Micro-RNA-Regulated SQUAMOSA-PROMOTER BINDING PROTEIN-LIKE (SPL) Gene Expression and Cytokinin Accumulation Distinguish Early-Developing Male and Female Inflorescences in Oil Palm (Elaeis guineensis).

Sexual differentiation of inflorescences and flowers is important for reproduction and affects crop plant productivity. We report here on a molecular study of the process of sexual differentiation in the immature inflorescence of oil palm (Elaeis guineensis). This species is monoecious and exhibits gender diphasy, producing male and female inflorescences separately on the same plant in alternation. Three main approaches were used: small RNA-seq to characterise and study the expression of miRNA genes; RNA-seq to monitor mRNA accumulation patterns; hormone quantification to assess the role of cytokinins and auxins in inflorescence differentiation. Our study allowed the characterisation of 30 previously unreported palm MIRNA genes. In differential gene and miRNA expression studies, we identified a number of key developmental genes and miRNA-mRNA target modules previously described in relation to their developmental regulatory role in the cereal panicle, notably the miR156/529/535-SQUAMOSA PROMOTER-BINDING PROTEIN-LIKE (SPL) gene regulatory module. Gene enrichment analysis highlighted the importance of hormone-related genes, and this observation was corroborated by the detection of much higher levels of cytokinins in the female inflorescence. Our data illustrate the importance of branching regulation within the developmental window studied, during which the female inflorescence, unlike its male counterpart, produces flower clusters on new successive axes by sympodial growth.

A Non-Shedding Fruit Elaeis oleifera Palm Reveals Perturbations to Hormone Signaling, ROS Homeostasis, and Hemicellulose Metabolism.

The developmentally programmed loss of a plant organ is called abscission. This process is characterized by the ultimate separation of adjacent cells in the abscission zone (AZ). The discovery of an American oil palm (Elaeis oleifera) variant that does not shed its has allowed for the study of the mechanisms of ripe fruit abscission in this species. A comparative transcriptome analysis was performed to compare the fruit AZs of the non-shedding E. oleifera variant to an individual of the same progeny that sheds its ripe fruit normally. The study provides evidence for widespread perturbation to gene expression in the AZ of the non-shedding variant, compared to the normal fruit-shedding control, and offers insight into abscission-related functions. Beyond the genes with known or suspected roles during organ abscission or indehiscence that were identified, a list of genes with hormone-related functions, including ethylene, jasmonic acid, abscisic acid, cytokinin and salicylic acid, in addition to reactive oxygen species (ROS) metabolism, transcriptional responses and signaling pathways, was compiled. The results also allowed a comparison between the ripe fruit abscission processes of the African and American oil palm species at the molecular level and revealed commonalities with environmental stress pathways.

Partial disintegration of vegetable cell wall during cooking improves vitamin K1 (phylloquinone) bioaccessibility in in vitro digestion.

Vegetables rich in vitamin K consumption could prevent bleeding and maintain bone status. The aims of the present work were to investigate i) the effect of household cooking (i.e., boiling for 5 min at 100 °C in distilled water and stir-frying for 3 min at 180 °C in hot canola oil) on phylloquinone bioaccessibility of five rich phylloquinone leafy vegetables, namely Water spinach (Ipomoea aquatic Forssk), Amaranth (Amaranthus blitum subsp. oleraceus L.), Chinese broccoli (Brassica alboglabra), Pak choi (Brassica rapa L.) and Drumstick (Moringa oleifera Lam.), and ii) the structural changes of these leaves before and after in vitro gastro-intestinal digestion. All the experiments were realized in triplicate for each vegetable. The amounts of phylloquinone in leafy vegetables were noticeable in almost all species and ranged from 94 to 182 µg/100 g DM. Their cell wall polysaccharide contents greatly varied from 4.3 to 8.4 g for 100 g. The content in bioaccessible phylloquinone was low in raw leaves (<25 µg/100 g DM) as well as its bioaccessibility (<15%). Leaf pectin content impaired phylloquinone bioaccessibility using principal component analysis. Boiling and stir-frying significantly improved the bioaccessibility of phylloquinone in leaves by a factor of three to twelve and two to seven respectively (p<0.05). These variations were associated with changes in leaf structure. Palisade and spongy cells appeared ruptured and disorganized after stir-frying. Given the estimated bioaccessibility of phylloquinones, the consumption of 500 g of cooked wet leaves per day would cover phylloquinone needs of an individual adult average body weight.

