Cellular dynamics during maturation-related decline of adventitious root formation in forest tree species

Slow release of a synthetic auxin induces formation of adventitious roots in recalcitrant woody plants

Clonal propagation of plants by induction of adventitious roots (ARs) from stem cuttings is a requisite step in breeding programs. A major barrier exists for propagating valuable plants that naturally have low capacity to form ARs. Due to the central role of auxin in organogenesis, indole-3-butyric acid is often used as part of commercial rooting mixtures, yet many recalcitrant plants do not form ARs in response to this treatment. Here we describe the synthesis and screening of a focused library of synthetic auxin conjugates in Eucalyptus grandis cuttings and identify 4-chlorophenoxyacetic acid-L-tryptophan-OMe as a competent enhancer of adventitious rooting in a number of recalcitrant woody plants, including apple and argan. Comprehensive metabolic and functional analyses reveal that this activity is engendered by prolonged auxin signaling due to initial fast uptake and slow release and clearance of the free auxin 4-chlorophenoxyacetic acid. This work highlights the utility of a slow-release strategy for bioactive compounds for more effective plant growth regulation.

Effects of Different Donor Ages on the Growth of Cutting Seedlings Propagated from Ancient Platycladus orientalis

The effects of tree age on the growth of cutting seedlings propagated from ancient trees have been an important issue in plant breeding and cultivation. In order to understand seedling growth and stress resistance stability, phenotypic measurements, physiological assays, and high-throughput transcriptome sequencing were performed on sown seedlings propagated from 5-year-old donors and cutting seedlings propagated from 5-, 300-, and 700-year-old Platycladus orientalis donors. In this study, the growth of cutting seedlings propagated from ancient trees was significantly slower; the soluble sugar and chlorophyll contents gradually decreased with the increase in the age of donors, and the flavonoid and total phenolic contents of sown seedlings were higher than those of cutting seedlings. Enrichment analysis of differential genes showed that plant hormone signal transduction, the plant-pathogen interaction, and the flavone and flavonol biosynthesis pathways were significantly up-regulated with the increasing age of cutting seedlings propagated from 300- and 700-year-old donors. A total of 104,764 differentially expressed genes were calculated using weighted gene co-expression network analysis, and 8 gene modules were obtained. Further, 10 hub genes in the blue module were identified, which revealed that the expression levels of JAZ, FLS, RPM1/RPS3, CML, and RPS2 increased with the increase in tree age. The results demonstrated that the age of the donors seriously affected the growth of P. orientalis cutting seedlings and that cutting propagation can preserve the resistance of ancient trees. The results of this study provide important insights into the effects of age on asexually propagated seedlings, reveal potential molecular mechanisms, and contribute to an improvement in the level of breeding and conservation of ancient germplasm resources of P. orientalis trees.

Transcriptomics Analysis Reveals a Putative Role for Hormone Signaling and MADS-Box Genes in Mature Chestnut Shoots Rooting Recalcitrance

Maturation imposes several changes in plants, which are particularly drastic in the case of trees. In recalcitrant woody species, such as chestnut (Castanea sativa Mill.), one of the major maturation-related shifts is the loss of the ability to form adventitious roots in response to auxin treatment as the plant ages. To analyze the molecular mechanisms underlying this phenomenon, an in vitro model system of two different lines of microshoots derived from the same field-grown tree was established. While juvenile-like shoots root readily when treated with exogenous auxin, microshoots established from the crown of the tree rarely form roots. In the present study, a transcriptomic analysis was developed to compare the gene expression patterns in both types of shoots 24 h after hormone and wounding treatment, matching the induction phase of the process. Our results support the hypothesis that the inability of adult chestnut tissues to respond to the inductive treatment relies in a deep change of gene expression imposed by maturation that results in a significant transcriptome modification. Differences in phytohormone signaling seem to be the main cause for the recalcitrant behavior of mature shoots, with abscisic acid and ethylene negatively influencing the rooting ability of the chestnut plants. We have identified a set of related MADS-box genes whose expression is modified but not suppressed by the inductive treatment in mature shoots, suggesting a putative link of their activity with the rooting-recalcitrant behavior of this material. Overall, distinct maturation-derived auxin sensibility and homeostasis, and the related modifications in the balance with other phytohormones, seem to govern the outcome of the process in each type of shoots.

