Upon a dark-to-light shift, the conditional fluorescent ( flu ) mutant of Arabidopsis releases singlet oxygen ( ¹ O ₂ ) within the plastid compartment. Distinct sets of nuclear genes are activated that are different from those induced by superoxide (O ₂ ^•− ) and/or hydrogen peroxide (H ₂ O ₂ ), suggesting that different types of reactive oxygen species activate distinct signaling pathways. It is not known whether the pathways operate separately or interact with each other. We have addressed this problem by modulating noninvasively the level of H ₂ O ₂ in plastids by means of a transgenic line that overexpresses the thylakoid-bound ascorbate peroxidase (tAPX). The overexpression of the H ₂ O ₂ -specific scavenger reduced strongly the activation of nuclear genes in plants treated with the herbicide paraquat that in the light leads to the enhanced generation of O ₂ ^•− and H ₂ O ₂ . In the flu mutant overexpressing tAPX, the intensity of ¹ O ₂ -mediated cell death and growth inhibition was increased when compared with the flu parental line. Also, the expression of most of the nuclear genes that were rapidly activated after the release of ¹ O ₂ was significantly higher in flu plants overexpressing tAPX, whereas in wild-type plants, overexpression of tAPX did not lead to visible stress responses and had only a very minor impact on nuclear gene expression. The results suggest that H ₂ O ₂ antagonizes the ¹ O ₂ -mediated signaling of stress responses as seen in the flu mutant. This cross-talk between H ₂ O ₂ - and ¹ O ₂ -dependent signaling pathways might contribute to the overall stability and robustness of wild-type plants exposed to adverse environmental stress conditions.

The effects of auxins 2,4-D (2,4-dichlorophenoxyacetic acid), NAA (1-naphthaleneacetic acid) or picloram (4-amino-3,5,6-trichloropicolinic acid; 9 µM) and cytokinin BA (benzyloadenine; 4.5 µM) applied in the early stages of somatic embryogenesis (SE) on specific stages of SE in Picea abies and P. omorika were investigated. The highest SE initiation frequency was obtained after 2,4-D application in P. omorika (22.00%) and picloram application in P. abies (10.48%). NAA treatment significantly promoted embryogenic tissue (ET) proliferation in P. abies, while 2,4-D treatment reduced it. This reduction was related to the oxidative stress level, which was lower with the presence of NAA in the proliferation medium and higher with the presence of 2,4-D. The reduced oxidative stress level after NAA treatment suggests that hydrogen peroxide (H₂O₂) acts as a signalling molecule and promotes ET proliferation. NAA and picloram in the proliferation medium decreased the further production and maturation of P. omorika somatic embryos compared with that under 2,4-D. The quality of the germinated P. abies embryos and their development into plantlets depended on the auxin type and were the highest in NAA-originated embryos. These results show that different auxin types can generate different physiological responses in plant materials during SE in both spruce species.