Apical meristem transcriptome analysis identifies a role for the blue light receptor gene GhFKF1 in cotton architecture development

Cotton architecture is determined by the differentiation fate transition of axillary meristem(AM),and influences cotton yield and the efficiency of mechanized harvesting.We observed that the initiation of flowering primordium was earlier in early-maturing than that in late-maturing cultivars during the differentiation and development of AM.The RNA-Seq and expression level analyses showed that genes FLAVIN BINDING,KELCH REPEAT,F-BOX1(GhFKF1),and GIGANTEA(GhGI)were in response to circadian rhythms,and involved in the regulation of cotton flowering.The gene structure,predicted protein structure,and motif content analyses showed that in Arabidopsis,cotton,rapseed,and soybean,proteins GhFKF1 and GhGI were functionally conserved and share evolutionary origins.Compared to the wild type,in GhFKF1 mutants that were created by the CRISPR/Cas9 system,the initiation of branch primordium was inhibited.Conversely,the knocking out of GhGI increased the number of AM differentiating into flower primordium,and there were much more lateral branch differentiation and development.Besides,we investigated that proteins GhFKF1 and GhGI can interact with each other.These results suggest that GhFKF1 and GhGI are key regulators of cotton architecture development,and may collaborate to regulate the differentiation fate transition of AM,ultimately influencing plant architecture.We describe a strategy for using the CRISPR/Cas9 system to increase cotton adaptation and productivity by optimizing plant architecture.

Barley FASCIATED EAR genes determine inflorescence meristem size and yield traits

In flowering plants,the inflorescence meristem(IM)provides founder cells to form successive floral meristems,which are precursors of fruits and seeds.The activity and developmental progression of IM are thus critical for yield production in seed crops.In some cereals,such as rice(Oryza sativa)and maize(Zea mays),the size of undifferentiated IM,which is located at the inflorescence apex,is positively associated with yield traits such as spikelet number.However,the relationship between IM size and yieldrelated spike traits remains unknown in the Triticeae tribe.Here we report that IM size has a negative correlation with yield traits in barley(Hordeum vulgare).Three FASCIATED EAR(FEA)orthologs,HvFEA2,HvFEA3,and HvFEA4,regulate IM size and spike morphogenesis and ultimately affect yield traits.Three HvFEAs genes are highly expressed in developing spikes,and all three loss-of-function mutants exhibit enlarged IM size,shortened spikes,and reduced spikelet number,which may lead to reduced grain yield.Natural variations identified in HvFEAs indicate selection events during barley domestication.We further reveal that HvFEA4,as a transcription factor,potentially targets multiple pathways during reproductive development,including transcriptional control,phytohormone signaling,and redox status.The roles of barley FEA genes in limiting IM size and promoting spikelet formation suggest the potential of increasing yield by manipulating IM activity.

B Class Floral Homeotic Genes are Involved in the Petal Identity and Flower Meristem Determinations in Chrysanthemum morifolium

Chrysanthemum morifolium,an ornamental crop with diverse forms of inflorescence,is a good model for studying flower development in Asteraceae.However,the genetic background is complex and the mechanisms of regulating flower development are still unclear.Here,we identified two natural mutant lines of chrysanthemum and named them M1 and M2 according to the severity of the phenotype.Both lines showed defects in petal identity,and the petals of the M1 line had a mild phenotype:partially loss of petal identity and conversion of petals into green,leaf-like organs.The M2 line had severe phenotypes:in addition to severe petal defects,secondary inflorescences were produced in the capitulum to replace the normal ray and disc florets,which indicated a transformation of a flower meristem into an inflorescence meristem.Transcriptome sequencing of WT and M2 inflorescences was performed and found altered expression of floral organ development A,B and E class genes,where B and E class genes were significantly down-regulated.qRT-PCR analysis in both M1 and M2 lines revealed that the expression of three chrysanthemum class B genes CmAP3.1,CmAP3.2 and CmPI,was negatively correlated with phenotypic severity.This suggests that class B genes in chrysanthemum not only have conserved functions in determining petal identity but also were involved in the determinacy of the flower meristem.This study provides insights into the functions of class B genes in flower development,and is informative for dissecting the molecular mechanisms of flower development in chrysanthemum.