Transcriptome Dynamics of Rooting Zone and Leaves during In Vitro Adventitious Root Formation in Eucalyptus nitens

Wood properties and agronomic traits associated with fast growth and frost tolerance make Eucalyptus nitens a valuable forest alternative. However, the rapid age-related decline in the adventitious root (AR) formation (herein, meaning induction, initiation, and expression stages) limits its propagation. We analyzed transcriptomic profile variation in leaves and stem bases during AR induction of microcuttings to elucidate the molecular mechanisms involved in AR formation. In addition, we quantified expressions of candidate genes associated with recalcitrance. We delimited the ontogenic phases of root formation using histological techniques and Scarecrow and Short-Root expression quantification for RNA sequencing sample collection. We quantified the gene expressions associated with root meristem formation, auxin biosynthesis, perception, signaling, conjugation, and cytokinin signaling in shoots harvested from 2- to 36-month-old plants. After IBA treatment, 702 transcripts changed their expressions. Several were involved in hormone homeostasis and the signaling pathways that determine cell dedifferentiation, leading to root meristem formation. In part, the age-related decline in the rooting capacity is attributable to the increase in the ARR1 gene expression, which negatively affects auxin homeostasis. The analysis of the transcriptomic variation in the leaves and rooting zones provided profuse information: (1) To elucidate the auxin metabolism; (2) to understand the hormonal and signaling processes involved; (3) to collect data associated with their recalcitrance.

Comparing adventitious root-formation and graft-unification abilities in clones of Argania spinosa

Argania spinosa trees have attracted attention in recent years due to their high resistance to extreme climate conditions. Initial domestication activities practiced in Morocco. Here we report on selection and vegetative propagation of A. spinosa trees grown in Israel. Trees yielding relatively high amounts of fruit were propagated by rooting of stem cuttings. High variability in rooting ability was found among the 30 clones selected. In-depth comparison of a difficult-to-root (ARS7) and easy-to-root (ARS1) clone revealed that the rooted cuttings of ARS7 have a lower survival rate than those of ARS1. In addition, histological analysis of the adventitious root primordia showed many abnormal fused primordia in ARS7. Hormone profiling revealed that while ARS1 accumulates more cytokinin, ARS7 accumulates more auxin, suggesting different auxin-to-cytokinin ratios underlying the different rooting capabilities. The hypothesized relationship between rooting and grafting abilities was addressed. Reciprocal grafting was performed with ARS1/ARS7 but no significant differences in the success of graft unification between the trees was detected. Accordingly, comparative RNA sequencing of the rooting and grafting zones showed more differentially expressed genes related to rooting than to grafting between the two trees. Clustering, KEGG and Venn analyses confirmed enrichment of genes related to auxin metabolism, transport and signaling, cytokinin metabolism and signaling, cell wall modification and cell division in both regions. In addition, the differential expression of some key genes in ARS1 vs. ARS7 rooting zones was revealed. Taken together, while both adventitious root-formation and graft-unification processes share response to wounding, cell reprogramming, cell division, cell differentiation and reconnection of the vasculature, there are similar, but also many different genes regulating the two processes. Therefore an individual genotype can have low rooting capacity but good graft-unification ability.

Overexpression of a SHORT-ROOT transcriptional factor enhances the auxin mediated formation of adventitious roots and lateral roots in poplar trees

SHORT-ROOT (SHR) defines root stem cells and maintains radial patterning, but its involvement in adventitious root (AR) formation has not been reported. In this study, we showed that PtSHR2 was transcriptionally upregulated by excision before the formation of AR and responded dynamically to auxin. PtSHR2 overexpression (SHR2B^OE) in hybrid poplars resulted in an increased number of ARs with an initial delay. Despite a lower endogenous content in the stems than in wild-type plants, indole-3-acetic acid (IAA) content at the SHR2B^OE basal stem increased rapidly after cutting and reached a higher maximum than in wild-type plants, which was accompanied by a more sustained and stronger induction of AR formation marker genes. In addition, the higher auxin content in SHR2B^OE ARs resulted in more and longer lateral roots (LRs). Application of auxin abolished the early delay in the formation of AR and largely other AR phenotypes of SHR2B^OE plants, whereas the polar auxin transport inhibitor N-1-naphthylphthalamic acid completely inhibited both AR and LR abnormalities. Since the enhanced rooting ability of SHR2B^OE stem cuttings in hydroponics was clearly confirmed, our results suggest a novel role of poplar SHR2 as a positive regulator during the organogenesis of AR and LR by affecting local auxin homeostasis.

