Incremental steps toward incompatibility revealed by Arabidopsis epistatic interactions modulating salicylic acid pathway activation

Abstract

Plant growth is influenced by genetic factors and environmental cues. Genotype-by-environment interactions are governed by complex genetic epistatic networks that are subject to natural selection. Here we describe a novel epistatic interaction modulating growth in response to temperature common to 2 Arabidopsis recombinant inbred line (RIL) populations (Ler x Kas-2 and Ler x Kond). At 14 degrees C, lines with specific allele combinations at interacting loci (incompatible interactions) have severe growth defects. These lines exhibit deregulated cell death programs and enhanced disease resistance. At 20 degrees C, growth defects are suppressed, but a positive trait of enhanced resistance is retained. Mapping of 1 interacting QTL to a cluster of RPP1-like TIR-NB-LRR genes on chromosome 3 is consistent with our finding that environmentally conditioned epistasis depends on activation of the salicylic acid (SA) stress signaling pathway. The nature of the epistatic interaction conforms to the Dobzhansky-Muller model of genetic incompatibility with incomplete penetrance for reproductive isolation. Variation in fitness of different incompatible lines reveals the presence of additional modifiers in the genetic background. We propose that certain interacting loci lead to an optimal balance between growth and resistance to pathogens by modulating SA signaling under specific environments. This could allow the accumulation of additional incompatibilities before reaching complete reproductive isolation.

Fig. 1.

Detection of QTL of rosette area and cell death at low temperature. (A) LOD trace for the QTL detection involved in variation of rosette area and cell death at low temperature in Ler × Kas-2 and Ler × Kond RIL. (B) Rosette area and cell death score of Ler × Kas-2 (Top) and Ler × Kond (Bottom) RILs sorted according to the allelic values at interacting loci QTL 3, QTL 4, and QTL 5. Values with different letters are significantly different at level P < 0.001 in a Student-Newman Keuls test (SNK). n, number of RIL from each class; bars, SD.

Fig. 2.

Developmental phenotypes of incompatible lines. (A) Five-week-old Ler × Kas-2 incompatible lines (14, 4, and NIL) and parental accessions Ler and Kas-2 at 14 °C (Left) and 20 °C (Right). (B) Eight-week-old NIL at 14 °C and at 20 °C (C) compared with Ler and Kas-2 parents. (D) Two-week-old NIL grown at 14 °C under high humidity conditions.

Fig. 3.

Comparison of the genetic architecture in QTL 3 between Ler and Col. Marker positions are indicated by arrows. Insertions are indicated in triangles (yellow in Ler, orange in Col). Duplications are shown in dotted blue lines. Genes and their orientations are represented. The 3 digits next to the genes are identifiers for the last corresponding AGI numbers (At3g44XXX). The Col At3g44630 and At3g44670 RPP1-like genes and homologs in Ler are shown in light blue. Inserted genes in Ler encoding TIR-NB-LRR proteins (R1-R5) or truncated forms (only TIR domain in R6) are indicated in dark blue. Transposable elements are shown in black boxes. C, copia; L, LINE; I, transposase IS4.

Fig. 4.

Cell death and disease-resistance phenotypes of incompatible lines. (A) Spontaneous cell death revealed by TB staining of 3-week-old leaves of incompatible lines (4, NIL) and parental accessions grown at 14 °C (Left) or 20 °C (Right). (B) Infection phenotypes of the same lines as in (A). Two-week-old plants grown at 14 °C (Left) or 20 °C (Right) were inoculated with virulent H. parasitica isolate Cala2 at 18 °C. Microscopic examination of TB-stained leaves to reveal dead plant cells and pathogen mycelium growth was performed 4 days postinoculation. (Scale bars, 500 μm.)

Fig. 5.

Suppression of incompatibility by SA depletion. (A) Percent rosette area relative to Ler of 3-week-old plants: incompatible line RIL4 at 14 °C (RIL4), F₁ progeny from a cross between RIL4 and NIL (RIL4 × NIL), RIL4 line transformed with Salicylate hydroxylase (RIL4-NahG), and F₁ progeny from a cross between RIL4-NahG and NIL (RIL4-NahG × NIL). (B) Infection phenotypes of RIL4, RIL4-NahG, RIL4 × NIL, and RIL4-NahG × NIL lines (see [A] for details of lines). Two-week-old plants grown at 14 °C were inoculated with virulent H. parasitica isolate Cala2 at 18 °C. Plant cell death and pathogen colonization was monitored as in Fig. 4. (Scale bars, 500 μm.)