The AGL6-like gene OsMADS6 regulates floral organ and meristem identities in rice

Although AGAMOUS-LIKE6 (AGL6) MADS-box genes are ancient with wide distributions in gymnosperms and angiosperms, their functions remain poorly understood. Here, we show the biological role of the AGL6-1ike gene, OsMADS6, in specifying floral organ and meristem identities in rice (Oryza sativa L.). OsMADS6 was strongly ex- pressed in the floral meristem at early stages. Subsequently, OsMADS6 transcripts were mainly detectable in paleas, lodicules, carpels and the integument of ovule, as well as in the receptacle. Compared to wild type plants, osmads6 mutants displayed altered palea identity, extra glume-like or mosaic organs, abnormal carpel development and loss of floral meristem determinacy. Strikingly, mutation of a SEPALLATA (SEP)-like gene, OsMADS1 (LHS1), enhanced the defect of osmads6 flowers, and no inner floral organs or glume-like structures were observed in whorls 2 and 3 of osmadsl-z osmads6-1 flowers. Furthermore, the osmadsl-z osmads6-1 double mutants developed severely indetermi- nate floral meristems. Our finding, therefore, suggests that the ancient OsMADS6 gene is able to specify "floral state" by determining floral organ and meristem identities in monocot crop rice together with OsMADS1.

AtCDC5 regulates the G2 to M transition of the cell cycle and is critical for the function of Arabidopsis shoot apical meristem

As a cell cycle regulator, the Myb-related CDC5 protein was reported to be essential for the G2 phase of the cell cycle in yeast and animals, but little is known about its function in plants. Here we report the functional characterization of the CDC5 gene in Arabidopsis thaliana. Arabidopsis CDC5 （AtCDC5） is mainly expressed in tissues with high cell division activity, and is expressed throughout the entire process of embryo formation. The AtCDC5 loss-of-function mutant is embryonic lethal. In order to investigate the function of AtCDC5 in vivo, we generated AtCDC5-RNAi plants in which the expression of AtCDC5 was reduced by RNA interference. We found that the G2 to M （G2/M） phase transition was affected in the AtCDC5-RNAi plants, and that endoreduplication was increased. Additionally, the maintenance of shoot apical meristem （SAM） function was disturbed in the AtCDC5-RNAi plants, in which both the WUSCHEL （WUS）- CLAVATA （CLV） and the SHOOT MERISTEMLESS （STM） pathways were impaired. In situ hybridization analysis showed that the expression of STMwas greatly reduced in the shoot apical cells of the AtCDC5-RNAi plants. Moreover, cyclinB1 or Histone4 was found to be expressed in some of these cells when the transcript of STM was undetectable. These results suggest that AtCDC5 is essential for the G2/M phase transition and may regulate the function of SAM by controlling the expression ofSTMand WUS.

Determination of the Photoperiod-Sensitive Inductive Phase in Maize with Leaf Numbers and Morphologies of Stem Apical Meristem

It is vital to determine the effective photoperiods of maize for making full use of tropical germplasm, which is the foundation for determining the effect of latitude and planting date on the development of photoperiod-sensitive maize cultivars. The objective of this study is to determine the photoperiod-sensitive inductive phase using reciprocal transfer between long- day （LD） （15 h d^-1） and short-day conditions （SD） （9 h d^-1）. For Huangzao 4 and CML288, days to tassel and pollen shedding were recorded, and stem apical meristems （SAM） were observed by a laser scanning confocal microscope. The results show that the seedlings are insensitive to photoperiod when they are very young （juvenile）. However, after this period, LD delays flowering and increases the leaf numbers below the inflorescence, and the length of the interval of the photoperiod-sensitive inductive phase is longer under LD conditions than under SD conditions. Transferred from SD to LD, plants show a sudden decrease in leaf numbers once sufficient SD has been received for flower commitment. While transferred from LD to SD, plants have a continuous increase in leaf numbers during the photoperiod sensitive inductive phase under LD conditions. At the same time, when plants are competent to flowers, the obvious morphology is the elongation of maize SAM. There is an obvious variance of the photoperiod sensitive phase under LD and SD conditions in different maize.

