AtPRK2 promotes ROP1 activation via RopGEFs in the control of polarized pollen tube growth

Abstract

The ROP1 GTPase-based signaling network controls tip growth in Arabidopsis pollen tubes. Our previous studies imply that ROP1 might be directly activated by RopGEF1, which belongs to a plant-specific family of Rho guanine nucleotide exchange factors (RopGEFs) and in turn may be activated by an unknown factor through releasing RopGEF1's auto-inhibition. In this study, we found that RopGEF1 forms a complex with ROP1 and AtPRK2, a receptor-like protein kinase previously shown to interact with RopGEFs. AtPRK2 phosphorylated RopGEF1 in vitro and the atprk1,2,5 triple mutant showed defective pollen tube growth, similar to the phenotype of the ropgef1,9,12,14 quadruple mutant. Overexpression of a dominant negative form of AtPRK2 (DN-PRK2) inhibited pollen germination in Arabidopsis and reduced pollen elongation in tobacco. The DN-PRK2-induced pollen germination defect was rescued by overexpressing a constitutively active form of RopGEF1, RopGEF1(90-457), implying that RopGEF1 acts downstream of AtPRK2. Moreover, AtPRK2 increased ROP1 activity at the apical plasma membrane whereas DN-PRK2 reduced ROP1 activity. Finally, two mutations at the C-terminal putative phosphorylation sites of RopGEF1 (RopGEF1S460A and RopGEF1S480A) eliminated the function of RopGEF1 in vivo. Taken together, our results support the hypothesis that AtPRK2 acts as a positive regulator of the ROP1 signaling pathway most likely by activating RopGEF1 through phosphorylation.

Keywords: AtPRK2; ROP GTPase; RopGEF1; auto-inhibition; polarity growth..

Figure 1.

Phylogenetic Tree of Selected AtPRKs and LePRKs. LePRK1–3 from tomato, six pollen-specific (AtPRK1–6), and several non-pollen-specific AtPRKs from Arabidopsis were included in this tree. Rice genes are included as outgroups.

Figure 2.

Pollen Tube Phenotypes of the Quadruple gef Mutant and Triple prk Mutant, and the DN-PRK2 OX/RopGEF1(90–457) Transgenic Plants. (A–C) Pollen tubes of Col0 (A), prk1 prk2 prk5 triple mutant (B), and gef1 gef9 gef12 gef14 quadruple mutant (C), after 5-h incubation in pollen germination medium.(D–G) Pollen germination assay of Col0 (D), DN-PRK2 (E), DNPRK2, RopGEF1 (F), and DNPRK2, RopGEF1(90–457) (G) plants.(H, I) Statistical analysis of pollen tube growth (H) and the germination rate of various genotypes (I).(J) RT–PCR results suggest that the expression level of DNPRK2 does not change in all of the tested lines. Bar = 100 μm. (A–C) same magnification and (D–G) same magnification.

Figure 3.

ROP1 Activity Is Positively Regulated by AtPRK2 and Negatively Regulated by DNPRK2 in Pollen Tube Growth.(A–E) ROP1 activity was increased by AtPRK2 and decreased by DN-PRK2. The distribution of the active ROP1 marker, GFP–RIC4 and control (A), AtPRK2 OX (B), and DN-PRK2 OX (C) pollen tubes are shown. Distribution of GFP–RIC4ΔC was enlarged in most AtPRK2 OX pollen tubes and narrowed in most DNPRK2 pollen tubes. (D) Statistical analysis of GFP–RIC4ΔC distribution in each background compared to wild-type pollen tube. Data were collected from three independent experiments.(F–I) DN-PRK2 suppresses RIC4-OX-induced defect in polar pollen tube growth. (E) GFP, (F) DN-PRK2+GFP, (G) DN-PRK2+GFP–RIC4, and (H) GFP–RIC4 were transiently expressed in tobacco pollen by bombardment and germinated for 5 h at RT before visualization. (I) Quantitative data of pollen tube widths from various backgrounds. Bar shows SD in (H) and (I). Scale bar = 5 μm in (A); Scale bar = 25 μm in (E).

Figure 4.

