The Response of Picea abies Somatic Embryos to UV-B Radiation Depends on the Phase of Maturation

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.

Early molecular events involved in Pinus pinaster Ait. somatic embryo development under reduced water availability: transcriptomic and proteomic analyses

Maritime pine somatic embryos (SEs) require a reduction in water availability (high gellan gum concentration in the maturation medium) to reach the cotyledonary stage. This key switch, reported specifically for pine species, is not yet well understood. To facilitate the use of somatic embryogenesis for mass propagation of conifers, we need a better understanding of embryo development. Comparison of both transcriptome (Illumina RNA sequencing) and proteome [two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis with mass spectrometry (MS) identification] of immature SEs, cultured on either high (9G) or low (4G) gellan gum concentration, was performed, together with analysis of water content, fresh and dry mass, endogenous abscisic acid (ABA; gas chromatography-MS), soluble sugars (high-pressure liquid chromatography), starch and confocal laser microscope observations. This multiscale, integrated analysis was used to unravel early molecular and physiological events involved in SE development. Under unfavorable conditions (4G), the glycolytic pathway was enhanced, possibly in relation to cell proliferation that may be antagonistic to SE development. Under favorable conditions (9G), SEs adapted to culture constraint by activating specific protective pathways, and ABA-mediated molecular and physiological responses promoting embryo development. Our results suggest that on 9G, germin-like protein and ubiquitin-protein ligase could be used as predictive markers of SE development, whereas protein phosphatase 2C could be a biomarker for culture adaptive responses. This is the first characterization of early molecular mechanisms involved in the development of pine SEs following an increase in gellan gum concentration in the maturation medium, and it is also the first report on somatic embryogenesis in conifers combining transcriptomic and proteomic datasets.

The anti-actin drugs latrunculin and cytochalasin affect the maturation of spruce somatic embryos in different ways

The role of the actin cytoskeleton in somatic embryo development was investigated using latrunculin B and cytochalasin D. Brief treatments (1h) with either drug at the start of maturation fragmented the actin in suspensor cells and/or depolymerized actin filaments in meristematic cells. The drugs targeted different cells: latB primarily affected the suspensor cells, but cchD damaged both suspensor and meristematic cells. Lethal damage to the meristematic and suspensor cells was observed when the drugs were applied throughout the maturation period, although the severity of this effect depended on their concentrations. The drugs' effects on the yield of mature somatic embryos were investigated by applying them to embryo cultures throughout the maturation period or for one week at three different points in the maturation process: immediately prior to the start of maturation, during the first week of maturation, and during the fourth week of maturation. The strongest effects were observed when the drugs were applied at the start of maturation. Under these conditions, latB destroyed the suspensors, eliminating the underdeveloped embryos that depend on them. This accelerated the development of embryos that were capable of separating from the suspensors. Thus, while the total number of embryos at the end of the maturation period was lower than in untreated control cultures, the surviving mature embryos were of high quality. cchD treatment at the start of maturation strongly inhibited embryo development. Drug treatment at the end of the maturation period did not significantly affect embryo development: latB caused no change in the yield of somatic embryos, but cchD treatment increased the number of malformed embryos compared to untreated controls.

Cryotolerance in Norway spruce and its association with growth rates, anatomical features and polyamines of embryogenic cultures

Our study focused on the possible association between the cryotolerance of Norway spruce (Picea abies (L.) Karst.) embryogenic cultures and the anatomical structures of their embryogenic suspensor mass (ESM), their growth rate and their content of endogenous polyamines (PAs). The anatomical characteristics and PA content during cryopreservation and regrowth were studied in the ESMs of AFO 541 and C110 cultures, which have comparable ESM anatomy but diverse growth rates, PA content and regeneration abilities after cryopreservation. Different levels of tolerance to exogenous treatment were already apparent after transfer of the ESMs to liquid media. The endogenous free PAs were maintained at high levels, with spermidine being the predominant PA in the ESM of AFO 541, while in the ESM of C110 the content of putrescine and spermidine was almost identical and rather low, the content of spermidine being approximately one-third that in the ESM of AFO 541. Osmotic pretreatment, using a double application of sorbitol followed by an application of dimethyl sulfoxide (DMSO) resulted in the continual disintegration of polyembryogenic centers and suspensors in both cell lines. A continual decrease in the level of PAs was observed during the cell osmotic pretreatment. The cells that retained their viability and regrowth ability after cryopreservation were the meristematic cells inside the embryonal heads and the cells in the intermediate area between suspensor and meristems. Restoration of AFO 541 growth after cryopreservation was almost immediate; however, the C110 ESM culture regrew with difficulty, often exhibiting callogenesis. High levels of PA-soluble conjugates and an increase in the amount of PAs bound to high-molecular-mass substances was observed in cells of AFO 541 on Day 6 after thawing and also to some extent on Day 11. On Day 21 after thawing, the amount of free putrescine and spermidine in the AFO 541 cells reached the level observed in the suspension culture before the cryotreatment. The extremely low level of PAs determined in the ESM of C110 3 weeks after thawing agreed with the cell viability and rate of regrowth observed in this culture. The possible role of PAs in the process of cryopreservation of Norway spruce cultures is discussed.