The GhREV transcription factor regulate the development of shoot apical meristem in cotton(Gossypium hirsutum)

Background:Manual topping is a routine agronomic practice for balancing the vegetative and reproductive growth of cotton(Gossypium hirsutum)in China,but its cost-effectiveness has decreased over time.Therefore,there is an urgent need to replace manual topping with new approaches,such as biological topping.In this study,we examined the function of Gh REV transcription factors(a classⅢhomeodomain-leucine zipper family,HD-ZIPⅢ)in regulating the development of shoot apical meristem(SAM)in cotton with the purpose of providing candidate genes for biological topping of cotton in the future.Results:We cloned four orthologous genes of At REV in cotton,namely Gh REV1,Gh REV2,Gh REV3,and Gh REV4.All the Gh REVs expressed in roots,stem,leaves,and SAM.Compared with Gh REV1 and Gh REV3,the expression level of Gh REV2 and Gh REV4 was higher in the SAM.However,only Gh REV2 had transcriptional activity.Gh REV2 is localized in the nucleus;and silencing it via virus-induced gene silencing(VIGS)produced an abnormal SAM.Two key genes,Gh WUSA10 and Gh STM,which involved in regulating the development of plant SAM,showed about 50%reduction in their transcripts in VIGS-Gh REV2 plants.Conclusion:Gh REV2 positively regulates the development of cotton SAM by regulating Gh WUSA10 and Gh STM potentially.

The Wheat Plastochron Mutant, fushi-darake, Shows Transformation of Reproductive Spikelet Meristem into Vegetative Shoot Meristem

In wheat plants at the vegetative growth stage, the shoot apical meristem (SAM) produces leaf primordia. When reproductive growth is initiated, the SAM forms an inflorescence meristem (IM) that differentiates a series of spikelet meristem (SM) as the branch. The SM then produces a series of floret meristem (FM) as the branch. To identify the mechanisms that regulate formation of the reproductive meristems in wheat, we have investigated a leaf initiation mutant, fushi-darake (fdk) which was developed by ion beam mutagenesis. The morphological traits were compared in wild type (WT) and fdk mutant plants grown in the experimental field. WT plants initiated leaves from SAM at regular intervals in spiral phyllotaxy, while fdk plants had 1/2 alternate phyllotaxy with rapid leaf emergence. The fdk plants have increased numbers of nodes and leaves compared with WT plants. The time interval between successive leaf initiation events (plastochron) was measured in plants grown in a growth chamber. The fdk plants clearly show the rapid leaf emergence, indicating a shortened plastochron. Each tiller in fdk plants branches at the upper part of the culm. The fine structure of organ formation in meristems of fdk plants was examined by scanning electron microscopy (SEM). The SEM analysis indicated that fdk plants show transformation of spikelet meristems into vegetative shoot meristems. In conclusion, the fdk mutant has a heterochronic nature, i.e., both reproductive and vegetative programs were simultaneously in operation during the reproductive phase, resulting in a shortened plastochron and transformation of reproductive spikelets into vegetative shoots.

