Crosstalk between reactive oxygen species and hormonal signalling pathways regulates grain dormancy in barley

Seed dormancy, defined as the inability to germinate under favourable conditions, is controlled by abscisic acid (ABA) and gibberellins (GAs). Phytohormone signalling interacts with reactive oxygen species (ROS) signalling regarding diverse aspects of plant physiology and is assumed to be important in dormancy alleviation. Using dormant barley grains that do not germinate at 30 degrees C in darkness, we analysed ROS content and ROS-processing systems, ABA content and metabolism, GA-responsive genes and genes involved in GA metabolism in response to hydrogen peroxide (H2O2) treatment. During after-ripening, the ROS content in the embryo was not affected, while the antioxidant glutathione (GSH) was gradually converted to glutathione disulphide (GSSG). ABA treatment up-regulated catalase activity through transcriptional activation of HvCAT2. Exogenous H2O2 partially alleviated dormancy although it was associated with a small increase in embryonic ABA content related to a slight induction of HvNCED transcripts. H2O2 treatment did not affect ABA sensitivity but up-regulated the expression of HvExpA11 (GA-induced gene), inhibited the expression of HvGA2ox3 involved in GA catabolism and enhanced the expression of HvGA20ox1 implicated in GA synthesis. In barley, H2O2 could be implicated in dormancy alleviation through activation of GA signalling and synthesis rather than repression of ABA signalling.