Involvement of Plant Stem Cells or Stem Cell-Like Cells in Dedifferentiation

Callus proliferation-induced hypoxic microenvironment decreases shoot regeneration competence in Arabidopsis

Plants are aerobic organisms that rely on molecular oxygen for respiratory energy production. Hypoxic conditions, with oxygen levels ranging between 1% and 5%, usually limit aerobic respiration and affect plant growth and development. Here, we demonstrate that the hypoxic microenvironment induced by active cell proliferation during the two-step plant regeneration process intrinsically represses the regeneration competence of the callus in Arabidopsis thaliana. We showed that hypoxia-repressed plant regeneration is mediated by the RELATED TO APETALA 2.12 (RAP2.12) protein, a member of the Ethylene Response Factor VII (ERF-VII) family. We found that the hypoxia-activated RAP2.12 protein promotes salicylic acid (SA) biosynthesis and defense responses, thereby inhibiting pluripotency acquisition and de novo shoot regeneration in calli. Molecular and genetic analyses revealed that RAP2.12 could bind directly to the SALICYLIC ACID INDUCTION DEFICIENT 2 (SID2) gene promoter and activate SA biosynthesis, repressing plant regeneration possibly via a PLETHORA (PLT)-dependent pathway. Consistently, the rap2.12 mutant calli exhibits enhanced shoot regeneration, which is impaired by SA treatment. Taken together, these findings uncover that the cell proliferation-dependent hypoxic microenvironment reduces cellular pluripotency and plant regeneration through the RAP2.12-SID2 module.

Coconut Callus Initiation for Cell Suspension Culture

The development of a cell suspension culture system for the scaling up of coconut embryogenic callus (EC) production would drastically improve efforts to achieve the large-scale production of high-quality clonal plantlets. To date, the hard nature of coconut EC appeared to be the main constraint for developing cell suspension cultures. Hence, this study attempted to acquire friable EC through the following approaches: The manipulation of (1) medium type and subculture frequency, (2) a reduced 2,4-dichlorophenoxy acetic acid concentration during subculture, (3) the nitrate level and the ammonium-to-nitrate ratio, and the addition of amino acid mixture, (4) the addition of L-proline, and (5) the reduction of medium nutrients. Unfortunately, none of these culture conditions produced friable coconut EC. Even though friable EC was not achieved via these approaches, some of the conditions were found to influence the formation of compact EC, therefore these results are important for further studies focused on somatic embryogenesis in coconut and other species.

Cellular responses of oil palm genotypes during somatic embryogenesis involve participation of procambial cells, DNA demethylation, and auxin accumulation

Cell markers of somatic embryogenesis initiation from leaf tissues in oil palm involve the participation of procambial cells, DNA demethylation, and auxin accumulation. Low callogenesis and genotype-dependent response have been mentioned in the development of somatic embryogenesis protocols of Elaeis oleifera × E. guineensis elite hybrids, which requires more detailed investigations of the process. Thus, the initial cellular responses of immature leaves of adult genotypes of this hybrid were investigated for the first time, emphasizing histological, epigenetic, and endogenous auxin changes. Leaf segments from two genotypes, one responsive to somatic embryogenesis (B351733) and another non-responsive (B352933), were inoculated in Murashige and Skoog medium with 450 µM of 4-amino-3, 5, 6-trichloropicolinic acid. For anatomical analysis, samples of both genotypes were collected at 0, 20, 90, and 105 days of cultivation. Samples of both genotypes were also taken at different cultivation periods to analyze DNA methylation status (% 5-mC-5 methylcytosine) via ELISA test. Immunolocalization assays were performed with anti-indole-3-acetic acid and anti-5-methyl-deoxycytosine antibodies from samples of hybrid B351733. We distinguished two groups of cells reactive to the induction of embryogenic callogenesis, parenchymatous sheath cells, and procambial cells; however, only the latter are directly involved with the formation of calluses. The data obtained indicate that the formation of calluses in hybrid B351733 is related to DNA hypomethylation, while the non-responsiveness of leaf explants in hybrid B352932 is related to DNA hypermethylation. The in situ immunolocalization enabled the identification of initial markers of the callogenic process, such as IAA accumulation and hypomethylation. Identifying these events brings the possibility of establishing strategies for efficient manipulation of somatic embryogenesis protocols in palm trees.

