The Arabidopsis thaliana FASCICLIN LIKE ARABINOGALACTAN PROTEIN 4 gene acts synergistically with abscisic acid signalling to control root growth

Abstract

Background and aims: The putative FASCICLIN-LIKE ARABINOGALACTAN PROTEIN 4 (At-FLA4) locus of Arabidopsis thaliana has previously been shown to be required for the normal growth of wild-type roots in response to moderately elevated salinity. However, the genetic and physiological pathway that connects At-FLA4 and normal root growth remains to be elucidated.

Methods: The radial swelling phenotype of At-fla4 was modulated with growth regulators and their inhibitors. The relationship of At-FLA4 to abscisic acid (ABA) signalling was analysed by probing marker gene expression and the observation of the At-fla4 phenotype in combination with ABA signalling mutants.

Key results: Application of ABA suppresses the non-redundant role of At-FLA4 in the salt response. At-FLA4 positively regulates the response to low ABA concentration in roots and is required for the normal expression of ABA- and abiotic stress-induced genes. The At-fla4 phenotype is enhanced in the At-abi4 background, while two genetic suppressors of ABA-induced gene expression are required for salt oversensitivity of At-fla4. Salt oversensitivity in At-fla4 is suppressed by the CYP707A inhibitor abscinazole E2B, and salt oversensitivity in At-fla4 roots is phenocopied by chemical inhibition of ABA biosynthesis.

Conclusions: The predicted lipid-anchored glycoprotein At-FLA4 positively regulates cell wall biosynthesis and root growth by modulating ABA signalling.

Keywords: Arabidopsis thaliana; At-FLA4; Fasciclin; abscisic acid; arabinogalactan protein; cell wall; plant cell wall signalling; root growth.

Fig. 1.

Synergistic effect of At-FLA4 and ABA signaling. The At-fla4 root phenotype is suppressed by (A) ABA and (B) pyrabactin. (C) The effect of ABA on root length requires At-FLA4. Root length measured after 48 h on media containing different concentrations of ABA (n = 20, ±confidence interval, α = 0·01). (D) The effect of salt on the expression of ABA-responsive transcripts in roots depends on At-FLA4. The indicated pairs were tested for statistically significant differences, and t-test values ≤0·05 and values ≤0·01 are indicated with lower case and uppercase letters, respectively, in the figures. A: Col vs. At-fla4 on standard medium (MS0), B: Col vs. At-fla4 on 100 mm NaCl, C: Col MS0 vs. Col NaCl, D: At-fla4 MS0 vs. At-fla4 NaCl.

Fig. 2.

Genetic interaction between At-FLA4 and positive ABA response regulators At-ABI4 and At-ABI5. Note that the abi4 fla4 double mutant shows abnormal root growth on NaCl-free medium. Scale bar = 1 mm.

Fig. 3.

Genetic interaction between At-FLA4 and ABA repressors At-CPL1, At-CPL3 and At-SAD1. Conditions are the same as for Fig. 2. Note that on 100 mm NaCl-containing medium, the At-cpl1 At-fla4 and the At-sad1 At-fla4 double mutants are comparable with the wild type, while the At-cpl3 At-fla4 double mutant is comparable with the At-fla4 single mutant. Scale bar = 2 mm.

Fig. 4.

Interaction of At-FLA4 with ABA metabolism. (A) Inhibition of ABA biosynthesis with fluridone phenocopies the At-fla4 phenotype. (B) Inhibition of ABA catabolism with abscinazole E2B suppresses the At-fla4 phenotype. Expression of genes involved in ABA (C) biosynthesis and (D) catabolism is not significantly different between Col and At-fla4.

Fig. 5.

Effect of ABA (5 µM) and fluridone (5 µm) on the transcript level of selected At-FLA genes and At-RD29B. To mark significant differences between standard medium (MS0) and media containing ABA or fluridone, t-test values ≤ 0·05 and ≤0·01 are indicated with lower case a and uppercase A, respectively.

Fig. 6.

Genetic model for the interaction of At-FLA4 with ABA. For an explanation, see the Discussion.