Circadian clock-regulated phosphate transporter PHT4;1 plays an important role in Arabidopsis defense

Abstract

The Arabidopsis accelerated cell death 6-1 (acd6-1) mutant shows constitutive defense, cell death, and extreme dwarf phenotypes. In a screen for acd6-1 suppressors, we identified a mutant that was disrupted by a T-DNA in the PHOSPHATE TRANSPORTER 4;1 (PHT4;1) gene. The suppressor mutant pht4;1-1 is dominant, expresses truncated PHT4;1 transcripts, and is more susceptible to virulent Pseudomonas syringae strains but not to several avirulent strains. Treatment with a salicylic acid (SA) agonist induced a similar level of resistance in Col-0 and pht4;1-1, suggesting that PHT4;1 acts upstream of the SA pathway. Genetic analysis further indicates that PHT4;1 contributes to SID2-dependent and -independent pathways. Transgenic expression of the DNA fragment containing the PHT4;1-1 region or the full-length PHT4;1 gene in wild-type conferred enhanced susceptibility to Pseudomonas infection. Interestingly, expression of PHT4;1 is regulated by the circadian clock. Together, these data suggest that the phosphate transporter PHT4;1 is critical for basal defense and also implicate a potential role of the circadian clock in regulating innate immunity of Arabidopsis.

Figure 1.

The pht4;1-1 Mutant Suppresses acd6-1-Conferred Phenotypes. (A) Picture of 25-day-old plants. (B) Northern blot analysis of PR1 expression. Total RNA was isolated from 25-day-old uninfected plants. rRNA was used as a loading control. (C) SA quantification. Uninfected 25-day-old plants were harvested for SA extraction followed by HPLC analysis. (D) Bacterial growth assay. 25-day-old plants were infected with Pseudomonas syringae pv. maculicola strain DG3 (PmaDG3) (OD₆₀₀ = 0.0001) and leaf discs were collected for bacterial growth assay 3 d after infection. CFU, colony forming unit. Different letters in (C) and (D) indicate significant difference among samples (P < 0.05; n = 3 in (C) and n = 6 in (D)). These experiments were repeated two times, with similar results.

Figure 2.

pht4;1-1 Is Dominant and Confers Enhanced Disease Susceptibility to Virulent Pseudomonas Strains. (A) Infection with PmaDG3 (OD₆₀₀ = 0.0001). (B) Infection with P. syringae pv. tomato DC3000 (DC3000) (OD₆₀₀ = 0.0001). (C) Infection with Pma avrRpt2 or Pma avrRpm1 (OD₆₀₀ = 0.0002). (D) Infection of heterozygous pht4;1-1 with PmaDG3 (OD₆₀₀ = 0.0001). 25-day-old plants were infected with the indicated Pseudomonas strains and assayed for bacterial growth. Significant difference between pht4;1-1 and Col-0 was observed at 2 and 3 days after infection in (A) and (B) and was indicated with different letters in (D) (P < 0.05; n = 6). No difference was observed in the two samples in (C) 3 d after infection with either avirulent strain. These experiments were repeated two times, with similar results.

Figure 3.

Two PHT4;1 Null Alleles Are Not Compromised in Disease Resistance. (A) Structure of the PHT4;1 gene and positions of the mutant alleles. Filled boxes indicate exons and horizontal lines indicate introns. Vertical lines indicate the positions of PHT4;1 alleles. The arrow pairs are primer sets used in RT–PCR in (B). (B) RT–PCR analysis. EF1α was used as a loading control. Note the PCR products amplified from a genomic DNA template included introns and therefore were larger than their corresponding RT–PCR products. (C) Bacterial growth assay. 25-day-old plants were infected with PmaDG3 (OD₆₀₀ = 0.0002). Six leaf discs from infected individual plants of each genotype were collected for bacterial growth assay 3 d after infection. No significant difference was observed among the genotypes. Experiments in (B) and (C) were repeated three times, with similar results.

Figure 4.

Transgenic Plants Expressing PHT4;1-1 or PHT4;1 Are More Susceptible to PmaDG3 Infection. A genomic fragment containing the PHT4;1-1 region or the full-length PHT4;1 gene was PCR amplified and cloned into the binary vector pGreenII 0029 (Hellens et al., 2000) for plant transformation. The PHT4;1 promoter (1329 bp) was used to drive the expression of these two constructs. 25-day-old plants were infected with PmaDG3 (OD₆₀₀ = 0.0001) and leaf discs were collected for bacterial growth assay 3 d after infection. Asterisks indicate significant difference between Col-0 and the transgenic plants (P < 0.05; n = 6). These experiments were repeated two times, with similar results.

Figure 5.

pht4;1-1-Conferred Susceptibility Can Be Suppressed by BTH Treatment. Plants were treated with 300 μM BTH for 36 h before PmaDG3 infection (OD₆₀₀ = 0.0001). Different letters indicate significant difference among samples (P < 0.05; n = 6). These experiments were repeated two times, with similar results.

Figure 6.

The pht4;1-1 Mutation Contributes to SID2-Dependent and -Independent Pathways. (A) Picture of 25-day-old plants. Note the triple mutants are larger than the double mutants. (B) Cell death phenotypes. The fifth leaves of the indicated genotypes were stained with trypan blue and photographed with a Photometrics CCD camera connected to a Leica dissecting microscope. Arrows indicate minor cell death in acd6-1sid2-1 and acd6-1pht4; 1-1sid2-1. These experiments were repeated two times, with similar results.

Figure 7.

Expression of PHT4;1 and PHT4;1-1 Is Light-Regulated. 25-day-old plants grown in a chamber with a 16-h light/8-h dark cycle were kept in the same condition or dark-treated for 24 h. Leaves were harvested for total RNA extraction followed by northern blotting. rRNA was used as a loading control. These experiments were repeated two times, with similar results.

Figure 8.

Expression of PHT4;1 Is Regulated by the Circadian Clock. (A) Circadian expression of PHT4;1 in acd6-1. 25-day-old acd6-1 plants grown in a chamber with a 12-h light/12-h dark cycle were kept in the same chamber or moved to a chamber with continuous light conditions. Starting from time 0 (9:00 am), plants were harvested every 6 h for 42 h. (B) Circadian expression of PHT4;1 during Pseudomonas infection. 25-day-old Col-0 plants grown in a chamber with a 12-h light/12-h dark cycle were infiltrated with PmaDG3 (OD = 0.0001) or 10 mM MgSO₄ as control. The infiltrated leaves were collected at 4-h interval for 24 h. Total RNA were extracted from the above samples and analyzed by northern blotting. White boxes indicate light periods and black boxes indicate dark periods. rRNA was used as a loading control. These experiments were repeated two times, with similar results.