Establishment of Stem Cell-like Cells of Sida hermaphrodita (L.) Rusby from Explants Containing Cambial Meristems

The in vitro cultures of plant stem cells and stem cell-like cells can be established from tissues containing meristematic cells. Chemical compounds—as well as their production potential—is among the emerging topics of plant biotechnology. We induced the callus cell biomass growth and characterized the parameters indicating the presence of stem cells or stem cell-like cells. Four types of explants (stem, petiole, leaf, root) from Sida hermaphrodita (L.) Rusby and various combinations of auxins and cytokinins were tested for initiation of callus, growth of sub-cultivated callus biomass, and establishment of stem cells or stem cell-like cells. Induction of callus and its growth parameters were significantly affected both by the explant type and the combination of used plant growth hormones and regulators. The responsibility for callus initiation and growth was the highest in stem-derived explants containing cambial meristematic cells. Growth parameters of callus biomass and specific characteristics of vacuoles confirmed the presence of stem cells or stem cell-like cells in sub-cultivated callus cell biomass. Establishment of in vitro stem cell or stem cell-like cell cultures in S. hermaphrodita can lead to the development of various applications of in vitro cultivation systems as well as alternative applications of this crop.

Wuschel2 enables highly efficient CRISPR/Cas-targeted genome editing during rapid de novo shoot regeneration in sorghum

For many important crops including sorghum, use of CRISPR/Cas technology is limited not only by the delivery of the gene-modification components into a plant cell, but also by the ability to regenerate a fertile plant from the engineered cell through tissue culture. Here, we report that Wuschel2 (Wus2)-enabled transformation increases not only the transformation efficiency, but also the CRISPR/Cas-targeted genome editing frequency in sorghum (Sorghum bicolor L.). Using Agrobacterium-mediated transformation, we have demonstrated Wus2-induced direct somatic embryo formation and regeneration, bypassing genotype-dependent callus formation and significantly shortening the tissue culture cycle time. This method also increased the regeneration capacity that resulted in higher transformation efficiency across different sorghum varieties. Subsequently, advanced excision systems and “altruistic” transformation technology have been developed to generate high-quality morphogenic gene-free and/or selectable marker-free sorghum events. Finally, we demonstrate up to 6.8-fold increase in CRISPR/Cas9-mediated gene dropout frequency using Wus2-enabled transformation, compared to without Wus2, across various targeted loci in different sorghum genotypes.

Che et al. use Wuschel2-enabled genome transformation to induce somatic embryo formation in sorghum, a grain used in human food. Their approach not only overcomes the genotype-dependent barrier for genetic transformation without the introduction of morphogenic genes, but also increases the frequency of CRISPR/Castargeted genome editing.

Wuschel2 enables highly efficient CRISPR/Cas-targeted genome editing during rapid de novo shoot regeneration in sorghum

For many important crops including sorghum, use of CRISPR/Cas technology is limited not only by the delivery of the gene-modification components into a plant cell, but also by the ability to regenerate a fertile plant from the engineered cell through tissue culture. Here, we report that Wuschel2 (Wus2)-enabled transformation increases not only the transformation efficiency, but also the CRISPR/Cas-targeted genome editing frequency in sorghum (Sorghum bicolor L.). Using Agrobacterium-mediated transformation, we have demonstrated Wus2-induced direct somatic embryo formation and regeneration, bypassing genotype-dependent callus formation and significantly shortening the tissue culture cycle time. This method also increased the regeneration capacity that resulted in higher transformation efficiency across different sorghum varieties. Subsequently, advanced excision systems and "altruistic" transformation technology have been developed to generate high-quality morphogenic gene-free and/or selectable marker-free sorghum events. Finally, we demonstrate up to 6.8-fold increase in CRISPR/Cas9-mediated gene dropout frequency using Wus2-enabled transformation, compared to without Wus2, across various targeted loci in different sorghum genotypes.