Redox-related gene expression and sugar accumulation patterns are altered in the edible inflorescence produced by the cultivated form of pacaya palm (Chamaedorea tepejilote).

BACKGROUND AND AIMS: The pacaya palm is a dioecious neotropical palm species that is exploited in Latin America for its male inflorescence, which is edible when immature. It is cultivated, in a non-intensive manner, in Guatemala, where a morphotype occurs that produces much larger, more highly branched inflorescences compared with wild palms. We sought to identify molecular factors underlying this phenotypic divergence, which is likely to be a product of domestication. METHODS: We performed RNA-seq-based studies on immature pacaya palm male inflorescences in order to identify genes that might be directly or indirectly affected in their expression in relation to domestication. We also measured the accumulation of a range of soluble sugar molecules to provide information on the biochemical status of the two different types of material. KEY RESULTS: A total of 408 genes were found to display significantly different expression levels between the wild and cultivated morphotypes. Three different functional categories were found to be enriched in the gene set that was upregulated in the cultivated morphotype: redox balance; secondary metabolism; and transport. Several sugars were found to accumulate at higher levels in inflorescences of the cultivated morphotype, in particular myo-inositol, fructose and glucose. CONCLUSIONS: The observed upregulation of redox-related genes in the cultivated morphotype is corroborated by the observation of higher myo-inositol accumulation, which has been shown to be associated with enhanced scavenging of reactive oxygen species in other plants and which may affect meristem activity.

Multi-scale comparative transcriptome analysis reveals key genes and metabolic reprogramming processes associated with oil palm fruit abscission.

BACKGROUND: Fruit abscission depends on cell separation that occurs within specialized cell layers that constitute an abscission zone (AZ). To determine the mechanisms of fleshy fruit abscission of the monocot oil palm (Elaeis guineensis Jacq.) compared with other abscission systems, we performed multi-scale comparative transcriptome analyses on fruit targeting the developing primary AZ and adjacent tissues. RESULTS: Combining between-tissue developmental comparisons with exogenous ethylene treatments, and naturally occurring abscission in the field, RNAseq analysis revealed a robust core set of 168 genes with differentially regulated expression, spatially associated with the ripe fruit AZ, and temporally restricted to the abscission timing. The expression of a set of candidate genes was validated by qRT-PCR in the fruit AZ of a natural oil palm variant with blocked fruit abscission, which provides evidence for their functions during abscission. Our results substantiate the conservation of gene function between dicot dry fruit dehiscence and monocot fleshy fruit abscission. The study also revealed major metabolic transitions occur in the AZ during abscission, including key senescence marker genes and transcriptional regulators, in addition to genes involved in nutrient recycling and reallocation, alternative routes for energy supply and adaptation to oxidative stress. CONCLUSIONS: The study provides the first reference transcriptome of a monocot fleshy fruit abscission zone and provides insight into the mechanisms underlying abscission by identifying key genes with functional roles and processes, including metabolic transitions, cell wall modifications, signalling, stress adaptations and transcriptional regulation, that occur during ripe fruit abscission of the monocot oil palm. The transcriptome data comprises an original reference and resource useful towards understanding the evolutionary basis of this fundamental plant process.

Long-term conservation of Tarenaya rosea (Cleomaceae) root cultures: histological and histochemical analyses during cryopreservation using the encapsulation-vitrification technique.

