Abstract
Ultraviolet-B (UV-B) radiation is a key environmental signal which initiates diverse responses that affect the metabolism, development, and viability of plants. In keeping with our previous studies, we concentrated primarily on how UV-B radiation affects Norway spruce [Picea abies (L.) Karst.] somatic embryo maturation and how phenolics and polyamines (PAs) are linked to the defense response invoked by UV-B irradiation. We treated clusters of Norway spruce embryogenic culture (EC) with UV-B during the five stages of embryo maturation (early, cylindrical, precotyledonary, cotyledonary, and mature embryos). For the first time, we take an advantage of the unique environmental scanning electron microscope AQUASEM II to characterize somatic embryos in their native state. The severity of the irradiation effect on embryonal cell viability was shown to be dependent on the intensity of radiation as well as the stage of embryo development, and might be related to the formation of protoderm. The response of early embryos was characterized by an increase in malondialdehyde (MDA), a marked decrease in PA contents and a decline in phenolics. The reduced ability to activate the defense system seems to be responsible not only for the severe cell damage and decrease in viability but also for the inhibition of embryo development. The significant reduction in spermidine (Spd), which has been reported to be crucial for the somatic embryo development of several coniferous species, may be causally linked to the limited development of embryos. The pronounced decrease in cell wall-bound ferulic acid might correspond to failure of somatic embryos to reach more advanced stages of development. Embryos at later stages of development showed stress defense responses that were more efficient against UV-B exposure.
Highlights
Ultraviolet-B (UV-B) radiation affects plants both directly and indirectly, and can, for example, damage DNA, proteins, and membranes, alter transpiration and photosynthesis, and lead to changes in growth, development, and morphology (Jansen et al, 1998)
The embryonal heads of early embryos are enlarged and individual embryos begin to release from the polyembryogenic complexes (Figure 1 – embryogenic culture (EC) Ib)
The exposure of Norway spruce EC to UV-B radiation at different stages of maturation had the strongest effect on cell viability of embryos at early stages of development and resulted in the inhibition of somatic embryo development and/or the substantial decrease in the number of embryos
Summary
Ultraviolet-B (UV-B) radiation affects plants both directly and indirectly, and can, for example, damage DNA, proteins, and membranes, alter transpiration and photosynthesis, and lead to changes in growth, development, and morphology (Jansen et al, 1998). The scavenging of active oxygen and other radical species, either through enzymatic or non-enzymatic systems, can alleviate UV stress (Jansen et al, 1996). Polyamines (PAs), which constitute a group of low molecular weight aliphatic amines, are key to regulating growth and developmental processes in plants, as well as the response to biotic and abiotic stresses (Kusano et al, 2008; Gill and Tuteja, 2010; Takahashi and Kakehi, 2010; Hussain et al, 2011). The radical-scavenging activity of PAs moderates UV-B radiation stress, as has been demonstrated for other free-radical scavengers (Jansen et al, 1996). Species-specific and age-dependent differences in the quantitative and qualitative composition of PAs could affect the susceptibility of a plant to abiotic stresses (Reifenrath and Müller, 2007)
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