Modulation of embryo-forming capacity in culture through the expression of Brassica genes involved in the regulation of the shoot apical meristem

Transcriptional changes of antioxidant responses, hormone signalling and developmental processes evoked by the Brassica napus SHOOTMERISTEMLESS during in vitro embryogenesis

Previous work showed that alterations in Brassica napus (Bn) SHOOTMERISTEMLESS (BnSTM) expression levels influence microspore-derived embryogenesis in B. napus. While over-expression of BnSTM increased microspore-derived embryo (MDE) yield and quality, down-regulation of BnSTM repressed embryo formation [16]. Transcriptional analyses were conducted to investigate the molecular mechanisms underpinning these responses. The induction of BnSTM resulted in a heavy transcriptional activation of genes involved in antioxidant responses, hormone signalling and developmental processes. Several antioxidant enzymes, including catalases, superoxide dismutases, and components of the Halliwell-Asada cycle were induced in embryos ectopically expressing BnSTM and contributed to the removal of reactive oxygen species (ROS). These changes were accompanied by elevated levels of ascorbate and glutathione, which have been shown to promote embryonic growth and development. Induction or repression of BnSTM altered the early cytokinin response, whereas late responses, modulated by Type-A Arabidopsis response regulators (ARRs), were induced in MDEs over-expressing BnSTM. Major differences between transgenic MDEs were also observed in the expression pattern of several auxin transporters and key developmental factors required for normal embryogenesis. While some of these factors, BABYBOOM1 (BBM1) and SOMATIC EMBRYOGENESIS RECEPTOR KINASE (SERK), play a key role during early embryogeny, others, CYP78A5, LEAFY COTYLEDON1 and 2 (LEC1 and LEC2), as well as WOX2 and 9, are required for proper embryo development. Collectively these results demonstrate the involvement of BnSTM in novel developmental processes which can be utilized to enhance in vitro embryogenesis.

Altered seed oil and glucosinolate levels in transgenic plants overexpressing the Brassica napus SHOOTMERISTEMLESS gene

SHOOTMERISTEMLESS (STM) is a homeobox gene conserved among plant species which is required for the formation and maintenance of the shoot meristem by suppressing differentiation and maintaining an undetermined cell fate within the apical pole. To assess further the role of this gene during seed storage accumulation, transgenic Brassica napus (Bn) plants overexpressing or down-regulating BnSTM under the control of the 35S promoter were generated. Overexpression of BnSTM increased seed oil content without affecting the protein and sucrose level. These changes were accompanied by the induction of genes encoding several transcription factors promoting fatty acid (FA) synthesis: LEAFY COTYLEDON1 (BnLEC1), BnLEC2, and WRINKLE1 (BnWRI1). In addition, expression of key representative enzymes involved in sucrose metabolism, glycolysis, and FA biosynthesis was up-regulated in developing seeds ectopically expressing BnSTM. These distinctive expression patterns support the view of an increased carbon flux to the FA biosynthetic pathway in developing transformed seeds. The overexpression of BnSTM also resulted in a desirable reduction of seed glucosinolate (GLS) levels ascribed to a transcriptional repression of key enzymes participating in the GLS biosynthetic pathway, and possibly to the differential utilization of common precursors for GLS and indole-3-acetic acid synthesis. No changes in oil and GLS levels were observed in lines down-regulating BnSTM. Taken together, these findings provide evidence for a novel function for BnSTM in promoting desirable changes in seed oil and GLS levels when overexpressed in B. napus plants, and demonstrate that this gene can be used as a target for genetic improvement of oilseed species.

In vitro shoot organogenesis and hormone response are affected by the altered levels of Brassica napus meristem genes

Arabidopsis shoot meristem activity is regulated by a molecular network involving the participation of several components, including SHOOTMERISTEMLESS (STM), CLAVATA1 (CLV1), and ZWILLE (ZLL). In an effort to identify the role of these genes during in vitro shoot formation Brassica and Arabidopsis plants were transformed with the Brassica napus (Bn) STM, CLV1, ZLL1 and ZLL2 identified in previous work [1]. In both systems shoot organogenesis was promoted by the over-expression of BnSTM, BnZLL1, and BnZLL2, and repressed by the over-expression of BnCLV1. This distinct regulation, analogous to that occurring during in vivo meristem formation where STM and ZLL encourage stem cell formation while CLV1 accelerates transition to differentiation, suggests similar regulatory mechanisms governing shoot formation in vivo and in vitro. While the BnZLL1 and BnZLL2 induction of shoot organogenesis correlated only to changes in auxin signaling, BnSTM and BnCLV1 evoked major transcriptional alterations in cytokinin response. Besides increasing the transcript levels of two cytokinin receptors, ARABIDOPSIS HISTIDINE KINASE4 (AHK4) and CYTOKININ INDEPENDENT KINASE (CKI1), ectopic expression of BnSTM induced Type-B ARABIDOPSIS RESPONSE REGULATORS (ARRs) and repressed Type-A ARRs. Opposite transcriptional patterns occurred in explants over-expressing BnCLV1, characterized by a decreased ability to produce shoots. The role played by Type-A and Type-B ARRs during shoot organogenesis was further examined using a genetic approach which revealed the requirement of ARR12 for the BnSTM positive regulation of shoot organogenesis. Collectively these results expand our knowledge on the function of meristem genes, and provide new tools for enhancing in vitro propagation systems.