Efficient Regeneration System for Genetic Transformation of Mulberry (<i>Morus indica</i>L. Cultivar S-36) Using <i>in Vitro</i>Derived Shoot Meristems

Shoot meristems used for the study were exercised from the in vitro regenerated shoots cultured on MS medium supplemented with 0.5 mg/L of BAP for multiplication. The sensitivity of the in vitro regenerated was studied using shoot meristems of 0.5 cm. Shoot meristems were cultured on medium containing 10-100 mg/l kanamycin to determine the concentration that was lethal for multiple shoot induction and root induction. The response of shoot multiplication decreased (66.2%-6.2%) as the concentration of kanamycin increased (10.0-70.0 mg/L) with complete inhibition of shoot proliferation at 100 mg/L kanamycin. The rooting phase was very sensitive to kanamycin compared to shoot multiplication. The percentage of shoots that rooted decreased (53.8%-4.8%) with increase in the concentration of kanamycin (10.0-70.0 mg/l) on IBA and 2,4-D supplemented medium. For transformation studies, the shoot tips that were infected with Agrobacterium strain were placed on selection medium containing MS medium with 0.5 mg/L BAP and 100 mg/L kanamycin and scored for the putative transformed shoots. An average of 62.2% of shoot tips developed shoot buds from the base and the shoots reached a length of 0.5-1.0 cm at the end of 30 days of culture on the selective medium in comparison to control which showed no response. An average of 66.7% of the regenerated plants showed GUS expression on selection medium where 43.2% and 65% of GUS expression was recorded in the leaves and callus. Leaves and callus induced from the controls did not show GUS activity. Stable integration of nptII gene with the genomic DNA from these transformed plants was confirmed through PCR analysis. Our result presents an efficient regeneration system using in vitro derived shoot meristems for Agrobacterium mediated gene transfer.

Floral Organogenesis and Ring Meristem in <i>Phytolacca</i>

To further study the floral organogenesis and discussing the floral origin of Phytolacca, the procedures of floral organogenesis were observed in Phytolacca esculenta and Phytolacca zhejiangensis. The results showed that the floral organogenesis was consistent in Phytolacca. Their sepals were 2/5 helix, and with counter-clockwise and clockwise, usually the first sepal located at non-median of abaxial side. The first sepal of Phytolacca esculenta was initiated at non-median of adaxial side. There was no evident relationship between sepal and stamen initiating position, and the stamens initiated on ring meristem, they initiated approximately at the same time, and when the androecium member was numerous, they initiated centrifugally, the outer stamen initiated irregularly. Carpel initiated alternately with inner stamens. And the carpels connected by septum, if the septum grew more, the carpel was syncarpous at morphology, otherwise the carpel was apocarpous at morphology. So the syncarpous and the apocarpous have no successively relationship on evolution. Ovule initiated inside the carpel and opposite to carpel. Androecium, carpel and ovule initiated at ring meristem.

Generation of Virus Free Potato Plantlets through Meristem Culture and Their Field Evaluation