Adventitious root cultures of Tarenaya rosea were successfully cryopreserved using the encapsulation-vitrification technique. Histological analysis revealed useful information on the successive steps of cryopreservation. Coupled with complementary histochemical approaches, these studies provided cellular and tissue descriptions of T. rosea root cultures during cryopreservation and contributed to an understanding of cellular stress responses, as well as characterization of the anatomical pattern of root regeneration. The effects of exposure duration to PVS3 solution (0-120 min), unloading treatment (direct and gradual), and recovery medium (liquid and solid) on recovery of cryopreserved roots were investigated. The highest recovery (91%) after cooling in liquid nitrogen (LN) was reached with PVS3 treatment for 90 min, gradual rehydration in unloading solution, and recovery on solid MS medium. The cryopreserved roots showed high multiplication capacity, which was maintained for up to four subcultures. The effect of cryopreservation on root structure was investigated by histological and histochemical studies. Plasmolysis intensified during exposure to loading and PVS3 solutions, but decreased after unloading treatment. The proportion of intercellular spaces increased progressively throughout the cryopreservation protocol, culminating in root cortex disruption. Histochemical analyses revealed polysaccharides, proteins, and both lipidic and pectic substances in intercellular spaces. The vascular cylinder remained intact, ensuring the formation of new roots from the pericycle, showing that proliferative capacity of cryopreserved roots had not diminished.

Histological characterization of Passiflora pohlii Mast. root tips cryopreserved using the V-Cryo-plate technique.

Cryopreservation stands out as the main strategy to ensure safe and cost efficient long-term conservation of plant germplasm, especially for biotechnological materials. However, the injuries associated with the procedure may result in structural damage and low recovery rates after cooling. Histological analysis provides useful information on the effects of osmotic dehydration, LN exposure, and recovery conditions on cellular integrity and tissue organization, allowing the determination of the critical steps of the cryopreservation protocol and, thus, the use of optimized treatments. Passiflora pohlii Mast. (Passifloraceae) is a native species from Brazil with potential agronomic interest. Recent studies showed the presence of saponins in its roots, which presented antioxidant activity. The goal of this work was to develop a cryopreservation technique for root tips of in vitro-derived plants of P. pohlii using the V-Cryo-plate technique and to characterize the anatomical alterations that occurred during the successive steps of the protocol. Root tips were excised from in vitro plants and precultured before adhesion to cryo-plates and then treated for different periods with the plant vitrification solutions PVS2 or PVS3. Treatment with PVS2 for 45 min resulted in higher recovery (79%) when compared with PVS3 (43%). The greatest number of adventitious roots per cryopreserved explant was also observed after a 45-min exposure to PVS2. Plasmolysis levels were higher in cortical cells of cryopreserved explants treated with PVS2, while pericycle and central cylinder cells were not damaged after this treatment. Thirty days after rewarming, no plasmolysis could be detected, regardless of the experimental conditions.

Transcriptome Analysis of Cell Wall and NAC Domain Transcription Factor Genes during Elaeis guineensis Fruit Ripening: Evidence for Widespread Conservation within Monocot and Eudicot Lineages.

