CUC2 as an early marker for regeneration competence in Arabidopsis root explants

Cloning and functional analysis of the BrCUC2 gene in Brassica rapa L

The CUP-SHAPED COTYLEDON2 (CUC2) gene plays an important role in the formation of apical meristem and organ edges in plants. The apical meristematic tissue of Brassica rapa (B. rapa) is associated with cold resistance, however, the role of the CUC2 gene in cold resistance of B.rapa is unclear. In this study, we used bioinformatics software to analyze the structure of BrCUC2 gene, real-time fluorescence quantitative PCR to detect the expression level of BrCUC2, constructed transgenic Arabidopsis thaliana by the flower dipping method and subcellular localization for functional validation. The results showed that, we isolated a 1104 bp open reading frame of BrCUC2 from the winter B. rapa cultivar 'Longyou 7'. The BrCUC2 contains a highly conserved domain belonging to the NAM superfamily. Its homologus CUC genes contain similar conserved motifs and are closely related to Brassica oleracea (B.oleracea), and the N-terminal of amino acid sequence contains NAC domain. BrCUC2 protein was localized in the nucleus and self-activation tests showed that pGBKT7-BrCUC2 had self-activation. Tissue-specific expression analysis and promoter β-Glucuronidase (GUS) activity showed that BrCUC2 had high expression levels in B. rapa growth points and A. thaliana leaf edges, stems and growth points. After low-temperature stress, BrCUC2 showed greater expression in 'Longyou 7,' which presents strong cold resistance and concave growth points, than in 'Longyou 99,' which presents weak cold resistance and protruding growth points. BrCUC2 promoter contains multiple elements related to stress responses. BrCUC2 overexpression revealed that the phenotype did not differ from that of the wild type during the seedling stage but showed weak growth and a dwarf phenotype during the flowering and mature stages. After low-temperature treatment, the physiological indexes and survival rate of BrCUC2-overexpression lines of Arabidopsis thaliana (A. thaliana) were better than those of the wild type within 12 h, although differences were not observed after 24 h. These results showed that BrCUC2 improved the low-temperature tolerance of transgenic A. thaliana within a short time. It can provide a foundation for the study of cold resistance in winter B. rapa.

Identification of Transcriptional Networks Involved in De Novo Organ Formation in Tomato Hypocotyl Explants

Some of the hormone crosstalk and transcription factors (TFs) involved in wound-induced organ regeneration have been extensively studied in the model plant Arabidopsis thaliana. In previous work, we established Solanum lycopersicum "Micro-Tom" explants without the addition of exogenous hormones as a model to investigate wound-induced de novo organ formation. The current working model indicates that cell reprogramming and founder cell activation requires spatial and temporal regulation of auxin-to-cytokinin (CK) gradients in the apical and basal regions of the hypocotyl combined with extensive metabolic reprogramming of some cells in the apical region. In this work, we extended our transcriptomic analysis to identify some of the gene regulatory networks involved in wound-induced organ regeneration in tomato. Our results highlight a functional conservation of key TF modules whose function is conserved during de novo organ formation in plants, which will serve as a valuable resource for future studies.

Enhancement of shoot regeneration by treatment with inhibitors of auxin biosynthesis and transport during callus induction in tissue culture of Arabidopsis thaliana

In two-step culture systems for efficient shoot regeneration, explants are first cultured on auxin-rich callus-inducing medium (CIM), where cells are activated to proliferate and form calli containing root-apical meristem (RAM)-type stem cells and stem cell niche, and then cultured on cytokinin-rich shoot-inducing medium (SIM), where stem cells and stem cell niche of the shoot apical meristem (SAM) are established eventually leading to shoot regeneration. In the present study, we examined the effects of inhibitors of auxin biosynthesis and polar transport in the two-step shoot regeneration culture of Arabidopsis and found that, when they were applied during CIM culture, although callus growth was repressed, shoot regeneration in the subsequent SIM culture was significantly increased. The regeneration-stimulating effect of the auxin biosynthesis inhibitor was not linked with the reduction in the endogenous indole-3-acetic acid (IAA) level. Expression of the auxin-responsive reporter indicated that auxin response was more uniform and even stronger in the explants cultured on CIM with the inhibitors than in the control explants. These results suggested that the shoot regeneration competence of calli was enhanced somehow by the perturbation of the endogenous auxin dynamics, which we discuss in terms of the transformability between RAM and SAM stem cell niches.

