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

Abstract

Background and AimsThe 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.MethodsThe 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 ResultsApplication 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.ConclusionsThe predicted lipid-anchored glycoprotein At-FLA4 positively regulates cell wall biosynthesis and root growth by modulating ABA signalling.

SALT-OVERLY SENSITIVE5 Mediates Arabidopsis Seed Coat Mucilage Adherence and Organization through Pectins

“…Like other AGPs, SOS5 has been shown to play a role in cell wall structure (Shi et al, 2003;Xu et al, 2008;Harpaz-Saad et al, 2011), and like other AGPs, the mechanism through which SOS5 acts is under debate. It has been suggested that SOS5 serves as a structural component of the wall (Shi et al, 2003), a cell-cell signaling molecule (Xu et al, 2008;Seifert et al, 2014), or a cofactor for CESA5 complexes (Harpaz-Saad et al, 2011). Here, we have shown that SOS5 is required for mucilage adherence and the formation of ray structure but seems to do so through an influence on pectin rather than for cellulose biosynthesis.…”

A speculation on the tandem fasciclin 1 repeat of FLA4 proteins in angiosperms

“…Under all conditions Fas1-2 is predicted to interact with FEI1 which might in turn control growth by interacting with the biosynthesis and perception of growth regulators. 7,23,24 Under some conditions such as low salt concentration, this interaction might be negatively modulated in an auto-inhibitory fashion by the Fas1-1 domain.…”

FASCICLIN LIKE ARABINOGALACTAN PROTEIN 4 and RESPIRATORY BURST OXIDASE HOMOLOG D and F independently modulate abscisic acid signaling

“…1A), as previously reported. 3,7 Moreover, the triple mutant shows shorter root length compared to the At-fla4 mutant and the double mutant (Fig. 1A).…”

“…2 The At-FLA4 locus of A. thaliana plays a non-redundant role for root growth and salt tolerance. [3][4][5] The root of At-fla4 shows a short and fat phenotype, which is caused by abnormal expansion of epidermal, cortical, and endodermal cells. 5 Externally applied ABA suppresses the Atfla4…”

“…However, the mechanistic role of At-FLA4 in ABA response is unclear. 3 Arabidopsis contains 10 RESPIRA-TORY BURST OXIDASE HOMOLOG (RBOH) genes and RBOH-dependent Reactive oxygen species (ROS) have now been established as an important second messenger that regulates expression of hundreds of genes in response to stress. 6 Reactive oxygen species generated by the partially redundant RBOH isoforms D and F play an important role in abscisic acid (ABA) signaling in stomatal guard cells and roots.…”