Study of soil-plant transfer of 226Ra under greenhouse conditions

A soil-plant transfer study was performed using soil from a former uranium ore processing factory in South Bohemia. We present the results from greenhouse experiments which include estimates of the time required for phytoremediation. The accumulation of (226)Ra by different plant species from a mixture of garden soil and contaminated substrate was extremely variable, ranging from 0.03 to 2.20 Bq (226)Ra/g DW. We found differences in accumulation of (226)Ra between plants from the same genus and between cultivars of the same plant species. The results of (226)Ra accumulation showed a linear relation between concentration of (226)Ra in plants and concentration of (226)Ra in soil mixtures. On the basis of these results we estimated the time required for phytoremediation, but this appears to be too long for practical purposes.

The role of actin isoforms in somatic embryogenesis in Norway spruce

BACKGROUND

Somatic embryogenesis in spruce is a process of high importance for biotechnology, yet it comprises of orchestrated series of events whose cellular and molecular details are not well understood. In this study, we examined the role of actin cytoskeleton during somatic embryogenesis in Norway spruce line AFO 541 by means of anti-actin drugs.

RESULTS

Application of low doses (50-100 nM) of latrunculin B (Lat B) during the maturation of somatic embryos predominantly killed suspensor cells while leaving the cells in meristematic centres alive, indicating differential sensitivity of actin in the two cell types. The treatment resulted in faster development of more advanced embryos into mature somatic embryos and elimination of insufficiently developed ones. In searching for the cause of the differential actin sensitivity of the two cell types, we analysed the composition of actin isoforms in the culture and isolated four spruce actin genes. Analysis of their expression during embryo maturation revealed that one actin isoform was expressed constitutively in both cell types, whereas three actin isoforms were expressed predominantly in suspensor cells and their expression declined during the maturation. The expression decline was greatly enhanced by Lat B treatment. Sequence analysis revealed amino-acid substitutions in the Lat B-binding site in one of the suspensor-specific actin isoforms, which may result in a different binding affinity for Lat B.

CONCLUSIONS

We show that manipulating actin in specific cell types in somatic embryos using Lat B treatment accelerated and even synchronized the development of somatic embryos and may be of practical use in biotechnology.

Polyamine profiles and biosynthesis in somatic embryo development and comparison of germinating somatic and zygotic embryos of Norway spruce

The polyamine (PA) contents and activities of PA biosynthetic enzymes in Norway spruce somatic embryos [Picea abies L. (Karst.), genotype AFO 541] were studied in relation to anatomical changes during their development, from proliferation to germination, and changes in these variables associated with the germination of mature somatic and zygotic embryos were compared. Activities of PA biosynthetic enzymes steadily increased during the development of somatic embryos, from embryogenic suspensor mass until early cotyledonary stages. In these stages, the spermidine (Spd) level was significantly higher than the putrescine (Put) level, and the increases coincided with the sharp increases in S-adenosylmethionine decarboxylase activity in the embryos. The biosynthetic enzyme activity subsequently declined in mature cotyledonary embryos, accompanied by sharp reductions in PA contents, especially in cellular Put contents in embryos from 6 weeks old through the desiccation phase (although the spermine level significantly increased during the desiccation phase), resulting in a shift in the Spd/Put ratio from ca. 2 in early cotyledonary embryos to around 10 after 3 weeks of desiccation. In mature zygotic embryos, Spd contents were twofold lower, but Put levels were higher, than in mature somatic embryos, hence their Spd/Put ratio was substantially lower (ca. 2, in both embryos and megagametophytes). In addition, the PA synthesis activity profiles in the embryos differed (ornithine decarboxylase and arginine decarboxylase activities predominating in mature somatic and zygotic embryos, respectively). The start of germination was associated with a rise in PA biosynthetic activity in the embryos of both origins, which was accompanied by a marked increase in Put contents in somatic embryos, resulting in the decline of Spd/Put ratio to about 2, similar to the ratio in mature and germinating zygotic embryos. The accumulation of high levels of PAs in somatic embryos may be causally linked to their lower germinability than in zygotic embryos.

Trichloroacetic acid in Norway spruce/soil-system. II. Distribution and degradation in the plant

Independently from its origin, trichloroacetic acid (TCA) as a phytotoxic substance affects coniferous trees. Its uptake, distribution and degradation were thus investigated in the Norway spruce/soil-system using 14C labeling. TCA is distributed in the tree mainly by the transpiration stream. As in soil, TCA seems to be degraded microbially, presumably by phyllosphere microorganisms in spruce needles. Indication of TCA biodegradation in trees is shown using both antibiotics and axenic plants.