The oil palm (Elaeis guineensis), a monocotyledonous species in the family Arecaceae, has an extraordinarily oil rich fleshy mesocarp, and presents an original model to examine the ripening processes and regulation in this particular monocot fruit. Histochemical analysis and cell parameter measurements revealed cell wall and middle lamella expansion and degradation during ripening and in response to ethylene. Cell wall related transcript profiles suggest a transition from synthesis to degradation is under transcriptional control during ripening, in particular a switch from cellulose, hemicellulose, and pectin synthesis to hydrolysis and degradation. The data provide evidence for the transcriptional activation of expansin, polygalacturonase, mannosidase, beta-galactosidase, and xyloglucan endotransglucosylase/hydrolase proteins in the ripening oil palm mesocarp, suggesting widespread conservation of these activities during ripening for monocotyledonous and eudicotyledonous fruit types. Profiling of the most abundant oil palm polygalacturonase (EgPG4) and 1-aminocyclopropane-1-carboxylic acid oxidase (ACO) transcripts during development and in response to ethylene demonstrated both are sensitive markers of ethylene production and inducible gene expression during mesocarp ripening, and provide evidence for a conserved regulatory module between ethylene and cell wall pectin degradation. A comprehensive analysis of NAC transcription factors confirmed at least 10 transcripts from diverse NAC domain clades are expressed in the mesocarp during ripening, four of which are induced by ethylene treatment, with the two most inducible (EgNAC6 and EgNAC7) phylogenetically similar to the tomato NAC-NOR master-ripening regulator. Overall, the results provide evidence that despite the phylogenetic distance of the oil palm within the family Arecaceae from the most extensively studied monocot banana fruit, it appears ripening of divergent monocot and eudicot fruit lineages are regulated by evolutionarily conserved molecular physiological processes.

Transcriptomic and histological responses of African rice (Oryza glaberrima) to Meloidogyne graminicola provide new insights into root-knot nematode resistance in monocots.

Background and Aims: The root-knot nematode Meloidogyne graminicola is responsible for production losses in rice ( Oryza sativa ) in Asia and Latin America. The accession TOG5681 of African rice, O. glaberrima , presents improved resistance to several biotic and abiotic factors, including nematodes. The aim of this study was to assess the cytological and molecular mechanisms underlying nematode resistance in this accession. Methods: Penetration and development in M. graminicola in TOG5681 and the susceptible O. sativa genotype 'Nipponbare' were compared by microscopic observation of infected roots and histological analysis of galls. In parallel, host molecular responses to M. graminicola were assessed by root transcriptome profiling at 2, 4 and 8 d post-infection (dpi). Specific treatments with hormone inhibitors were conducted in TOG5681 to assess the impact of the jasmonic acid and salicylic acid pathways on nematode penetration and reproduction. Key Results: Penetration and development of M. graminicola juveniles were reduced in the resistant TOG5681 in comparison with the susceptible accession, with degeneration of giant cells observed in the resistant genotype from 15 dpi onwards. Transcriptome changes were observed as early as 2 dpi, with genes predicted to be involved in defence responses, phenylpropanoid and hormone pathways strongly induced in TOG5681, in contrast to 'Nipponbare'. No specific hormonal pathway could be identified as the major determinant of resistance in the rice-nematode incompatible interaction. Candidate genes proposed as involved in resistance to M. graminicola in TOG5681 were identified based on their expression pattern and quantitative trait locus (QTL) position, including chalcone synthase, isoflavone reductase, phenylalanine ammonia lyase, WRKY62 transcription factor, thionin, stripe rust resistance protein, thaumatins and ATPase3. Conclusions: This study provides a novel set of candidate genes for O. glaberrima resistance to nematodes and highlights the rice- M. graminicola pathosystem as a model to study plant-nematode incompatible interactions.

Cellular and Pectin Dynamics during Abscission Zone Development and Ripe Fruit Abscission of the Monocot Oil Palm.