Different aspects of micropropagation through meristem culture for the production of virus indexed source plants, in vitro tuberization and field evaluation of the in vitro regenerated plants were studied on four commercial cultivars of potato (Solanum tuberosum L.) viz., Diamant, Cardinal, Shilbilati and Lalpakri. The investigation was conducted at Rajshahi, Bangladesh from December 2010 to March 2012 to produce virus-free potato plantlets through meristem culture, shoot multiplications with root induction as well as their acclimatization and evaluation of morphological characters and tuber yield under field condition. Shoot tips of 25 - 30 day old field-grown plants of above mentioned four cultivars were used for meristem isolation. After isolation, meristems of these varieties of potato were cultured on “M” shaped filter paper bridge in Murashige and Skoog (MS) liquid medium. Four different treatments of media formulations viz. 0.1 mg/L KIN + 0.1 mg/L GA3, 0.1 mg/L KIN + 0.5 mg/L GA3, 0.5 mg/L KIN + 0.1 mg/L GA3 and 0.5 mg/L KIN + 0.5 mg/L GA3 were used as plant growth regulators. From these formulations MS + 0.1 mg/L KIN + 0.5 mg/L GA3 was found to be the best for the primary establishment of meristem culture. The primarily established meristems were subcultured on to MS semisolid basal medium supplemented with four different treatment combinations of hormones viz. 0.5 mg/L BA + 1.0 mg/L IBA;0.1 mg/L KIN + 0.1 mg/L GA3;0.5 mg/L BA + 0.5 mg/L GA3 and 0.5 mg/L KIN + 0.5 mg/L GA3 were used to identify the suitable media compositions for shoot proliferation. Results showed that out of these four media treatments the formulation 0.5 mg/L BA + 0.5 mg/L GA3 was found to be the best suitable for shoot generation. Among the four cultivars of potato higher frequency of shoot proliferation (number of shoots/explant and longest shoot length) was observed in Diamant, though the highest shoot formation (76%) was recorded in Cardinal. Virus free in vitro grown potato plantlets were obtained through DAS-ELISA test and used substantially for micro-propagation. After gradual acclimatization of rooted plantlets of four potato cultivars, they were transferred into the field for cultivation and established successfully. It was observed from the field study of in vitro meristem-derived plantlets that there were no virus-affected plants. The virus-free exotic varieties were much superior in all vegetative attributes and yield compared to those of indigenous varieties with producing potato plants of normal height. In contrast, the indigenous varieties took a longer time to tuber initiation and maturity, lower plant height and number of leaves per plant, a higher number of tubers but a lower amount of tuber weight per plant, and poorer tuber grade than the exotic varieties. However, the variety Cardinal exposed the best performances in the context of survival percentage of plantlets (90%), days to tuber initiation (DTI), a number of leaves per plant (NL), tuber weight per plant (343.40%) and the percentage of rich tuber grade.

The Effects of Lead on the Meristem of Wheat Seedlings

The ultrastructure of apical meristem cells was studied in Triticum aestivum L. cv. “Trizo” seedlings grown on soil without or enriched with selenium and survived 14 days’ stress caused by lead pollution in the soil. The soil treatments: control—the original soil;(Pb1)—50 mg&middot;kg&minus;1;(Pb2)—100 mg&middot;kg&minus;1;(Pb1 + Se1) —0.4 mg&middot;kg&minus;1 Se added to Pb1 treated soil;(Pb1 + Se2)—0.8 mg&middot;kg&minus;1 Se added to Pb1 treated soil;(Pb2 + Se1)—0.4 mg&middot;kg&minus;1 Se added to Pb2 treated soil;(Pb2 + Se2)—0.8 mg&middot;kg&minus;1 Se added to Pb2 treated soil were used. Light and other conditions were optimal for plant growth. A distinctive feature of the cells of the apical meristem of control plants was the absence of nuclear membranes. Proplastids were membrane vesicles 1 - 2 microns in diameter, filled with contents of varying degrees of density, from membrane vesicles containing only plastid DNA up to a fully formed structure of proplastids. In (Pb1)-plants, the amount of cytoplasmic ribosomes and proplastids in the meristematic cells was less than in the control. The structure of the forming proplastids was almost the same as that of the control plants. Signs of degradation of meristematic proplastids, such as a decrease of their diameter, observed in (Pb2)-plants. The introduction of selenium into lead contaminated soil increased the accumulation of Pb in plants, especially in the roots and apical meristem. In (Pb1 + Se1)-, (Pb1 + Se2)-, (Pb2 + Se1)-, and (Pb2 + Se2)-plants, the number of cytoplasmic ribosomes in meristematic cells increased, which indirectly indicates an increase in protein synthesis. Based on our concept about the formation (assembly) of proplastids in the cells of the apical meristem, we believe that toxic agents, such as lead, which inhibit the development of proplastids into chloroplasts in mesophyll cells, act on apical meristem cells at the stage when plastid DNA is replicated in the cytoplasm and is not yet surrounded by a plastid membrane.