A systems genetics approach reveals PbrNSC as a regulator of lignin and cellulose biosynthesis in stone cells of pear fruit

BACKGROUND

Stone cells in fruits of pear (Pyrus pyrifolia) negatively influence fruit quality because their lignified cell walls impart a coarse and granular texture to the fruit flesh.

RESULTS

We generate RNA-seq data from the developing fruits of 206 pear cultivars with a wide range of stone cell contents and use a systems genetics approach to integrate co-expression networks and expression quantitative trait loci (eQTLs) to characterize the regulatory mechanisms controlling lignocellulose formation in the stone cells of pear fruits. Our data with a total of 35,897 expressed genes and 974,404 SNPs support the identification of seven stone cell formation modules and the detection of 139,515 eQTLs for 3229 genes in these modules. Focusing on regulatory factors and using a co-expression network comprising 39 structural genes, we identify PbrNSC as a candidate regulator of stone cell formation. We then verify the function of PbrNSC in regulating lignocellulose formation using both pear fruit and Arabidopsis plants and further show that PbrNSC can transcriptionally activate multiple target genes involved in secondary cell wall formation.

CONCLUSIONS

This study generates a large resource for studying stone cell formation and provides insights into gene regulatory networks controlling the formation of stone cell and lignocellulose.

The evolution of root branching: increasing the level of plasticity

Plant roots and root systems are indispensable for water and nutrient foraging, and are a major evolutionary achievement for plants to cope with dry land conditions. The ability of roots to branch contributes substantially to their capacity to explore the soil for water and nutrients, and led ~400 million years ago to the successful colonization of land by plants, eventually even in arid regions. During this colonization, different forms of root branching evolved, reinforcing step by step the phenotypic plasticity of the root system. Whereas the lycophytes, the most ancient land plants with roots, only branch at the root tip, ferns are able to form roots laterally in a fixed pattern along the main root. Finally, roots of seed plants show the highest phenotypic plasticity, because lateral roots can possibly, dependent on internal and/or external signals, be produced at almost any position along the main root. The competence to form lateral roots in seed plants is based on the presence of internal cell files with stem cell-like features. Despite the dissimilarities between the different clades, a number of genetic modules seem to be co-opted in order to acquire root branching capacity. In this review, starting from the lateral root pathways in seed plants, we review root branching in the different land plant lineages and discuss the hitherto described genetic modules that contribute to their root branching capacity. We try to obtain insight into how land plants have acquired an increasing root branching plasticity during evolution that contributed to the successful colonization of our planet by seed plants.

To regenerate or not to regenerate: factors that drive plant regeneration

Plants have a remarkable regenerative capacity, but it varies widely among species and tissue types. Whether plant cells/tissues initiate regeneration largely depends on the extent to which they are constrained to their original tissue fate. Once cells start the regeneration program, they acquire a new fate, form meristems, and develop into organs. During these processes, the cells must continuously overcome various barriers to the progression of the regeneration program until the organ (or whole plant) is complete. Recent studies have revealed key factors and signals affecting cell fate during plant regeneration. Here, we review recent research on: (i) environmental signal inputs and physical stimuli that act as initial triggers of regeneration; (ii) epigenetic and transcriptional cellular responses to those triggers leading to cellular reprograming; and (iii) molecules that direct the formation and development of the new stem cell niche. We also discuss differences and similarities between regeneration and normal development.

Genome wide identification and predicted functional analyses of NAC transcription factors in Asian pears

BACKGROUND

NAC proteins contribute to diverse plant developmental processes as well as tolerances to biotic and abiotic stresses. The pear genome had been decoded and provided the basis for the genome-wide analysis to find the evolution, duplication, gene structures and predicted functions of PpNAC transcription factors.

RESULTS

A total of 185 PpNAC genes were found in pear, of which 148 were mapped on chromosomes while 37 were on unanchored scaffolds. Phylogeny split the NAC genes into 6 clades (Group1- Group6) with their sub clades (~ subgroup A to subgroup H) and each group displayed common motifs with no/minor change. The numbers of exons in each group varied from 1 to 12 with an average of 3 while 44 pairs from all groups showed their duplication events. qPCR and RNA-Seq data analyses in different pear cultivars/species revealed some predicted functions of PpNAC genes i.e. PpNACs 37, 61, 70 (2A), 53, 151(2D), 10, 92, 130 and 154 (3D) were potentially involved in bud endodormancy, PpNACs 61, 70 (2A), 172, 176 and 23 (4E) were associated with fruit pigmentations in blue light, PpNACs 127 (1E), 46 (1G) and 56 (5A) might be related to early, middle and late fruit developments respectively. Besides, all genes from subgroups 2D and 3D were found to be related with abiotic stress (cold, salt and drought) tolerances by targeting the stress responsive genes in pear.