Involvement of the auxin-cytokinin homeostasis in adventitious root formation of rose cuttings as affected by their nodal position in the stock plant

Enhanced levels of indole-3-acetic and raised auxin to cytokinin ratios in the stem base contribute to the positive acropetal gradient in rooting capacity of leafy single-node stem cuttings of rose. Cuttings excised from different nodal positions in stock plants can differ in subsequent adventitious root formation. We investigated the involvement of the auxin-cytokinin balance in position-affected rooting of Rosa hybrida. Leafy single-node stem cuttings of two rose cultivars were excised from top versus bottom positions. Concentrations of IAA and cytokinins were monitored in the bud region and the stem base during 8 days after planting using chromatography-MS/MS technology. The effects of nodal position and external supply of indole-butyric acid on rooting were analyzed. Most cytokinins increased particularly in the bud region and peaked at day two before the bud break was recorded. IAA increased in both tissues between day one and day eight. Top versus bottom cuttings revealed higher levels of isopentenyladenosine (IPR) in both tissues as well as higher concentrations of IAA and a higher ratio of IAA to cytokinins particularly in the stem base. The dynamic of hormones and correlation analysis indicated that the higher IPR contributed to the enhanced IAA in the bud region which served as auxin source for the auxin homeostasis in the stem base, where IAA determined the auxin-cytokinin balance. Bottom versus top cuttings produced lower numbers and lengths of roots, whereas this deficit was counterbalanced by auxin application. Further considering other studies of rose, it is concluded that cytokinin-, sucrose- and zinc-dependent auxin biosynthesis in the outgrowing buds is an important factor that contributes to the enhanced IAA levels and auxin/cytokinin ratios in the stem base of apical cuttings, promoting root induction.

An HD-ZIP transcription factor, MxHB13, integrates auxin-regulated and juvenility-determined control of adventitious rooting in Malus xiaojinensis

Adventitious root (AR) formation is a critical factor in the vegetative propagation of forestry and horticultural plants. Competence for AR formation declines in many species during the miR156/SPL-mediated vegetative phase change. Auxin also plays a regulatory role in AR formation. In apple rootstock, both high miR156 expression and exogenous auxin application are prerequisites for AR formation. However, the mechanism by which the miR156/SPL module interacts with auxin in controlling AR formation is unclear. In this paper, leafy cuttings of juvenile (Mx-J) and adult (Mx-A) phase Malus xiaojinensis were used in an RNA-sequencing experiment. The results revealed that numerous genes involved in phytohormone signaling, carbohydrate metabolism, cell dedifferentiation, and reactivation were downregulated in Mx-A cuttings in response to indole butyric acid treatment. Among the differentially expressed genes, an HD-ZIP transcription factor gene, MxHB13, was found to be under negative regulation of MdSPL26 by directly binding to MxHB13 promoter. MxTIFY9 interacts with MxSPL26 and may play a role in co-repressing the expression of MxHB13. The expression of MxTIFY9 was induced by exogenous indole butyric acid. MxHB13 binds to the promoter of MxABCB19-2 and positively affects the expression. A model is proposed in which MxHB13 links juvenility-limited and auxin-limited AR recalcitrance mechanisms in Mx-A.

The microRNA476a-RFL module regulates adventitious root formation through a mitochondria-dependent pathway in Populus

For woody plants, clonal propagation efficiency is largely determined by adventitious root (AR) formation at the bases of stem cuttings. However, our understanding of the molecular mechanisms contributing to AR morphogenesis in trees remains limited, despite the importance of vegetative propagation, currently the most common practice for tree breeding and commercialization. Here, we identified Populus-specific miR476a as a regulator of wound-induced adventitious rooting that acts by orchestrating mitochondrial homeostasis. MiR476a exhibited inducible expression during AR formation and directly targeted several Restorer of Fertility like (RFL) genes encoding mitochondrion-localized pentatricopeptide repeat proteins. Genetic modification of miR476a-RFL expression revealed that miR476a/RFL-mediated dynamic regulation of mitochondrial homeostasis influences AR formation in poplar. Mitochondrial perturbation via exogenous application of a chemical inhibitor indicated that miR476a/RFL-directed AR formation depends on mitochondrial regulation that acts via auxin signaling. Our results thus establish a microRNA-directed mitochondrion-auxin signaling cascade required for AR development, providing insights into the role of mitochondrial regulation in the developmental plasticity of plants.

Proteomic analysis reveals key proteins involved in ethylene-induced adventitious root development in cucumber (Cucumis sativus L.)