Recent progress in the understanding of tissue culture-induced genome level changes in plants and potential applications

In vitro cell and tissue-based systems have tremendous potential in fundamental research and for commercial applications such as clonal propagation, genetic engineering and production of valuable metabolites. Since the invention of plant cell and tissue culture techniques more than half a century ago, scientists have been trying to understand the morphological, physiological, biochemical and molecular changes associated with tissue culture responses. Establishment of de novo developmental cell fate in vitro is governed by factors such as genetic make-up, stress and plant growth regulators. In vitro culture is believed to destabilize the genetic and epigenetic program of intact plant tissue and can lead to chromosomal and DNA sequence variations, methylation changes, transposon activation, and generation of somaclonal variants. In this review, we discuss the current status of understanding the genomic and epigenomic changes that take place under in vitro conditions. It is hoped that a precise and comprehensive knowledge of the molecular basis of these variations and acquisition of developmental cell fate would help to devise strategies to improve the totipotency and embryogenic capability in recalcitrant species and genotypes, and to address bottlenecks associated with clonal propagation.

Distinct fluctuations in nucleotide metabolism accompany the enhanced in vitro embryogenic capacity of Brassica cells over-expressing SHOOTMERISTEMLESS

Besides regulating meristem formation and maintenance in vivo, SHOOTMERISTEMLESS (STM) has been shown to affect embryogenesis. While the over-expression of Brassica napus (Bn)STM enhances the number of microspore-derived embryos produced in culture and their ability to regenerate viable plants, a down-regulation of this gene represses the embryogenic process (Elhiti et al., J Exp Bot, 61:4069-4085, 2010). Synthesis and degradation of pyrimidine and purine nucleotides were measured in developing microspore-derived embryos (MDEs) generated from B. napus lines ectopically expressing or down-regulating BnSTM. Pyrimidine metabolism was investigated by following the metabolic fate of exogenously supplied (14)C-uridine, uracil and orotic acid, whereas purine metabolism was estimated by using (14)C-adenine, adenosine and inosine. The improvement in embryo number and quality affected by the ectopic expression of BnSTM was linked to the increased pyrimidine and purine salvage activity during the early phases of embryogenesis and the enlargement of the adenylate pool (ATP + ADP) required for the active growth of the embryos. This was due to an increase in transcriptional and enzymatic activity of several salvage enzymes, including adenine phosphoribosyltransferase (APRT) and adenosine kinase (ADK). The highly operative salvage pathway induced by the ectopic expression of BnSTM was associated with a slow catabolism of nucleotides, suggesting the presence of an antagonist mechanism controlling the rate of salvage and degradation pathways. During the second half of embryogenesis utilization of uridine for UTP + UDPglucose (UDPG) synthesis increased in the embryos over-expressing BnSTM, and this coincided with a better post-germination performance. All these events were precluded by the down-regulation of BnSTM which repressed the formation of the embryos and their post-embryonic performance. Overall, this work provides evidence that precise metabolic changes are associated with proper embryo development in culture.

Manipulation of hemoglobin expression affects Arabidopsis shoot organogenesis

Over the past few years non-symbiotic plant hemoglobins have been described in a variety of plant species where they fulfill several functions ranging from detoxification processes to basic aspects of plant growth and post-embryonic development. To date no information is available on the role of hemoglobins during in vitro morphogenesis. Shoot organogenesis was induced in Arabidopsis lines constitutively expressing class 1, 2 and 3 hemoglobins (GLB1, 2 and 3) and lines in which the respective genes were either downregulated by RNAi (GLB1) or knocked out (GLB2 and GLB3). The process was executed by culturing root explants on an initial auxin-rich callus induction medium (CIM) followed by a transfer onto a cytokinin-containing shoot induction medium (SIM). While the repression of GLB2 inhibited organogenesis the over-expression of GLB1 or GLB2 enhanced the number of shoots produced in culture, and altered the transcript levels of genes participating in cytokinin perception and signalling. The up-regulation of GLB1 or GLB2 activated CKI1 and AHK3, genes encoding cytokinin receptors and affected the transcript levels of cytokinin responsive regulators (ARRs). The expression of Type-A ARRs (ARR4, 5, 7, 15, and 16), feed-back repressors of the cytokinin pathway, was repressed in both hemoglobin over-expressors whereas that of several Type-B ARRs (ARR2, 12, and 13), transcription activators of cytokinin-responsive genes, was induced. Such changes enhanced the sensitivity of the root explants to cytokinin allowing the 35S::GLB1 and 35S::GLB2 lines to produce shoots at low cytokinin concentrations which did not promote organogenesis in the WT line. These results show that manipulation of hemoglobin can modify shoot organogenesis in Arabidopsis and possibly in those systems partially or completely unresponsive to applications of exogenous cytokinins.