CONCLUSIONS

The present genome-wide analysis provided valuable information for understanding the classification, motif and gene structure, evolution and predicted functions of NAC gene family in pear as well as in higher plants. NAC TFs play diverse and multifunctional roles in biotic and abiotic stresses, growth and development and fruit ripening and pigmentation through multiple pathways in pear.

Cytokinin-overinduced transcription factors and thalianol cluster genes in CARBOXYL-TERMINAL DOMAIN PHOSPHATASE-LIKE 4-silenced Arabidopsis roots during de novo shoot organogenesis

Cytokinin (CK) is one of key phytohormones for de-differentiation and de novo organogenesis in plants. During the CK-mediated organogenesis not only genes in CK homeostasis, perception and signal transduction, but also factors regulating basic transcription, splicing and chromatin remodeling contribute to coordinate a sequence of events leading to formation of new organs. We have found that silencing of RNA polymerase II CTD-phosohatase-like 4 (CPL4_RNAi) in Arabidopsis induces CK-oversensitive de novo shoot organogenesis (DNSO) from roots, partly by early activation of transcription factors such as WUSCHEL and SHOOT MERISTEMLESS during pre-incubation on callus induction media. Here we show that a cluster of thalianol-biogenesis genes is highly expressed in the CPL4_RNAi during DNSO, implying involvement of CPL4 in transcriptional regulation of the thalianol pathway in DNSO.

Epigenetic reprogramming by histone acetyltransferase HAG1/AtGCN5 is required for pluripotency acquisition in Arabidopsis

Shoot regeneration can be achieved in vitro through a two-step process involving the acquisition of pluripotency on callus-induction media (CIM) and the formation of shoots on shoot-induction media. Although the induction of root-meristem genes in callus has been noted recently, the mechanisms underlying their induction and their roles in de novo shoot regeneration remain unanswered. Here, we show that the histone acetyltransferase HAG1/AtGCN5 is essential for de novo shoot regeneration. In developing callus, it catalyzes histone acetylation at several root-meristem gene loci including WOX5, WOX14, SCR, PLT1, and PLT2, providing an epigenetic platform for their transcriptional activation. In turn, we demonstrate that the transcription factors encoded by these loci act as key potency factors conferring regeneration potential to callus and establishing competence for de novo shoot regeneration. Thus, our study uncovers key epigenetic and potency factors regulating plant-cell pluripotency. These factors might be useful in reprogramming lineage-specified plant cells to pluripotency.

ARR12 promotes de novo shoot regeneration in Arabidopsis thaliana via activation of WUSCHEL expression

Auxin and cytokinin direct cell proliferation and differentiation during the in vitro culture of plant cells, but the molecular basis of these processes, especially de novo shoot regeneration, has not been fully elucidated. Here, we describe the regulatory control of shoot regeneration in Arabidopsis thaliana (L.) Heynh, based on the interaction of ARABIDOPSIS RESPONSE REGULATOR12 (ARR12) and WUSCHEL (WUS). The major site of ARR12 expression coincided with the location where the shoot apical meristem (SAM) initiated. The arr12 mutants showed severely impaired shoot regeneration and reduced responsiveness to cytokinin; consistent with this, the overexpression of ARR12 enhanced shoot regeneration. Certain shoot meristem specification genes, notably WUSCHEL (WUS) and CLAVATA3, were significantly downregulated in the arr12 explants. Chromatin immunoprecipitation (ChIP) and transient activation assays demonstrated that ARR12 binds to the promoter of WUS. These observations indicate that during shoot regeneration, in vitro, ARR12 functions as a molecular link between cytokinin signaling and the expression of shoot meristem specification genes.

