Induction and signaling of an apoptotic response typified by DNA laddering in the defense response of oats to infection and elicitors

Cells in the primary leaves of oats displayed internucleosomal DNA cleavage in response to incompatible crown rust infection. DNA laddering also was evident in leaves treated with calcium ionophore A23187, nonspecific elicitors such as chitin and chitosan oligomers, and victorin, which functions as a specific elicitor in Pc-2/Vb containing oat leaves. The nuclei in a victorin-treated susceptible oat line were positive for the TUNEL assay. These elicitors clearly induced a 28-kDa nuclease (p28) in addition to three constitutive nucleases of 33, 24, and 22 kDa, Activation of p28 preceded the appearance of DNA laddering and possibly was mediated by de novo synthesis and/or cysteine protease activity. Pharmacological studies showed that the induction of DNA laddering was associated with oxidative stress, Ca2+ influx, and serine and cysteine proteases, Protein kinase and calmodulin activities did not seem to be involved in the induction of DNA laddering by victorin, whereas kinase-mediated signals were involved in DNA laddering induced by A23187. Protein kinase, calmodulin, G-protein activities, and Ca2+ influx, however, are involved in phytoalexin production. Our results imply that p28 is a possible nuclease candidate responsible for the induction of DNA laddering. The results also demonstrated that the mediators involved in the induction of apoptosis depended on the type of stimuli, whereas p28 and serine and cysteine proteases commonly are associated with each elicitor-induced apoptosis.