An epigenetic view of plant cells cultured in vitro: somaclonal variation and beyond

Epigenetic mechanisms are highly dynamic events that modulate gene expression. As more accurate and powerful tools for epigenetic analysis become available for application in a broader range of plant species, analysis of the epigenetic landscape of plant cell cultures may turn out to be crucial for understanding variant phenotypes. In vitro plant cell and tissue culture methodologies are important for many ongoing plant propagation and breeding programmes as well as for cutting-edge research in several plant model species. Although it has long been known that in vitro conditions induce variation at several levels, most studies using such conditions rely on the assumption that in vitro cultured plant cells/tissues mostly conform genotypically and phenotypically. However, when large-scale clonal propagation is the aim, there has been a concern in confirming true-to-typeness using molecular markers for evaluating stability. While in most reports genetic variation has been found to occur at relatively modest frequencies, variation in DNA methylation patterns seems to be much more frequent and in some cases it has been directly implicated in phenotypic variation. Recent advances in the field of epigenetics have uncovered highly dynamic mechanisms of chromatin remodelling occurring during cell dedifferentiation and differentiation processes on which in vitro adventitious plant regeneration systems are based. Here, an overview of recent findings related to developmental switches occurring during in vitro culture is presented. Additionally, an update on the detection of epigenetic variation in plant cell cultures will be provided and discussed in the light of recent progress in the plant epigenetics field.

Androgenesis in recalcitrant solanaceous crops

Tomato, eggplant, and pepper are three solanaceous crops of outstanding importance worldwide. For hybrid seed production in these species, a fast and cheap method to obtain pure (homozygous) lines is a priority. Traditionally, pure lines are produced by classical inbreeding and selection techniques, which are time consuming (several years) and costly. Alternatively, it has become possible to accelerate the production of homozygous lines through a biotechnological approach: the induction of androgenesis to generate doubled haploid (homozygous) plants. This biotechnological in vitro tool reduces the process to only one generation, which implies important time and costs savings. These facts make androgenic doubled haploids the choice in a number of important crops where the methodology is well set up. Unfortunately, recalcitrant solanaceous crops such as tomato, eggplant, and pepper are still far from an efficient and reliable technology to be applied on a routine basis to different genotypes in breeding programs. In eggplant and pepper, only anther cultures are known to work relatively well. Unfortunately, a more efficient and promising technique, the culture of isolated microspores, is not sufficiently developed yet. In tomato, none of these methods is available nowadays. However, recent advances in the knowledge of embryo development are filling the gaps and opening new ways to achieve the final goal of an efficient protocol in these three recalcitrant species. In this review, we outline the state of the art on androgenic induction in tomato, eggplant, and pepper, and postulate new experimental ways in order to overcome current limitations.

Ectopic expression of the Brassica SHOOTMERISTEMLESS attenuates the deleterious effects of the auxin transport inhibitor TIBA on somatic embryo number and morphology

The auxin transport inhibitor 2,3,5-triiodobenzoic acid (TIBA) is a useful compound for investigating the role of auxin flow during plant growth and development. In Arabidopsis lines, applications of TIBA during the induction phase of somatic embryogenesis inhibit embryo development and induce the differentiation of the meristematic cells of the shoot apical meristem (SAM), leading to the fusion of the cotyledons. These abnormalities were associated to changes in the expression levels of auxin transporter genes (PINs) and endogenous distribution of IAA. Treatments of TIBA caused a rapid accumulation of IAA within the epidermal and cortical root cells of the explants (bent-cotyledon zygotic embryos), as well as in the apical and sub-apical cells of the callus generated by the surface of the cotyledons of the explants. Within the callus only a few cells acquired meristematic characteristics, and this was associated to low expression levels of genes involved in embryogenic cell fate acquisition, such as WUSCHEL (WUS), LEAFY COTYLEDON 1 and 2. All these deleterious effects were attenuated when TIBA was administered to lines over-expressing SHOOT MERISTEMLESS (STM) isolated from Brassica oleracea (Bo), B. napus (Bn), and B. rapa (Br). Of interest, TIBA-treated explants of Arabidopsis lines over-expressing the Brassica STM were able to produce a large number of embryogenic cells and somatic embryos which exhibited a normal morphology and two distinct cotyledons. A proposed reason for this behaviour was ascribed to the ability of the transformed tissue to retain a normal distribution of auxin in the presence of TIBA. Proper localization of auxin might be required for the normal expression of several genes needed for the acquisition of embryogenic competence and formation of somatic embryos.