Indirect somatic embryogenesis of Piper hispidinervum L. and evaluation of the regenerated plants by flow cytometry

Background

Piper hispidinervum is a species native from the Amazon region with great economic potential, given its scientifically proven insecticidal properties. In this study, an efficient protocol of plant regeneration via indirect somatic embryogenesis has been established for the first time. In a first experiment, for the induction of calluses, foliar explants of non-discriminated accesses of P. hispidinervum were inoculated in MS medium supplemented with α-naphtalenacetic acid (NAA) and 6-benzylaminopurine (BAP), in different combinations. For a second experiment, foliar explants from five different accesses of P. hispidinervum (PH17, PH21, PH28, PH37, and PH39) were analyzed regarding the formation of calluses when cultivated in MS medium with 5 mg L−1 NAA + 2.5 mg L−1 BAP. To obtain somatic embryos-like structures, calluses were cultivated in MS medium with 10 mg L−1 NAA + 2.5 mg L−1 of BAP. The somatic embryos-like structures obtained were inoculated in MS medium devoid of growth regulators and the plantlets were subjected to acclimatization. Calluses and somatic embryos-like structures were subjected to anatomical analysis and genetic stability of regenerated plants was analyzed by flow cytometry.

Results

The treatments 2.5 mg L−1 BAP and 5 mg L−1 NAA + 2.5 mg L−1 BAP, after 60 days of cultivation, provided each 32% of primary callus, not being verified the formation of calluses in medium devoid of BAP. It was found that accesses differed among them with respect to the formation of primary calluses, with emphasis on accesses PH28, PH37, and PH39, with mean percentage of 95.3%. Regarding the percentage of embryogenic calluses and formation of somatic embryos-like structures, there were no statistical differences between accesses, with mean values of 90.6% and 77.3%, respectively. The somatic embryos-like structures of P. hispidinervum have conspicuous morphoanatomical similarities with the zygotic embryo, and flow cytometry analysis showed no significant variation in nuclear DNA size among plants regenerated in vitro and plants coming from seed germination, which indicates ploidy level stability.

Conclusion

This protocol is the first cited in the literature that demonstrates an efficient micropropagation process by somatic embryogenesis of P. hispidinervum. It can be used either to enable large-scale vegetative production or to subsidize germplasm conservation or genetic engineering of P. hispidinervum.

Variation burst during dedifferentiation and increased CHH-type DNA methylation after 30 years of in vitro culture of sweet orange

Somaclonal variation arising from tissue culture may provide a valuable resource for the selection of new germplasm, but may not preserve true-to-type characteristics, which is a major concern for germplasm conservation or genome editing. The genomic changes associated with dedifferentiation and somaclonal variation during long-term in vitro culture are largely unknown. Sweet orange was one of the earliest plant species to be cultured in vitro and induced via somatic embryogenesis. We compared four sweet orange callus lines after 30 years of constant tissue culture with newly induced calli by comprehensively determining the single-nucleotide polymorphisms, copy number variations, transposable element insertions, methylomic and transcriptomic changes. We identified a burst of variation during early dedifferentiation, including a retrotransposon outbreak, followed by a variation purge during long-term in vitro culture. Notably, CHH methylation showed a dynamic pattern, initially disappearing during dedifferentiation and then more than recovering after 30 years of in vitro culture. We also analyzed the effects of somaclonal variation on transcriptional reprogramming, and indicated subgenome dominance was evident in the tetraploid callus. We identified a retrotransposon insertion and DNA modification alternations in the potential regeneration-related gene CLAVATA3/EMBRYO SURROUNDING REGION-RELATED 16. This study provides the foundation to harness in vitro variation and offers a deeper understanding of the variation introduced by tissue culture during germplasm conservation, somatic embryogenesis, gene editing, and breeding programs.

Nanog/NFATc1/Osterix signaling pathway-mediated promotion of bone formation at the tendon-bone interface after ACL reconstruction with De-BMSCs transplantation

Background

Bone formation plays an important role in early tendon–bone healing after anterior cruciate ligament reconstruction (ACLR). Dedifferentiated osteogenic bone marrow mesenchymal stem cells (De-BMSCs) have enhanced osteogenic potential. This study aimed to investigate the effect of De-BMSCs transplantation on the promotion of bone formation at the tendon–bone interface after ACLR and to further explore the molecular mechanism of the enhanced osteogenic potential of De-BMSCs.