The mechanisms involved in adventitious root formation reflect the adaptability of plants to the environment. Moreover, the rooting process is regulated by endogenous hormone signals. Ethylene, a signaling hormone molecule, has been shown to play an essential role in the process of root development. In the present study, in order to explore the relationship between the ethylene-induced adventitious rooting process and photosynthesis and energy metabolism, the iTRAQ technique and proteomic analysis were employed to ascertain the expression of different proteins that occur during adventitious rooting in cucumber (Cucumis sativus L.) seedlings. Out of the 5,014 differentially expressed proteins (DEPs), there were 115 identified DEPs, among which 24 were considered related to adventitious root development. Most of the identified proteins were related to carbon and energy metabolism, photosynthesis, transcription, translation and amino acid metabolism. Subsequently, we focused on S-adenosylmethionine synthase (SAMS) and ATP synthase subunit a (AtpA). Our findings suggest that the key enzyme, SAMS, upstream of ethylene synthesis, is directly involved in adventitious root development in cucumber. Meanwhile, AtpA may be positively correlated with photosynthetic capacity during adventitious root development. Moreover, endogenous ethylene synthesis, photosynthesis, carbon assimilation capacity, and energy material metabolism were enhanced by exogenous ethylene application during adventitious rooting. In conclusion, endogenous ethylene synthesis can be improved by exogenous ethylene additions to stimulate the induction and formation of adventitious roots. Moreover, photosynthesis and starch degradation were enhanced by ethylene treatment to provide more energy and carbon sources for the rooting process.

Expression Levels of Genes Encoding Proteins Involved in the Cell Wall-Plasma Membrane-Cytoskeleton Continuum Are Associated With the Maturation-Related Adventitious Rooting Competence of Pine Stem Cuttings

Stem cutting recalcitrance to adventitious root formation is a major limitation for the clonal propagation or micropropagation of elite genotypes of many forest tree species, especially at the adult stage of development. The interaction between the cell wall-plasma membrane and cytoskeleton may be involved in the maturation-related decline of adventitious root formation. Here, pine homologs of several genes encoding proteins involved in the cell wall-plasma membrane-cytoskeleton continuum were identified, and the expression levels of 70 selected genes belonging to the aforementioned group and four genes encoding auxin carrier proteins were analyzed during adventitious root formation in rooting-competent and non-competent cuttings of Pinus radiata. Variations in the expression levels of specific genes encoding cell wall components and cytoskeleton-related proteins were detected in rooting-competent and non-competent cuttings in response to wounding and auxin treatments. However, the major correlation of gene expression with competence for adventitious root formation was detected in a family of genes encoding proteins involved in sensing the cell wall and membrane disturbances, such as specific receptor-like kinases (RLKs) belonging to the lectin-type RLKs, wall-associated kinases, Catharanthus roseus RLK1-like kinases and leucine-rich repeat RLKs, as well as downstream regulators of the small guanosine triphosphate (GTP)-binding protein family. The expression of these genes was more affected by organ and age than by auxin and time of induction.

Identifying Molecular Chechkpoints for Adventitious Root Induction: Are We Ready to Fill the Gaps?

The molecular mechanisms underlying de novo root organogenesis have been under intense study for the last decades. As new tools and resources became available, a comprehensive model connecting the processes and factors involved was developed. Separate phases that allow for specific analyses of individual checkpoints were well defined. Physiological approaches provided information on the importance of metabolic processes and long-distance signaling to balance leaf and stem status and activation of stem cell niches to form new root meristems. The study of plant hormones revealed a series of sequential roles for cytokinin and auxin, dynamically interconnected and modulated by jasmonic acid and ethylene. The identification of genes specifying cell identity uncovered a network of sequentially acting transcriptional regulators that link hormonal control to cell fate respecification. Combined results from herbaceous model plants and the study of recalcitrant woody species underscored the need to understand the limiting factors that determine adventitious rooting competence. The relevance of epigenetic control was emphasized by the identification of microRNAs and chromatin remodeling agents involved in the process. As the different players are set in place and missing pieces become apparent, findings in related processes can be used to identify new candidates to complete the picture. Molecular knobs connecting the balance cell proliferation/differentiation to hormone signaling pathways, transcriptional control of cell fate or metabolic modulation of developmental programs can offer clues to unveil new elements in the dynamics of adventitious rooting regulatory networks. Mechanisms for cell non-autonomous signaling that are well characterized in other developmental processes requiring establishment and maintenance of meristems, control of cell proliferation and cell fate specification can be further explored. Here, we discuss possible candidates and approaches to address or elude the limitations that hinder propagation programs requiring adventitious rooting.