Genome-wide analysis of transcription factors during somatic embryogenesis in banana (Musa spp.) cv. Grand Naine

Transcription factors BABY BOOM (BBM), WUSCHEL (WUS), BSD, LEAFY COTYLEDON (LEC), LEAFY COTYLEDON LIKE (LIL), VIVIPAROUS1 (VP1), CUP SHAPED COTYLEDONS (CUC), BOLITA (BOL), and AGAMOUS LIKE (AGL) play a crucial role in somatic embryogenesis. In this study, we identified eighteen genes of these nine transcription factors families from the banana genome database. All genes were analyzed for their structural features, subcellular, and chromosomal localization. Protein sequence analysis indicated the presence of characteristic conserved domains in these transcription factors. Phylogenetic analysis revealed close evolutionary relationship among most transcription factors of various monocots. The expression patterns of eighteen genes in embryogenic callus containing somatic embryos (precisely isolated by Laser Capture Microdissection), non-embryogenic callus, and cell suspension cultures of banana cultivar Grand Naine were analyzed. The application of 2, 4-dichlorophenoxyacetic acid (2, 4-D) in the callus induction medium enhanced the expression of MaBBM1, MaBBM2, MaWUS2, and MaVP1 in the embryogenic callus. It suggested 2, 4-D acts as an inducer for the expression of these genes. The higher expression of MaBBM2 and MaWUS2 in embryogenic cell suspension (ECS) as compared to non-embryogenic cells suspension (NECS), suggested that these genes may play a crucial role in banana somatic embryogenesis. MaVP1 showed higher expression in both ECS and NECS, whereas MaLEC2 expression was significantly higher in NECS. It suggests that MaLEC2 has a role in the development of non-embryogenic cells. We postulate that MaBBM2 and MaWUS2 can be served as promising molecular markers for the embryogencity in banana.

Direct conversion of root primordium into shoot meristem relies on timing of stem cell niche development

To understand how the identity of an organ can be switched, we studied the transformation of lateral root primordia (LRP) into shoot meristems in Arabidopsis root segments. In this system, the cytokinin-induced conversion does not involve the formation of callus-like structures. Detailed analysis showed that the conversion sequence starts with a mitotic pause and is concomitant with the differential expression of regulators of root and shoot development. The conversion requires the presence of apical stem cells, and only LRP at stages VI or VII can be switched. It is engaged as soon as cell divisions resume because their position and orientation differ in the converting organ compared with the undisturbed emerging LRP. By alternating auxin and cytokinin treatments, we showed that the root and shoot organogenetic programs are remarkably plastic, as the status of the same plant stem cell niche can be reversed repeatedly within a set developmental window. Thus, the networks at play in the meristem of a root can morph in the span of a couple of cell division cycles into those of a shoot, and back, through transdifferentiation.

Plant regeneration: cellular origins and molecular mechanisms

Compared with animals, plants generally possess a high degree of developmental plasticity and display various types of tissue or organ regeneration. This regenerative capacity can be enhanced by exogenously supplied plant hormones in vitro, wherein the balance between auxin and cytokinin determines the developmental fate of regenerating organs. Accumulating evidence suggests that some forms of plant regeneration involve reprogramming of differentiated somatic cells, whereas others are induced through the activation of relatively undifferentiated cells in somatic tissues. We summarize the current understanding of how plants control various types of regeneration and discuss how developmental and environmental constraints influence these regulatory mechanisms.

Combining linkage and association mapping identifies RECEPTOR-LIKE PROTEIN KINASE1 as an essential Arabidopsis shoot regeneration gene

De novo shoot organogenesis (i.e., the regeneration of shoots on nonmeristematic tissue) is widely applied in plant biotechnology. However, the capacity to regenerate shoots varies highly among plant species and cultivars, and the factors underlying it are still poorly understood. Here, we evaluated the shoot regeneration capacity of 88 Arabidopsis thaliana accessions and found that the process is blocked at different stages in different accessions. We show that the variation in regeneration capacity between the Arabidopsis accessions Nok-3 and Ga-0 is determined by five quantitative trait loci (QTL): REG-1 to REG-5. Fine mapping by local association analysis identified RECEPTOR-LIKE PROTEIN KINASE1 (RPK1), an abscisic acid-related receptor, as the most likely gene underlying REG-1, which was confirmed by quantitative failure of an RPK1 mutation to complement the high and low REG-1 QTL alleles. The importance of RPK1 in regeneration was further corroborated by mutant and expression analysis. Altogether, our results show that association mapping combined with linkage mapping is a powerful method to discover important genes implicated in a biological process as complex as shoot regeneration.

