Expression of ABA signalling genes and ABI5 protein levels in imbibed Sorghum bicolor caryopses with contrasting dormancy and at different developmental stages

Background and AimsPre-harvest sprouting susceptibility in grain sorghum (Sorghum bicolor) is related to low seed dormancy and reduced embryo sensitivity to inhibition of germination by abscisic acid (ABA). Intra-specific variability for pre-harvest sprouting might involve differential regulation of ABA signalling genes.MethodsSorghum genes encoding homologues for ABA signalling components from other species (ABI5, ABI4, VP1, ABI1 and PKABA1) were studied at the transcriptional and protein level (ABI5) during grain imbibition for two sorghum lines with contrasting sprouting phenotypes and in response to hormones.Key ResultsTranscript levels of these genes and protein levels of ABI5 were higher in imbibed immature caryopses of the more dormant line. Dormancy loss was related to lower transcript levels of these genes and lower ABI5 protein levels in both genotypes. Exogenous ABA inhibited germination of isolated embryos but failed to prevent ABI5 rapid decrease supporting a role for the seed coat in regulating ABI5 levels.ConclusionsSeveral genes involved in ABA signalling are regulated differently in imbibed caryopses from two sorghum lines with contrasting pre-harvest sprouting response before - but not after - physiological maturity. A role for ABI5 in the expression of dormancy during grain development is discussed.Fil:Rodríguez, M.V. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Mendiondo, G.M. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Maskin, L. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Gudesblat, G.E. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Iusem, N.D. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

SPEECHLESS integrates brassinosteroid and stomata signalling pathways.

Stomatal formation is regulated by multiple developmental and environmental signals, but how these signals are integrated to control this process is not fully understood1. In Arabidopsis thaliana, the basic helix-loop-helix transcription factor SPEECHLESS (SPCH) regulates the entry, amplifying and spacing divisions that occur during stomatal lineage development. SPCH activity is negatively regulated by mitogen-activated protein kinase (MAPK)-mediated phosphorylation2. Here, we show that in addition to MAPKs, SPCH activity is also modulated by brassinosteroid (BR) signalling. The GSK3/SHAGGY-like kinase BIN2 (BR INSENSITIVE2) phosphorylates residues overlapping those targeted by the MAPKs, as well as four residues in the amino-terminal region of the protein outside the MAPK target domain. These phosphorylation events antagonize SPCH activity and limit epidermal cell proliferation. Conversely, inhibition of BIN2 activity in vivo stabilizes SPCH and triggers excessive stomatal and non-stomatal cell formation. We demonstrate that through phosphorylation inputs from both MAPKs and BIN2, SPCH serves as an integration node for stomata and BR signalling pathways to control stomatal development in Arabidopsi