A haplotype-specific Resistance gene regulated by BONZAI1 mediates temperature-dependent growth control in Arabidopsis

Abstract

Plant growth homeostasis and defense responses are regulated by BONZAI1 (BON1), an evolutionarily conserved gene. Here, we show that growth regulation by BON1 is mediated through defense responses. BON1 is a negative regulator of a haplotype-specific Resistance (R) gene SNC1. The bon1-1 loss-of-function mutation activates SNC1, leading to constitutive defense responses and, consequently, reduced cell growth. In addition, a feedback amplification of the SNC1 gene involving salicylic acid is subject to temperature control, accounting for the regulation of growth and defense by temperature in bon1-1 and many other mutants. Thus, plant growth homeostasis involves the regulation of an R gene by BON1 and the intricate interplay between defense responses and temperature responses.

Figure 1.

bon1-2 Has Wild-Type Morphology. Col, bon1-1, Ws, and bon1-2 plants grown at 22°C before (A) and after (B) bolting. bon1-1 is dwarf with dark-green and twisted leaves and short inflorescence stems. bon1-2 resembles wild-type Ws.

Figure 2.

The Col-Specific SNC1 Gene Mediates the bon1-1 Phenotype. (A) A genetic map of the region carrying the MOB locus. The corresponding nucleotide positions (in Col) are numbered in kilobases below the line. MOB was positioned between markers VRN2 and TGCAPS2. The number of recombinants confirmed is indicated underneath the markers. (B) A molecular map of the Col RPP5 complex locus. Predicted genes are shown by arrows indicating the direction of transcription. The locus name, the complex locus name (Noel et al., 1999), and the genetic name (if known) are given for each gene. T-DNA insertion lines from the Salk Institute are indicated below the corresponding genes. (C) SNC1 RNA expression in bon1-1 and bon1-1 snc1-11. SNC1 transcript is ∼4.5 kb in the wild type and <1 kb in snc1-11. (D) Suppression of bon1-1 growth phenotype by snc1-11. Col, bon1-1, and bon1-1 snc1-11 grown at 22°C are shown. bon1-1 snc1-11 resembles wild-type Col.

Figure 3.

Sequence Comparison among SNC1, SNW, and RPP4. TIR domain and NB-Apaf-1, R proteins, and Ced4 domain are labeled. Identical amino acids are indicated by white letters with black background. High sequence identity between SNC1 and SNW is found mostly in the TIR and NB-Apaf-1, R proteins, and Ced4 domains. The C-terminal LRR region of SNW is not more similar to that of SNC1 than to that of RPP4.

Figure 4.

The bon1-1 Growth Defect Is Induced by Constitutive Defense Responses. (A) Disease resistance phenotype of bon1-1, bon1-2, and bon1-1snc1-11. No growth of P. parasitica is observed in bon1-1, whereas a similar amount of growth is found between Col and bon1-1 snc1-11 and between the Ws wild type and bon1-2. The experiment was repeated twice, and similar trends were observed. (B) Resistance to virulent P. parasitica is suppressed by nahG, eds1-22, pad4-1, and snc1-11. Sporulations on representative leaves are shown. (C) Suppression of PR1 gene upregulation in bon1-1 by snc1-11, eds1-22, and pad4-1 mutations and by the transgene nahG. rRNAs detected by ethidium bromide staining were used as a control. (D) Suppression of the dwarf phenotype of bon1-1 by the transgene nahG and mutations of eds1-22 and pad4-1.

Figure 5.

SNC1 Is under a Positive Feedback Regulation. (A) Upregulation of the SNC1 RNA transcripts in bon1-1 and snc1-1 compared with Col. (B) Induction of the SNC1 transcript by SA application. RNAs were extracted 2 d after plants were sprayed with 0, 0.5, and 1 mM of SA. (C) Suppression of SNC1 upregulation in bon1-1 by pad4-1 and nahG. Wild-type level of the SNC1 transcripts is observed in bon1-1 pad4-1 and bon1-1 nahG.

Figure 6.

Temperature Regulation of the bon1-1 Phenotype. (A) PR1 expression in bon1-1 at 22°C and 28°C. The PR1 gene is upregulated in bon1-1 at 22°C but not at 28°C. (B) Phenotypes of Col, bon1-1, cpr1-1, snc1-1, and snc1-2 grown at 22°C and 28°C. Higher temperature (28°C) suppresses the dwarf phenotype of these mutants exhibited at 22°C. (C) SNC1 RNA expression in the wild type, bon1-1, cpr1-1, and snc1-2 grown at 22°C and 28°C. Induction of the SNC1 transcript in these mutants at 22°C is abolished at 28°C. (D) EDS1 RNA expression in the wild type and bon1-1 at 22°C and 28°C. The EDS1 transcript level is higher at 22°C than at 28°C in both the wild type and bon1-1.

Figure 7.

Model for Growth Homeostasis Controlled by BON1. BON1 is a negative regulator of the R gene SNC1. SNC1, when activated, can induce SA accumulation and defense responses that in turn inhibit cell growth. A positive feedback regulation exists between SNC1 and SA, and high temperature suppresses this feedback amplification or components in the feedback regulation.