Abstract
Rac/Rop proteins play important roles in the regulation of cell growth and plant defense responses. However, the function of Rac/Rop proteins in wheat remains largely unknown. In this study, a small G protein gene, designated as TaRac6, was characterized from wheat (Triticum aestivum) in response to Puccinia striiformis f. sp. tritici (Pst) and was found to be highly homologous to the Rac proteins identified in other plant species. Transient expression analyses of the TaRac6-GFP fusion protein in Nicotiana benthamiana leaves showed that TaRac6 was localized in the whole cell. Furthermore, transient expression of TaRac6 inhibited Bax-triggered plant cell death (PCD) in N. benthamiana. Transcript accumulation of TaRac6 was increased at 24 h post-inoculation (hpi) in the compatible interaction between wheat and Pst, while it was not induced in an incompatible interaction. More importantly, silencing of TaRac6 by virus induced gene silencing (VIGS) enhanced the resistance of wheat (Suwon 11) to Pst (CYR31) by producing fewer uredinia. Histological observations revealed that the hypha growth of Pst was markedly inhibited along with more H2O2 generated in the TaRac6-silenced leaves in response to Pst. Moreover, transcript levels of TaCAT were significantly down-regulated, while those of TaSOD and TaNOX were significantly up-regulated. These results suggest that TaRac6 functions as a potential susceptibility factor, which negatively regulate the reactive oxygen species (ROS) burst in the wheat–Pst interaction.
Highlights
Small GTP-binding proteins are proteins that have a molecular weight of 20–40 kd
Analysis of cDNA library data revealed a Rac-like or Rop-like (Rac/Rop) homologous gene in wheat that was up regulated in a compatible interaction (Ma et al, 2009). The function of this Rac/Rop gene in wheat’s response to Puccinia striiformis f. sp. tritici (Pst) is still unknown. We report on this Rac/Rop family gene, designated as TaRac6, which was located to the whole cell and inhibited cell death induced by Bax
As a protein in the Rac/Rop GTPase family, TaRac6 contains five G boxes and a CxxL motif, which is the typical motif of the Rac/Rop protein belonging to Type I (Figure 1B)
Summary
Small GTP-binding proteins are proteins that have a molecular weight of 20–40 kd. They constitute a superfamily with five families—Ras, Ran, Rab, Rho, and Arf—which includes more than 100 members (Takai et al, 2001). TaRac Functions Negatively in Wheat-Pst. The Rac/Rop protein family contains five highly conserved G-boxes and a C-terminal motif (Wennerberg et al, 2005). Based on its C-terminal motifs, Rac/Rop proteins may be divided into two types (Winge et al, 2000): Type I have a conserved CaaL motif (a: aliphatic amino acid), while Type II lack the CaaL motif (Lavy et al, 2002) but have a cysteine domain to the membrane (Kawano et al, 2014). Prenylation is required for membrane attachment and function of type I Rops, while type II Rops with the cysteine domain are attached to the plasma membrane by S-acylation. The prenylation of type I Rops has only a small effect on ROP function. Type I ROPs affect the cell structure, primarily on the adaxial side, while type II ROPs induce a novel cell division phenotype (Sorek et al, 2011)
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