Methods

BMSCs from the femurs and tibias of New Zealand white rabbits were subjected to osteogenic induction and then cultured in medium without osteogenic factors; the obtained cell population was termed De-BMSCs. De-BMSCs were induced to undergo osteo-, chondro- and adipo-differentiation in vitro to examine the characteristics of primitive stem cells. An ACLR model with a semitendinosus tendon was established in rabbits, and the animals were divided into a control group, BMSCs group, and De-BMSCs group. At 12 weeks after surgery, the rabbits in each group were sacrificed to evaluate tendon–bone healing by histologic staining, micro-computed tomography (micro-CT) examination, and biomechanical testing. During osteogenic differentiation of De-BMSCs, an siRNA targeting nuclear factor of activated T-cells 1 (NFATc1) was used to verify the molecular mechanism of the enhanced osteogenic potential of De-BMSCs.

Results

De-BMSCs exhibited some properties similar to BMSCs, including multiple differentiation potential and cell surface markers. Bone formation at the tendon–bone interface in the De-BMSCs group was significantly increased, and biomechanical strength was significantly improved. During the osteogenic differentiation of De-BMSCs, the expression of Nanog and NFATc1 was synergistically increased, which promoted the interaction of NFATc1 and Osterix, resulting in increased expression of osteoblast marker genes such as COL1A, OCN, and OPN.

Conclusions

De-BMSCs transplantation could promote bone formation at the tendon–bone interface after ACLR and improve the biomechanical strength of the reconstruction. The Nanog/NFATc1/Osterix signaling pathway mediated the enhanced osteogenic differentiation efficiency of De-BMSCs.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02643-9.

The DME demethylase regulates sporophyte gene expression, cell proliferation, differentiation, and meristem resurrection

The angiosperm life cycle has alternating diploid (sporophyte) and haploid (gametophyte) generations. The sporophyte generation begins with fertilization of haploid gametes and the gametophyte generation begins after meiosis. In Arabidopsis, the DEMETER (DME) DNA demethylase is essential for reproduction and is expressed in the central cell and vegetative cell of the female and male gametophyte, respectively. Little is known about DME function in the sporophyte. We show that DME activity is required for sporophyte development—seed germination, root hair growth, and cellular proliferation and differentiation during development—and we identify sporophytic genes whose proper expression requires DME activity. Together, our study provides important clues about the genetic circuits regulated by the DME DNA demethylase that control Arabidopsis sporophyte development.

The flowering plant life cycle consists of alternating haploid (gametophyte) and diploid (sporophyte) generations, where the sporophytic generation begins with fertilization of haploid gametes. In Arabidopsis, genome-wide DNA demethylation is required for normal development, catalyzed by the DEMETER (DME) DNA demethylase in the gamete companion cells of male and female gametophytes. In the sporophyte, postembryonic growth and development are largely dependent on the activity of numerous stem cell niches, or meristems. Analyzing Arabidopsis plants homozygous for a loss-of-function dme-2 allele, we show that DME influences many aspects of sporophytic growth and development. dme-2 mutants exhibited delayed seed germination, variable root hair growth, aberrant cellular proliferation and differentiation followed by enhanced de novo shoot formation, dysregulation of root quiescence and stomatal precursor cells, and inflorescence meristem (IM) resurrection. We also show that sporophytic DME activity exerts a profound effect on the transcriptome of developing Arabidopsis plants, including discrete groups of regulatory genes that are misregulated in dme-2 mutant tissues, allowing us to potentially link phenotypes to changes in specific gene expression pathways. These results show that DME plays a key role in sporophytic development and suggest that DME-mediated active DNA demethylation may be involved in the maintenance of stem cell activities during the sporophytic life cycle in Arabidopsis.

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.

