Pine somatic embryogenesis using zygotic embryos as explants

Comparative analysis of element and hormone content in zygotic embryos of Pinus elliottii and P. elliottii × P. caribaea

Somatic embryogenesis is a crucial method for achieving clonal forestry in conifers. Understanding the development of zygotic embryos is essential not only for enhancing the efficiency and quality of somatic embryogenesis, but also for advancing forestry breeding programs. This study investigated dynamic changes of element and hormone contents during ZE development of Pinus elliottii and its hybrid P. elliottii × P. caribaea. Significant differences in embryo development speed among different clones were observed. Elemental analysis was conducted using inductively coupled plasma mass spectrometry (ICP-MS) and identified 68 elements, including major, minor, and beneficial elements. In both species, the contents of potassium (K), calcium (Ca), iron (Fe), boron (B) and five beneficial elements decreased during early ZE development, while phosphorus (P) and copper (Cu) increased. Significantly higher levels of K, Ca and Fe at the initial stage, and sulfur (S) and nickel (Ni) decreased at later stages were detected in P. elliottii than in the hybrid. For the other elements, except for very few significant differences at certain stages, most differences between the two species did not reach a significant level. The contents of endogenous hormones were determined and different accumulation patterns were detected in most hormones between the two species, except abscisic acid (ABA) which simultaneously decreased with developments by stage 8. Significant differences were found in indole-3-acetic acid (IAA) contents at most stages between species, while higher levels of total cytokinin (CK) at each stage were detected in the hybrid in comparison with those in P. elliottii. As a result, lower IAA to CK ratios in the hybrid than in P. elliottii. Methyl jasmonate (JA-me) and gibberellin A3 (GA3) contents showed a similar pattern and exhibited an M-shaped fluctuation in the hybrid. Furthermore, JA-me, GA3, gibberellin A4 (GA4) and brassinolide (BR) showed significantly higher levels in the hybrid than in P. elliottii. K-means clustering and correlation analyses were used to explore relationships between elements and hormones during embryo development, revealing complex interplay in both species. These data indicate different requirement in element and hormone contents for embryogenesis and suggest species-specific media composition for each step in somatic embryogenesis. The findings provide insights into their developmental processes and informing future research and applications in somatic embryogenesis and forestry breeding.

Modulating ascorbic acid levels to optimize somatic embryogenesis in Picea abies (L.) H. Karst. Insights into oxidative stress and endogenous phytohormones regulation

Global warming has adversely affected Picea abies (L.) H. Karst. forests in Europe, prompting the need for innovative forest-breeding strategies. Somatic embryogenesis (SE) offers promise but requires protocol refinement. Understanding the molecular mechanisms governing somatic embryo development is essential, as oxidative stress plays a crucial role in SE regulation. Ascorbic acid (ASA), is a vital antioxidant that can potentially control oxidative stress. In the present study, we normalized ASA concentrations in induction and proliferation media to enhance embryogenic tissue (ET) regeneration and proliferation capacity of mature explants. The media were supplemented with ASA at 0 mg l-1, 25 mg l-1, 50 mg l-1, 100 mg l-1, and 200 mg l-1. The accumulation of hydrogen peroxide (H2O2) and endogenous phytohormones, including auxins, cytokinins, brassinosteroids, abscisic acid, and gibberellin, was measured in non-embryonic calli and ET. Subsequently, their impact on ET induction and multiplication was analyzed. Our results demonstrate that application of ASA at concentrations of 25 mg l-1 and 200 mg l-1 led to increased H2O2 levels, potentially inducing oxidative stress while simultaneously reducing the levels of all endohormone groups. Notably, the highest ET induction frequency (approximately 70%) was observed for ASA at 50 mg l-1. These findings will enhance SE induction procedures, particularly in more resistant explants, underscoring the significance of ASA application to culture media.

Cytokinin Perception in Ancient Plants beyond Angiospermae

Cytokinins (CKs) control many plant developmental processes and responses to environmental cues. Although the CK signaling is well understood, we are only beginning to decipher its evolution. Here, we investigated the CK perception apparatus in early-divergent plant species such as bryophyte Physcomitrium patens, lycophyte Selaginella moellendorffii, and gymnosperm Picea abies. Of the eight CHASE-domain containing histidine kinases (CHKs) examined, two CHKs, PpCHK3 and PpCHK4, did not bind CKs. All other CHK receptors showed high-affinity CK binding (K_D of nM range), with a strong preference for isopentenyladenine over other CK nucleobases in the moss and for trans-zeatin over cis-zeatin in the gymnosperm. The pH dependences of CK binding for these six CHKs showed a wide range, which may indicate different subcellular localization of these receptors at either the plasma- or endoplasmic reticulum membrane. Thus, the properties of the whole CK perception apparatuses in early-divergent lineages were demonstrated. Data show that during land plant evolution there was a diversification of the ligand specificity of various CHKs, in particular, the rise in preference for trans-zeatin over cis-zeatin, which indicates a steadily increasing specialization of receptors to various CKs. Finally, this distinct preference of individual receptors to different CK versions culminated in vascular plants, especially angiosperms.

Toward establishing a morphological and ultrastructural characterization of proembryogenic masses and early somatic embryos of Araucaria angustifolia (Bert.) O. Kuntze

Somatic embryogenesis is a morphogenetic route useful for the study of embryonic development, as well as the large-scale propagation of endangered species, such as the Brazilian pine (Araucaria angustifolia). In the present study, we investigated the morphological and ultrastructural organization of A. angustifolia somatic embryo development by means of optical and electron microscopy. The proembryogenic stage was characterized by the proliferation of proembryogenic masses (PEMs), which are cellular aggregates composed of embryogenic cells (ECs) attached to suspensor-like cells (SCs). PEMs proliferate through three developmental stages, PEM I, II, and III, by changes in the number of ECs and SCs. PEM III-to-early somatic embryo (SE) transition was characterized by compact clusters of ECs growing out of PEM III, albeit still connected to it by SCs. Early SEs showed a dense globular embryonic mass (EM) and suspensor region (SR) connected by embryonic tube cells (TCs). By comparison, early somatic and zygotic embryos showed similar morphology. ECs are round with a large nucleus, nucleoli, and many cytoplasmic organelles. In contrast, TCs and SCs are elongated and vacuolated with cellular dismantling which is associated with programmed cell death of SCs. Abundant starch grains were observed in the TCs and SCs, while proteins were more abundant in the ECs. Based on the results of this study, a fate map of SE development in A. angustifolia is, for the first time, proposed. Additionally, this study shows the cell biology of SE development of this primitive gymnosperm which may be useful in evolutionary studies in this area.