A Perspective on Adventitious Root Formation in Tree Species

Adventitious root formation is an organogenic process, regulated at several levels, that is crucial for the successful vegetative propagation of numerous plants. In many tree species, recalcitrance to adventitious root formation is a major limitation in the clonal propagation of elite germplasms. Information on the mechanisms underlying the competence for adventitious root formation is still limited. Therefore, increasing our understanding of the mechanisms that enable differentiated somatic cells to switch their fates and develop into root meristematic cells, especially those involved in cell developmental aging and maturation, is a priority in adventitious root-related research. The dynamic cell wall-cytoskeleton, along with soluble factors, such as cellular signals or transcriptional regulators, may be involved in adult cell responses to intrinsic or extrinsic factors, resulting in maintenance, induction of root meristematic cell formation, or entrance into another differentiating pathway.

Recent Advances in Adventitious Root Formation in Chestnut

The genus Castanea includes several tree species that are relevant because of their geographical extension and their multipurpose character, that includes nut and timber production. However, commercial exploitation of the trees is hindered by several factors, particularly by their limited regeneration ability. Regardless of recent advances, there exists a serious limitation for the propagation of elite genotypes of chestnut due to decline of rooting ability as the tree ages. In the present review, we summarize the research developed in this genus during the last three decades concerning the formation of adventitious roots (ARs). Focusing on cuttings and in vitro microshoots, we gather the information available on several species, particularly C. sativa, C. dentata and the hybrid C.sativa × C. crenata, and analyze the influence of several factors on the achievements of the applied protocols, including genotype, auxin treatment, light regime and rooting media. We also pay attention to the acclimation phase, as well as compile the information available about biochemical and molecular related aspects. Furthermore, we considerate promising biotechnological approaches that might enable the improvement of the current protocols.

Coexpression analysis of a large-scale transcriptome identified a calmodulin-like protein regulating the development of adventitious roots in poplar

Poplars are important woody plants, and the ability to form adventitious roots (ARs) is the key factor for their cultivation because most poplars are propagated by cloning. In previous studies, Ca2+ was confirmed to regulate AR formation in poplar. In this study, wild-type poplar cuttings grown in 1.0 mM Ca2+ solution showed the best visible performance of AR development. Coexpression analysis of a large-scale RNA-Seq transcriptome was conducted to identify Ca2+-related genes that regulate AR development in poplar. A total of 15 coexpression modules (CMs) were identified, and two CMs showed high association with AR development. Functional analysis identified a number of biological pathways, including 'oxidation-reduction process', 'response to biotic stimulus' and 'metabolic process', in tissues of AR development. The Ca2+-related pathway was specifically selected, and its regulation in poplar AR development was predicted. A Ca2+ sensor, PdeCML23-1, which is a member of the calmodulin-like protein (CML) family, was found to promote AR development by phenotypic assay of overexpressed PdeCML23-1 transgenic lines at various growing conditions. By measuring cytosolic Ca2+ in AR tips, PdeCML23-1 seemed to play a role in decreasing cytosolic Ca2+ concentration. Additionally, the expression profiles of some genes and phytohormone indole acetic acid (IAA) were also changed in the overexpressed PdeCML23-1 transgenic lines. According to this study, we were able to provide a global view of gene regulation for poplar AR development. Moreover, we also observed the regulation of cytosolic Ca2+ concentration by PdeCML23-1, and this regulation was involved in AR development in poplar. We also predicted that PdeCML23-1 possibly regulates AR development by modulating IAA content in poplar.

The Induction of Adventitious Roots Regeneration before Transplanting Rootless Ficus elastica Heritage Tree