Suppression of RNA-dependent RNA polymerase 6 in tomatoes allows potato spindle tuber viroid to invade basal part but not apical part including pluripotent stem cells of shoot apical meristem

RNA-dependent RNA polymerase 6 (RDR6) is one of the key factors in plant defense responses and suppresses virus or viroid invasion into shoot apical meristem (SAM) in Nicotiana benthamiana. To evaluate the role of Solanum lycopersicum (Sl) RDR6 upon viroid infection, SlRDR6-suppressed (SlRDR6i) ‘Moneymaker’ tomatoes were generated by RNA interference and inoculated with intermediate or lethal strain of potato spindle tuber viroid (PSTVd). Suppression of SlRDR6 did not change disease symptoms of both PSTVd strains in ‘Moneymaker’ tomatoes. Analysis of PSTVd distribution in shoot apices by in situ hybridization revealed that both PSTVd strains similarly invade the basal part but not apical part including pluripotent stem cells of SAM in SlRDR6i plants at a low rate unlike a previous report in N. benthamiana. In addition, unexpectedly, amount of PSTVd accumulation was apparently lower in SlRDR6i plants than in control tomatoes transformed with empty cassette in early infection especially in the lethal strain. Meanwhile, SlRDR6 suppression did not affect the seed transmission rates of PSTVd. These results indicate that RDR6 generally suppresses PSTVd invasion into SAM in plants, while suppression of RDR6 does not necessarily elevate amount of PSTVd accumulation. Additionally, our results suggest that host factors such as RDR1 other than RDR6 may also be involved in the protection of SAM including pluripotent stem cells from PSTVd invasion and effective RNA silencing causing the decrease of PSTVd accumulation during early infection in tomato plants.

Histology, histochemistry and ultrastructure of pre-embryogenic cells determined for direct somatic embryogenesis in the palm tree Syagrus oleracea

Somatic embryogenesis in palm trees is, in general, a slow and highly complex process, with a predominance of the indirect route and, consequently, a lack of knowledge about the direct route. We present new knowledge related to the morphological, histochemical and ultrastructural aspects of the transition from somatic to embryogenic cells and direct formation of somatic embryos from mature zygotic embryos of Syagrus oleracea, a palm tree. The results support the general concept that 2,4-dichlorophenoxyacetic acid plays a critical role for the formation of somatic embryos of direct and multicellular origin. Seven days in medium with auxin were enough for the identification of embryogenic cells. These cells had a set of characteristics corresponding to totipotent stem cells. At 14 days on induction medium, nodular formations were observed in the distal region of inoculated embryos, which evolved into globular somatic embryos. At 120 days on induction medium, the quality of the somatic embryos was compromised. The dynamics of the mobilization of reserve compounds was also demonstrated, with emphasis on starch and protein as energy sources required for the embryogenic process. This study shows for the first time the anatomical and ultrastructural events involved in direct somatic embryogenesis in a palm tree and incites the scientific community to return to the discussion of classical concepts related to direct somatic embryogenesis, especially regarding the characteristics and location of determined pre-embryogenic cells.

Hypomethylated drm1 drm2 cmt3 mutant phenotype of Arabidopsis thaliana is related to auxin pathway impairment

DNA methylation carried out by different methyltransferase classes is a relevant epigenetic modification of DNA which plays a relevant role in the development of eukaryotic organisms. Accordingly, in Arabidopsis thaliana loss of DNA methylation due to combined mutations in genes encoding for DNA methyltransferases causes several developmental abnormalities. The present study describes novel growth disorders in the drm1 drm2 cmt3 triple mutant of Arabidopsis thaliana, defective both in maintenance and de novo DNA methylation, and highlights the correlation between DNA methylation and the auxin hormone pathway. By using an auxin responsive reporter gene, we discovered that auxin accumulation and distribution were affected in the mutant compared to the wild type, from embryo to adult plant stage. In addition, we demonstrated that the defective methylation status also affected the expression of genes that regulate auxin hormone pathways from synthesis to transport and signalling and a direct relationship between differentially expressed auxin-related genes and altered auxin accumulation and distribution in embryo, leaf and root was observed. Finally, we provided evidence of the direct and organ-specific modulation of auxin-related genes through the DNA methylation process.

