Signalling in the cereal aleurone: hormones, reactive oxygen and cell death

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Abstract
The cereal aleurone is widely used as a model system to study hormonal signalling. Abscisic acid (ABA) and gibberellins (GAs) elicit distinct responses in aleurone cells, ranging from those occurring within minutes of hormone addition to those that require several hours or days to complete. Programmed cell death is an example of a response in aleurone layers that is hormonally regulated. GAs promote cell death and cells in intact aleurone layers begin to die 24 h after GA treatment, whereas cell death of aleurone protoplasts begins 4 d after GA treatment. ABA prevents aleurone cell death and addition of ABA to cells pretreated with GA can delay cell death. Aleurone cells do not follow the apoptotic route of programmed cell death. Cells treated with GA, but not ABA, develop large, acidic vacuoles containing a spectrum of hydrolases typical of lytic compartments. Enzymes that metabolize reactive oxygen species are also present in aleurone cells, but ascorbate peroxidase, catalase and superoxide dismutase become less abundant after treatment with GA; activity of these enzymes increases or remains unchanged in ABA-treated cells. We propose a model whereby reactive oxygen species accumulate in GA-treated cells and lead to peroxidation of membrane lipids and plasma membrane rupture. RO, reactive oxygen species; HR, hypersensitive response; PSV, protein storage vacuole; PCD, programmed cell death; CAT, catalase; SOD, superoxide dismutase; APX, ascorbate peroxidase.