The transcription factor HSFA7b controls thermomemory at the shoot apical meristem by regulating ethylene biosynthesis and signaling in Arabidopsis

The shoot apical meristem(SAM)is responsible for overall shoot growth by generating all aboveground structures.Recent research has revealed that the SAM displays an autonomous heat stress(HS)memory of a previous non-lethal HS event.Considering the importance of the SAM for plant growth,it is essential to determine how its thermomemory is mechanistically controlled.Here,we report that HEAT SHOCK TRAN-SCRIPTION FACTOR A7b(HSFA7b)plays a crucial role in this process in Arabidopsis,as the absence of functional HSFA7b results in the temporal suppression of SAM activity after thermopriming.We found that HSFA7b directly regulates ethylene response at the SAM by binding to the promoter of the key ethylene signaling gene ETHYLENE-INSENSITIVE 3 to establish thermotolerance.Moreover,we demonstrated that HSFA7b regulates the expression of ETHYLENE OVERPRODUCER 1(ETO1)and ETO1-LIKE 1,both of which encode ethylene biosynthesis repressors,thereby ensuring ethylene homeostasis at the SAM.Taken together,these results reveal a crucial and tissue-specic role for HSFA7b in thermomemory at the Arabidopsis SAM.

Heat stress impairs floral meristem termination and fruit development by affecting the BR-SlCRCa cascade in tomato

Floral meristem termination is a key step leading to carpel initiation and fruit development.The frequent occurrence of heat stress due to global warming often disruptsfloral determinacy,resulting in defective fruit formation.However,the detailed mechanism behind this phenomenon is largely unknown.Here,we identify CRABS CLAW a(SlCRCa)as a key regulator offloral meristem termination in tomato.SlCRCa func-tions as an indispensablefloral meristem terminator by suppressing SlWUS activity through the TOMATO AGAMOUS 1(TAG1)–KNUCKLES(SlKNU)–INHIBITOR OF MERISTEM ACTIVITY(SlIMA)network.A direct binding assay revealed that SlCRCa specifically binds to the promoter and second intron of WUSCHEL(SlWUS).We also demonstrate that SlCRCa expression depends on brassinosteroid homeostasis in theflo-ral meristem,which is repressed by heat stress via the circadian factor EARLY FLOWERING 3(SlELF3).These results provide new insights intofloral meristem termination and the heat stress response inflowers and fruits of tomato and suggest that SlCRCa provides a platform for multiple protein interactions that may epigenetically abrogate stem cell activity at the transition fromfloral meristem to carpel initiation.

Condensation of STM is critical for shoot meristem maintenance and salt tolerance in Arabidopsis

The shoot meristem generates the entire shoot system and is precisely maintained throughout the life cycle under various environmental challenges.In this study,we identified a prion-like domain(PrD)in the key shoot meristem regulator SHOOT MERISTEMLESS(STM),which distinguishes STM from other related KNOX1 proteins.We demonstrated that PrD stimulates STM to form nuclear condensates,which are required for maintaining the shoot meristem.STM nuclear condensate formation is stabilized by selected PrD-containing STM-interacting BELL proteins in vitro and in vivo.Moreover,condensation of STM promotes its interaction with the Mediator complex subunit MED8 and thereby enhances its transcriptional activity.Thus,condensate formation emerges as a novel regulatory mechanism of shoot meristem functions.Furthermore,we found that the formation of STM condensates is enhanced upon salt stress,which allows enhanced salt tolerance and increased shoot branching.Our findings highlight that the transcription factor partitioning plays an important role in cell fate determination and might also act as a tunable environmental acclimation mechanism.

High-resolution anatomical and spatial transcriptome analyses reveal two types of meristematic cell pools within the secondary vascular tissue of poplar stem