Uprising the antioxidant power of Argania spinosa L. callus through abiotic elicitation

This study was carried out in order to investigate the ability of tissues of Argania spinosa (L.) to undergo unlimited cell divisions by triggering their proliferative potential via callogenesis. Axenic cultures were efficiently established using axillary buds cultured on half-strength Murashige and Skoog (MS) medium after 20min of surface sterilization with sodium hypochlorite 6% (v/v). The highest callus rate was achieved with 1.0mgL^-1 of naphthaleneacetic acid (NAA) and 1.0mgL^-1 of 2,4-dichlorophenoxyacetic acid (2,4D) or similarly with 0.01mgL^-1 of 6-benzylaminopurine (BAP) and 1.0mgL^-1 of 2,4D at pH of 5.8, under dark conditions. The results of this study show also a significant increase in the callus's antioxidant power under abiotic pressure induced by NaCl. Catalase (CAT), peroxidase (PO), and superoxide dismutase (SOD) activities were significantly triggered, which protected the cells from the stimulated oxidative stress, under hydrogen peroxide (H₂O₂) significant release. This reaction favors subsequently the tissue recover process linked to the low abundance of polyphenol oxidase (PPO) activity and malondialdehyde (MDA) content. This work proves the efficiency of salt stress in boosting the argan cell's antioxidant status, which could be commercially applied in the field of cells regenerative therapy.

Arabidopsis TOR signaling is essential for sugar-regulated callus formation

Dedifferentiation is a remarkable process that produces pluripotent stem cells from differentiated somatic cells to ensure developmental plasticity. Plants have evolved the ability of cellular dedifferentiation, and signaling cascades related to auxin and cytokinin-dependent callus formation have been extensively investigated. However, the molecular mechanism underlying sugar-dependent callus formation remains unknown. Here, we show that sugar-dependent callus formation is mainly regulated by the TOR-E2Fa module in Arabidopsis. Sugar-activated TOR kinase phosphorylates and stabilizes E2Fa proteins to transcriptionally activate S-phase genes during callus formation. In parallel, E2Fa is transcriptionally regulated by the ARF-LBD transcription cascade. Multi-layered regulation of E2Fa by sugar and auxin is likely to shape balanced cellular dedifferentiation capability in Arabidopsis.

Inducible Transposition of a Heat-Activated Retrotransposon in Tissue Culture

A transposition of a heat-activated retrotransposon named ONSEN required compromise of a small RNA-mediated epigenetic regulation that includes RNA-directed DNA methylation (RdDM) machinery after heat treatment. In the current study, we analyzed the transcriptional and transpositional activation of ONSEN to better understand the underlying molecular mechanism involved in the maintenance and/or induction of transposon activation in plant tissue culture. We found the transposition of heat-primed ONSEN during tissue culture independently of RdDM mutation. The heat activation of ONSEN transcripts was not significantly up-regulated in tissue culture compared with that in heat-stressed seedlings, indicating that the transposition of ONSEN was regulated independently of the transcript level. RdDM-related genes were up-regulated by heat stress in both tissue culture and seedlings. The level of DNA methylation of ONSEN did not show any change in tissue culture, and the amount of ONSEN-derived small RNAs was not affected by heat stress. The results indicated that the transposition of ONSEN was regulated by an alternative mechanism in addition to the RdDM-mediated epigenetic regulation in tissue culture. We applied the tissue culture-induced transposition of ONSEN to Japanese radish, an important breeding species of the family Brassicaceae. Several new insertions were detected in a regenerated plant derived from heat-stressed tissues and its self-fertilized progeny, revealing the possibility of molecular breeding without genetic modification.

Plastid osmotic stress influences cell differentiation at the plant shoot apex

The balance between proliferation and differentiation in the plant shoot apical meristem is controlled by regulatory loops involving the phytohormone cytokinin and stem cell identity genes. Concurrently, cellular differentiation in the developing shoot is coordinated with the environmental and developmental status of plastids within those cells. Here, we employ an Arabidopsis thaliana mutant exhibiting constitutive plastid osmotic stress to investigate the molecular and genetic pathways connecting plastid osmotic stress with cell differentiation at the shoot apex. msl2 msl3 mutants exhibit dramatically enlarged and deformed plastids in the shoot apical meristem, and develop a mass of callus tissue at the shoot apex. Callus production in this mutant requires the cytokinin receptor AHK2 and is characterized by increased cytokinin levels, downregulation of cytokinin signaling inhibitors ARR7 and ARR15, and induction of the stem cell identity gene WUSCHEL Furthermore, plastid stress-induced apical callus production requires elevated plastidic reactive oxygen species, ABA biosynthesis, the retrograde signaling protein GUN1, and ABI4. These results are consistent with a model wherein the cytokinin/WUS pathway and retrograde signaling control cell differentiation at the shoot apex.