"Salicylic Acid Mutant Collection" as a Tool to Explore the Role of Salicylic Acid in Regulation of Plant Growth under a Changing Environment

Abstract

The phytohormone salicylic acid (SA) has a crucial role in plant physiology. Its role is best described in the context of plant response to pathogen attack. During infection, SA is rapidly accumulated throughout the green tissues and is important for both local and systemic defences. However, some genetic/metabolic variations can also result in SA overaccumulation in plants, even in basal conditions. To date, more than forty Arabidopsis thaliana mutants have been described as having enhanced endogenous SA levels or constitutively activated SA signalling pathways. In this study, we established a collection of mutants containing different SA levels due to diverse genetic modifications and distinct gene functions. We chose prototypic SA-overaccumulators (SA-OAs), such as bon1-1, but also "non-typical" ones such as exo70b1-1; the selection of OA is accompanied by their crosses with SA-deficient lines. Here, we extensively studied the plant development and SA level/signalling under various growth conditions in soil and in vitro, and showed a strong negative correlation between rosette size, SA content and PR1/ICS1 transcript signature. SA-OAs (namely cpr5, acd6, bon1-1, fah1/fah2 and pi4kβ1β2) had bigger rosettes under high light conditions, whereas WT plants did not. Our data provide new insights clarifying a link between SA and plant behaviour under environmental stresses. The presented SA mutant collection is thus a suitable tool to shed light on the mechanisms underlying trade-offs between growth and defence in plants.

Keywords: Arabidopsis mutants; Salicylic acid; gene transcription; growth; light.

Figure 1

Rosette size and SA content of plants cultivated under long-day conditions. (A) Representative images of 4 week old plants cultivated at 22 °C, 16 h light/ 8 h dark. (B) Rosette size (area). Data are from three biological replicates, n ≥ 70. Central line of the boxplot represents the median occupancy, cross represents the mean, bottom and top edges of the box are 25 and 75% of distribution and the ends of whiskers are set at 1.5 times the interquartile range. (C) SA content in the leaves, n = 4. Data represent means + SEM, asterisks indicate variants different from WT, one-way ANOVA with Tukey’s HSD post hoc test, * p < 0.05, ** p < 0.01.

Figure 2

Transcription of ICS1 and PR1 in soil-grown plants cultivated under LD conditions. Samples were collected from four 4 week old plants. Values were normalized to WT at the respective conditions. TIP41 was used as a reference gene. Data represent means + SEM, asterisks indicate values different from WT, t-test, * p < 0.05, n = 4.

Figure 3

In vitro growth of SA collection mutants under different light intensities. Two week old seedlings were cultivated on ½ MS medium under 450 μE.m⁻².s⁻¹ or 170 μE.m⁻².s⁻¹ under 12 h light /12 h dark photoperiod. (A) Rosette weight. (B) Primary root length. Data represent four biological repetitions; at least 10 seedlings were measured for each variant in each biological repetition. Central line of the boxplot represents the median occupancy, cross represents the mean, bottom and top edges of the box are 25 and 75% of distribution and the ends of whiskers are set at 1.5 times the interquartile range, asterisks indicates variants different from those for the 450 μE.m⁻².s⁻¹ intensity the same genotype, * p < 0.01, t-test.

Figure 4

Correlation table of SA effects on growth. The matrix was built using the Pearson correlation for 12 parameters (rosette size, SA content and SA-related gene expression (ICS1 and PR1) for soil-grown plants under short-day (SD) and long-day (LD) conditions; and rosette weight and primary root length for in vitro grown plants grown under an LD photoperiod at 450 uE or 170 uE light intensity). Measurements were taken for 15 genotypes (listed in Table 1). Data are from three biological repetitions for each variant. Positive correlations are displayed in blue and negative correlations in red. Correlation coefficients are indicated. Only results that passed the 0.05 threshold for significance are displayed in colour.