The secondary vascular tissue emanating from meristems is central to understanding how vascular plants such as forest trees evolve,grow,and regulate secondary radial growth.However,the overall molecular characterization of meristem origins and developmental trajectories from primary to secondary vascular tissues in woody tree stems is technically challenging.In this study,we combined high-resolution anatomic analysis with a spatial transcriptome(ST)technique to define features of meristematic cells in a developmental gradient from primary to secondary vascular tissues in poplar stems.The tissue-specific gene expression of meristems and derived vascular tissue types were accordingly mapped to specific anatomical domains.Pseudotime analyses were used to track the origins and changes of meristems throughout the development from primary to secondary vascular tissues.Surprisingly,two types of meristematic-like cell pools within secondary vascular tissues were inferred based on high-resolution microscopy combined with ST,and the results were confirmed by in situ hybridization of,transgenic trees,and single-cell sequencing.The rectangle shape procambium-like(PCL)cells develop from procambium meristematic cells and are located within the phloem domain to produce phloem cells,whereas fusiform shape cambium zone(CZ)meristematic cells develop from fusiform metacambium meristematic cells and are located inside the CZ to produce xylem cells.The gene expression atlas and transcriptional networks spanning the primary transition to secondary vascular tissues generated in this work provide new resources for studying the regulation of meristem activities and the evolution of vascular plants.A web server(https://pgx.zju.edu.cn/stRNAPal/)was also established to facilitate the use of ST RNA-seq data.

Receptor-like protein kinase-mediated signaling in controlling root meristem homeostasis

Generation of the root greatly benefits higher plants living on land.Continuous root growth and development are achieved by the root apical meristem,which acts as a reservoir of stem cells.The stem cells,on the one hand,constantly renew themselves through cell division.On the other hand,they differentiate into functional cells to form diverse tissues of the root.The balance between the maintenance and consumption of the root apical meristem is governed by cell-to-cell communications.Receptor-like protein kinases(RLKs),a group of signaling molecules localized on the cell surface,have been implicated in sensing multiple endogenous and environmental signals for plant development and stress adaptation.Over the past two decades,various RLKs and their ligands have been revealed to participate in regulating root meristem homeostasis.In this review,we focus on the recent studies about RLK-mediated signaling in regulating the maintenance and consumption of the root apical meristem.

Auxin-Cytokinin Interaction Regulates Meristem Development

Plant hormones regulate many aspects of plant growth and development. Both auxin and cytokinin have been known for a long time to act either synergistically or antagonistically to control several significant developmental processes, such as the formation and maintenance of meristem. Over the past few years, exciting progress has been made to reveal the molecular mechanisms underlying the auxin-cytokinin action and interaction. In this review, we shall briefly discuss the major progress made in auxin and cytokinin biosynthesis, auxin transport, and auxin and cytokinin signaling. The frameworks for the complicated interaction of these two hormones in the control of shoot apical meristem and root apical meristem formation as well as their roles in in vitro organ regeneration are the major focus of this review.

Isolation and identification of genes expressed differentially in rice inflorescence meristem with suppression subtractive hybridization

A subtracted cDNA library of rice (Oryza sativa L.) inflorescence meristem (IM) was constructed using the suppression subtractive hybridization (SSH) method. The cDNAs of the rice shoot apical meristem (SAM) were used as 'driver' and inflorescence meristem (IM) as 'tester' in the experiment, respectively. Forty of 250 randomly chosen cDNA clones were identified by differential screening, which were IM-specific or IM-highly expressed. Most of the rice IM cDNAs cloned by SSH appear to represent rare transcripts, 40% of which were derived from truly differentially expressed genes. Of all the forty sequenced cDNA inserts, eleven contain the regions with 60%-90% identity to their homolog in GenBank, eighteen are expected to be new genes, only two correspond to published rice genes.

Plant stem cells and their regulations in shoot apical meristems

Stem cells in plants,established during embry-ogenesis,are located in the centers of the shoot apical meristem(SAM)and the root apical meristem(RAM).Stem cells in SAM have a capacity to renew themselves and to produce new organs and tissues indefinitely.Although fully differentiated organs such as leaves do not contain stem cells,cells in such organs do have the capacity to re-establish new stem cells,especially under the induction of phytohormones in vitro.Cytokinin and auxin are critical in creating position signals in the SAM to maintain the stem cell organizing center and to position the new organ primordia,respectively.This review addresses the distinct features of plant stem cells and focuses on how stem cell renewal and differentiation are regulated in SAMs.