice Innate Immunity by Interacting with the Rac1 Immune Complex  

Abstract

A small GTPase, Rac1, plays a key role in rice (Oryza sativa) innate immunity as part of a complex of regulatory proteins. Here, we used affinity column chromatography to identify rice RACK1 (for Receptor for Activated C-Kinase 1) as an interactor with Rac1. RACK1 functions in various mammalian signaling pathways and is involved in hormone signaling and development in plants. Rice contains two RACK1 genes, RACK1A and RACK1B, and the RACK1A protein interacts with the GTP form of Rac1. Rac1 positively regulates RACK1A at both the transcriptional and posttranscriptional levels. RACK1A transcription was also induced by a fungal elicitor and by abscisic acid, jasmonate, and auxin. Analysis of transgenic rice plants and cell cultures indicates that RACK1A plays a role in the production of reactive oxygen species (ROS) and in resistance against rice blast infection. Overexpression of RACK1A enhances ROS production in rice seedlings. RACK1A was shown to interact with the N terminus of NADPH oxidase, RAR1, and SGT1, key regulators of plant disease resistance. These results suggest that RACK1A functions in rice innate immunity by interacting with multiple proteins in the Rac1 immune complex.

Plasma Membrane Microdomains Are Essential for Rac1-RbohB/H-Mediated Immunity in Rice

“…These results indicate that Os-Rac1 and Os-Rbohs are specific PM microdomain proteins that respond to chitin elicitor and that PM microdomains are necessary for their localization and dynamics in the PM. Moreover, Os-RACK1 and Os-HSP70, which interact with Os-Rac1 (Thao et al, 2007;Nakashima et al, 2008), and Os-flotillin, a PM microdomain protein (Li et al, 2012), were detected in only DRM fraction of the wild type (Supplemental Figure 10). On the other hand, Os-CEBiP, a chitin receptor, was present in most fractions of both the wild type and OsFAH1/2-KD1 and declined with chitin treatment ( Figure 5B; Supplemental Figure 10).…”

“…We have previously proposed that rice Rac1 participates in a large protein network, termed the defensome, including HSP70, HSP90, the cochaperone Hop/Sti1a, the scaffold protein RACK1, and the lignin biosynthesis enzyme CCR1 (Kawasaki et al, 2006;Thao et al, 2007;Nakashima et al, 2008;Chen et al, 2010b). Fujiwara et al (2009) demonstrated that RACK1 shifts to the DRM fraction in response to chitin treatment, similar to Rac1.…”

“…Antibodies a-OsRac1, a-RACK1, and a-flotillin antibody were described previously (Lieberherr et al, 2005;Nakashima et al, 2008;Ishikawa et al, 2015). a-StRbohB, a-OsCEBiP, and a-CHC antibodies were kindly provided by Hirofumi Yoshioka (Nagoya University), Naoto Shibuya (Meiji University), and Masaru Fujimoto (University of Tokyo), respectively.…”

Rice RING protein OsBBI1 with E3 ligase activity confers broad-spectrum resistance against Magnaporthe oryzae by modifying the cell wall defence

“…Two R genes, Pi9 and Pi21, confer broad-spectrum resistance against multiple races of M. oryzae [9,10]. Rac GTPasemediated events (e.g., generation of ROS), activation of MAP kinase, and other key components, such as RAR1, SGT1, Hsp90, and WRKY45, play important roles in regulating the immune response against M. oryzae [11][12][13][14][15][16][17][18][19][20][21][22][23][24]. Endogenous SA is involved in defence against M. oryzae [25].…”

Phosphorylation of RACK1 in plants

“…8,9 Similar to mammalian RACK1, plant RACK1 interacts with a large group of proteins 10,11 and is involved in diverse biological processes, ranging from seed germination, leaf and root development to flowering, 8,12 and is also involved in responses to plant hormones, 8,13 and biotic and abiotic stresses. [13][14][15] At the molecular level, RACK1 protein is associated with ribosomes [16][17][18] and is involved in the regulation of protein translation 18 and miRNA abundance. 19 It is less clear how RACK1 is able to participate in so many different biological processes.…”