Heritage trees carry both botanical and historical value for a city’s resilience and sustainability and hence are precious and unique. Their transplant is costly and very rare due to tremendous cost and 100% survival requirement by law. Rootless transplant is even more detrimental to the heritage tree due to removal of roots infected by brown root rot (BRR) before transplanting. This study examined the adventitious roots (AR) induction ability of the Ficus elastica Roxb. heritage tree infected with BRR. The experimental design considered three factors: root diameter (RD), wounding method (WM), and auxin solution on aerial roots under fractional factorial experiment in completely randomized design (CRD). There were four RD groups: RDI (RD < 2 cm), RDII (2 ≤ RD ≤ 4.3 cm), RDIII (4.3 < RD ≤ 22), and RDIV (RD > 22); three WMs: cutting off (CF), girdling (GD), and rectangular shape peeling (RP) of aerial roots; and three auxin solutions: 2000 mg·L−1 IBA(Indole-3-butyric acid) (2B), 2000 mg·L−1 IBA + 2000 mg·L−1 NAA(1-Naphthaleneacetic acid) (2NB), and 4000 mg·L−1 IBA (4B) plus water as control (C). The number of rooting wounds, number of roots, and the mean length of the three longest adventitious roots in each wound were recorded to evaluate the AR rooting performance. Twenty four treatment combinations including 328 wounds were tested. The results showed that rooting ability was significantly correlated with RD and WM. Smaller RDs had better rooting and declined with increased RDs. CF had the best rooting followed by GD and then RP. Auxin solution did not significantly affect the rooting ability. It may be due to the abundant endogenous auxin in the heritage tree, which mitigated the effect of exogenous auxin for AR induction. We conclude that cutting off small-diameter aerial roots is the best approach to induce ARs from rootless F. elastica heritage trees to enhance transplantation success.

Some Urea Derivatives Positively Affect Adventitious Root Formation: Old Concepts and the State of the Art

The success of vegetative propagation programmes strongly depends on adventitious rooting, a postembryonic developmental process whereby new roots can be induced from differentiated cells in positions where normally they do not arise. This auxin-dependent organogenesis has been studied at molecular, cellular, and developmental levels, and our knowledge of the process has improved in recent years. However, bioactive compounds that enhance adventitious root formation and possibly reduce undesirable auxinic side effects are still needed to ameliorate this process. From this point of view, our structure–activity relationship studies concerning urea derivatives revealed that some of them, more specifically, the N,N′-bis-(2,3-methylenedioxyphenyl)urea (2,3-MDPU), the N,N′-bis-(3,4-methylenedioxyphenyl)urea (3,4-MDPU), the 1,3-di(benzo[d]oxazol-5-yl)urea (5-BDPU), and the 1,3-di(benzo[d]oxazol-6-yl)urea (6-BDPU), constitute a category of adventitious rooting adjuvants. The results of our studies are presented here, in order either to highlight the positive effects of the supplementation of these urea derivatives, or to better understand the nature of their interaction with auxin.

Comprehensive analysis of dynamic gene expression and investigation of the roles of hydrogen peroxide during adventitious rooting in poplar

BACKGROUND

Adventitious roots (ARs) are roots that are generated from nonrooting tissues. ARs are usually produced both during normal development and in response to stress conditions, such as flooding, nutrient deprivation, heavy metal stress and wounding. The ability of plants to form ARs is a key trait that enables plant propagation, especially for most tree species.

RESULTS

Here, the kinetics of AR formation in a tissue culture of a hybrid variety of poplar were investigated. AR formation mainly occurred during the first 8 days and both pre- and newly- formed primordia contributed to AR formation in poplar by histological study. RNA-Seq-based transcriptome analysis was performed for stem bases collected at 0, 2, 4, 6 and 8 days after excision (DAE). Based on the data, the expression patterns of 8 phytohormone-related genes were investigated, and their influences on AR formation were considered. Subsequent gene expression cluster analysis showed a number of biological processes involved in AR formation. Among these biological pathways, genes involved in H₂O₂ homeostasis showed enrichment in one cluster that was highly upregulated from DAE0 to DAE8. Pharmacological assay confirmed that an appropriate content of H₂O₂ in stem bases could accelerate the formation of ARs in poplar.

CONCLUSIONS

Based on the results of this study, we were able to predict a regulatory network for 7 phytohormones that are involved in poplar AR formation. The influence of H₂O₂ on AR formation was also confirmed_. These results enhance our understanding of the regulation of AR formation in tree species.

Molecular Dissection of the Regenerative Capacity of Forest Tree Species: Special Focus on Conifers

Somatic embryogenesis (SE) and organogenesis have become leading biotechnologies for forest tree improvement and the implementation of multi-varietal forestry. Despite major advances in clonal propagation using these technologies, many forest tree species, such as conifers, show a low regeneration capacity. Developmental factors such as genotype, the type and age of the explant or tissue, and the age and maturity of the mother tree are limiting factors for the success of propagation programs. This review summarizes recent research on the molecular pathways involved in the regulation of key steps in SE and organogenesis of forest tree species, mainly conifers. The interaction between auxin and stress conditions, the induction of cell identity regulators and the role of cell wall remodeling are reviewed. This information is essential to develop tools and strategies to improve clonal propagation programs for forest tree species.