A set of Arabidopsis thaliana miRNAs involve shoot regeneration in vitro

Plant miRNAs, the critical regulator of gene expression, involve many development processes in vivo. However, the roles of miRNAs in plant cell proliferation and redifferntiation in vitro remain unknown. To determine better the molecular mechanism of these processes, we have recently reported that a set of miRNAs with different expression patterns between cells of totipotent and non-totipotent Arabidopsis calli. Some of these were specifically up- or downregulated during callus formation or shoot regeneration, and other development. Among them, miR160, and one of its target genes, ARF10, regulated Arabidopsis in vitro shoot regeneration via WUS, CLV3 and CUC1/ 2. The miR160-overexpressing, 35S transgenic lines, exhibited reduced shoot regeneration efficiency. The mARF10, a miR160-resistant form of ARF10, showed a high level of shoot regeneration ability. In the transgenic, expression of the above shoot meristem-specific genes was elevated, which is consistent with the improved shoot regeneration. In contrast, the ARF10 deficient knockout mutant produced fewer regenerated shoot. However, overexpressors of ARF10 were only marginally more efficient than the wild type with the respect to shoot regeneration. Our observation strongly supports that proper shoot regeneration from in vitro cultured cells requires the miR160-directed negative influence of ARF10. The enhanced expression of ARF10 is likely to have contributed to the improved regeneration ability.

Novel natural genetic variation controlling the competence to form adventitious roots and shoots from the tomato wild relative Solanum pennellii

Tomato (Solanum lycopersicum L.) is an attractive model to study the genetic basis of adventitious organ formation capacity, since there is considerable natural genetic variation among wild relatives. Using a set of 46 introgression lines (ILs), each containing a small chromosomal segment of Solanum pennellii LA716 introgressed and mapped into the tomato cultivar M82, we characterized a high shoot-regeneration capacity for ILs 3-2, 6-1, 7-1, 7-2, 8-2, 8-3, 9-1, 9-2, 10-2 and 10-3, when cotyledon explants were cultivated on medium containing 5.0μM BAP. F1 seedlings from the crosses 'Micro-Tom×ILs' and 'ILs×ILs' demonstrated that the shoot regeneration capacity of most ILs was dominant and that the regeneration ability of IL8-3 enhanced that of the other ILs in an additive manner. The ILs 3-2, 7-1, 8-3, and 10-2 also exhibited enhanced root formation on MS medium containing 0.4μM NAA, indicating that these chromosomal segments may contain genes controlling the competence to assume distinct cell fates, rather than the induction of a specific organ. We also performed the introgression of the genes controlling competence into the model system 'Micro-Tom'. The further isolation of such genes will improve our understanding of the molecular basis of organogenic capacity.

The tomato (Solanum lycopersicum cv. Micro-Tom) natural genetic variation Rg1 and the DELLA mutant procera control the competence necessary to form adventitious roots and shoots

Despite the wide use of plant regeneration for biotechnological purposes, the signals that allow cells to become competent to assume different fates remain largely unknown. Here, it is demonstrated that the Regeneration1 (Rg1) allele, a natural genetic variation from the tomato wild relative Solanum peruvianum, increases the capacity to form both roots and shoots in vitro; and that the gibberellin constitutive mutant procera (pro) presented the opposite phenotype, reducing organogenesis on either root-inducing medium (RIM) or shoot-inducing medium (SIM). Mutants showing alterations in the formation of specific organs in vitro were the auxin low-sensitivity diageotropica (dgt), the lateral suppresser (ls), and the KNOX-overexpressing Mouse ears (Me). dgt failed to form roots on RIM, Me increased shoot formation on SIM, and the high capacity for in vitro shoot formation of ls contrasted with its recalcitrance to form axillary meristems. Interestingly, Rg1 rescued the in vitro organ formation capacity in proRg1 and dgtRg1 double mutants and the ex vitro low lateral shoot formation in pro and ls. Such epistatic interactions were also confirmed in gene expression and histological analyses conducted in the single and double mutants. Although Me phenocopied the high shoot formation of Rg1 on SIM, it failed to increase rooting on RIM and to rescue the non-branching phenotype of ls. Taken together, these results suggest REGENERATION1 and the DELLA mutant PROCERA as controlling a common competence to assume distinct cell fates, rather than the specific induction of adventitious roots or shoots, which is controlled by DIAGEOTROPICA and MOUSE EARS, respectively.

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.

De novo shoot organogenesis: from art to science

In vitro shoot organogenesis and plant regeneration are crucial for both plant biotechnology and the fundamental study of plant biology. Although the importance of auxin and cytokinin has been known for more than six decades, the underlying molecular mechanisms of their function have only been revealed recently. Advances in identifying new Arabidopsis genes, implementing live-imaging tools and understanding cellular and molecular networks regulating de novo shoot organogenesis have helped to redefine the empirical models of shoot organogenesis and plant regeneration. Here, we review the functions and interactions of genes that control key steps in two distinct developmental processes: de novo shoot organogenesis and lateral root formation.