RopGEF1 Is Phosphorylated by AtPRKs and Forms a Complex with AtPRK2 and ROP1.(A) Both full-length RopGEF1 and RopGEF1(90–457) interact with the kinase domains of AtPRK2, AtPRK3, and AtPRK4.(B) RopGEF1 is phosphorylated by AtPRK2, AtPRK3, and possibly AtPRK4. SOS2 was used as a positive control and GST alone was used as the negative control.(C) The kinase domain of AtPRK2 interacts with various forms of ROP1. GDP/GTP-bound form ROP1, GTP–DP2, and DN-ROP1 were used in each reaction as indicated.(D) The HIS-tagged kinase domain of AtPRK2, MBP–RopGEF1, and various forms of ROP1 form complexes in vitro.(E) Interaction assay between the kinase domain of AtPRK2, RopGEF1, and ROP1 after incubation for 3, 6, 9, and 12 h with/without alkaline phosphatase.(F) Pull-down assay to verify the interaction between ROP1, MBP-tagged ROPGEF1, and wild-type or mutated kinase domain of AtPRK2.

Figure 5.

The C-Terminal Region of RopGEF1 Is Important for Its Interaction with AtPRK2 and ROP1.(A) A diagram of RopGEF1 motifs. The PRONE domain, S1-3 subdomains, N-terminal, and C-terminal regions of RopGEF1 are indicated.(B) MBP-tagged truncated RopGEF1 was used in pull-down assays with the His-tagged kinase domain of AtPRK2 (His–PRK2) and various forms of GST-tagged ROP1. His–PRK2 or various GST–ROP1 included in the assay are indicated on the left. Different truncated RopGEF1 forms used in each assay are indicated above and signals were detected by anti-MBP antibodies. Proteins loaded in the assay are also shown.

Figure 6.

Phosphorylation in C-Terminal Regions Is Critical for RopGEF1 Activity.Serine-to-alanine mutation of several sites in the RopGEF1 C-terminal region decreases RopGEF1 activity. Wild-type and mutant RopGEF1 were transiently expressed in tobacco pollen for 5 h before observation. Compared to GFP (A), GFP–RopGEF1wt (F) caused severe depolarization of pollen tubes with balloon-like tips. The GFP–S460A mutation (B) and GFP–S480A mutation (C) show greatly reduced RopGEF1-induced depolarization; pollen tubes overexpressing the GFP–S460A mutant are much longer than GFP–RopGEF1wt. Overexpression of GFP–S484A (D), GFP–S488A (E), GFP–S458A (G), and GFP–S501A (H) mutations caused severely depolarized pollen tubes as wild-type RopGEF1 does. Scale bars shown, and images (A–C) with the same magnification and (D–H) with the same magnification. Arrowhead indicates the tip of pollen tubes.

Figure 7.

The S480 Site of RopGEF1 Is Conserved in RopGEF1 Orthologs and S460 Is Conserved in Brassicaceae.(A) A phylogenetic tree of RopGEF genes in dicots and monocots. Arabidopsis RopGEFs are highlighted in dark pink squares, other dicots genes are labeled by blue rhombuses, and the monocots genes are indicated by green triangles. Selaginella genes were included to root the tree.(B) Alignment of the C-terminal region of RopGEF1 orthologous genes. The S460 and S480 sites are indicated by red arrows.

Figure 8.

A Model for AtPRK2-Induced Release of RopGEF1’s Auto-Inhibition and Activation of ROP1 Signaling Pathways. RopGEF1 is subjected to auto-inhibitory regulation, which is controlled by its C-terminal region. Auto-inhibition is released through the phosphorylation of the serine amino acids in its C-terminal region upon RopGEF1 interaction with AtPRK2. The phosphorylation-induced conformational change in RopGEF1 exposes the PRONE domain. Active RopGEF1 can then promote the conversion of the GDP-bound inactive form of ROP1 into its GTP-bound active form, thereby enhancing the ROP1 signaling pathways and regulating pollen germination and pollen tube growth. We also propose that the GTP form of ROP1 interacts with both RopGEF1 and AtPRK2 and somehow, through positive feedback, regulates RopGEF1 activity. The PRONE domain of RopGEF1 is shown as brown cylinders, the N-terminal region is shown as a brown curve, and the C-terminal region is shown as a purple curve. Green arrow indicates positive regulation.