BRI1 Signaling in the Root is Mediated through the SERK1 and SERK3 Co-receptors

Brassinosteroid (BR) is a class of polyhydroxysteroids plant hormones known to regulate shoot and root growth. Genetic and molecular analyses demonstrate that receptor kinase BRI1 protein acts as a perceiver for BR. One of the characteristics of bri1 mutant’s phenotypes is the complete BR insensitivity in the root. Biochemical evidences of the BRI1 protein complex indicates that somatic embryogenesis receptor kinase 1 (SERK1) and SERK3 participate in the BR pathway in Arabidopsis root. While only serk3 mutants show partial reduction to BR sensitivity, serk1 presents a normal BR penetration phenotype compared to the wild type. Interestingly, the double mutant serk1serk3 displays more, but not full resistance to BR in root length assay. In this study, we aimed to enhance the BR insensitivity of the double mutant serk1serk3 by crossing serk1 mutant allele with a strong serk3 and bri1 mutant alleles. In our study, by generating serk1-3serk3-2 double mutants, a complete insensitivity to BR that phenocopied bri1-301 mutant was recorded. However, we were unable to increase BR resistance in the root of serk1-3serk3-2 double mutant by crossing with bri1 mutant allele in the triple mutant serk1-3serk3-2bri1. As a result, all the BRI1 signaling in the root was mediated through the SERK1 and SERK3 co-receptors. Additionally, we established that based on conventional BR assays, the At1g27190 protein was also involved in BR signaling. Preliminary data indicated that the triple mutant serk1serk3-2At1g27190 showed a dwarfed phenotype. Whether or not this dwarfed phenotype is linked to BRI1 signaling impairment needs to be further investigated.

Qa-SNARE Protein SYP22 Negatively Regulates Brassinosteroid Signaling in the Dark

Soluble N-ethyl-maleimide sensitive factor attachment adaptor protein receptor (SNARE) domain-containing proteins were mainly involved in vesicle-associated membrane fusion. Genetic screening has revealed the function of SNARE in different aspects of plant biology. Among them, Synthaxin-22 (SYP22) a Qa-SNARE has been reported to have a pleiotropic function in plant development including regulation of leaf waving, shoot gravitropism and flowering time. In this study, we identified a new role of SYP22 in regulation of brassinosteroid (BR) signaling, especially in the dark. SYP22 interacts with BR receptor, brassinosteroid insensitive 1 (BRI1), and overexpression of SYP22 enhanced a weak BRI1 mutant bri1-5 phenotype. syp22 mutant exhibits short hypocotyl and it is sensitive to exogenously treated BR while slightly insensitive to BR-biosynthesis inhibitor propiconazole (PCZ) in the dark. Expression levels of BR signaling maker genes ACS5, SAUR15 and IAA19 were slightly higher, while BR6OX2, a BR biosynthesis marker gene, was lower in syp22 compared to the wild-type. In addition, syp22 was sensitive to 2,4-D, a synthetic auxin, in the dark. In conclusion, SYP22 is involved in BR- and auxin-mediated hypocotyl growth inhibition in the dark, which might be via interaction with BR and auxin key regulators to alter their internalization in Arabidopsis.