The oil palm (Elaeis guineensis Jacq.) fruit primary abscission zone (AZ) is a multi-cell layered boundary region between the pedicel (P) and mesocarp (M) tissues. To examine the cellular processes that occur during the development and function of the AZ cell layers, we employed multiple histological and immunohistochemical methods combined with confocal, electron and Fourier-transform infrared (FT-IR) microspectroscopy approaches. During early fruit development and differentiation of the AZ, the orientation of cell divisions in the AZ was periclinal compared with anticlinal divisions in the P and M. AZ cell wall width increased earlier during development suggesting cell wall assembly occurred more rapidly in the AZ than the adjacent P and M tissues. The developing fruit AZ contain numerous intra-AZ cell layer plasmodesmata (PD), but very few inter-AZ cell layer PD. In the AZ of ripening fruit, PD were less frequent, wider, and mainly intra-AZ cell layer localized. Furthermore, DAPI staining revealed nuclei are located adjacent to PD and are remarkably aligned within AZ layer cells, and remain aligned and intact after cell separation. The polarized accumulation of ribosomes, rough endoplasmic reticulum, mitochondria, and vesicles suggested active secretion at the tip of AZ cells occurred during development which may contribute to the striated cell wall patterns in the AZ cell layers. AZ cells accumulated intracellular pectin during development, which appear to be released and/or degraded during cell separation. The signal for the JIM5 epitope, that recognizes low methylesterified and un-methylesterified homogalacturonan (HG), increased in the AZ layer cell walls prior to separation and dramatically increased on the separated AZ cell surfaces. Finally, FT-IR microspectroscopy analysis indicated a decrease in methylesterified HG occurred in AZ cell walls during separation, which may partially explain an increase in the JIM5 epitope signal. The results obtained through a multi-imaging approach allow an integrated view of the dynamic developmental processes that occur in a multi-layered boundary AZ and provide evidence for distinct regulatory mechanisms that underlie oil palm fruit AZ development and function.

The embryo and the endosperm contribute equally to argan seed oil yield but confer distinct lipid features to argan oil.

In the perspective of studying lipid biosynthesis in the argan seed, the anatomy, ploidy level and lipid composition of mature seed tissues were investigated using an experimental design including two locations in Algeria and four years of study. Using flow cytometry, we determined that mature argan seeds consist of two well-developed tissues, the embryo and the endosperm. The lipid content of the embryo was higher than that of the endosperm, but the dry weight of the endosperm was higher. Consequently, both tissues contribute equally to seed oil yield. Considerable differences in fatty acid composition were observed between the two tissues. In particular, the endosperm 18:2 percentage was twofold higher than that of the embryo. The tocopherol content of the endosperm was also markedly higher than that of the embryo. In contrast, the endosperm and the embryo had similar sterol and triterpene alcohol contents and compositions.

Floral structure and development in the monoecious palm Gaussia attenuata (Arecaceae; Arecoideae).

BACKGROUND AND AIMS: Sexual dimorphism, at both the flower and plant level, is widespread in the palm family (Arecaceae), in contrast to the situation in angiosperms as a whole. The tribe Chamaedoreeae is of special interest for studies of the evolution of sexual expression since dioecy appears to have evolved independently twice in this group from a monoecious ancestor. In order to understand the underlying evolutionary pathways, it is important to obtain detailed information on flower structure and development in each of the main clades. METHODS: Dissection and light and scanning electron microscopy were performed on developing flowers of Gaussia attenuata, a neotropical species belonging to one of the three monoecious genera of the tribe. KEY RESULTS: Like species of the other monoecious genera of the Chamaedoreeae (namely Hyophorbe and Synechanthus), G. attenuata produces a bisexual flower cluster known as an acervulus, consisting of a row of male flowers with a basal female flower. Whereas the sterile androecium of female flowers terminated its development at an early stage of floral ontogeny, the pistillode of male flowers was large in size but with no recognizable ovule, developing for a longer period of time. Conspicuous nectary differentiation in the pistillode suggested a possible role in pollinator attraction. CONCLUSIONS: Gaussia attenuata displays a number of floral characters that are likely to be ancestral to the tribe, notably the acervulus flower cluster, which is conserved in the other monoecious genera and also (albeit in a unisexual male form) in the dioecious genera (Wendlandiella and a few species of Chamaedorea). Comparison with earlier data from other genera suggests that large nectariferous pistillodes and early arrest in staminode development might also be regarded as ancestral characters in this tribe.

DNA methylation and expression of the EgDEF1 gene and neighboring retrotransposons in mantled somaclonal variants of oil palm.

The mantled floral phenotype of oil palm (Elaeis guineensis) affects somatic embryogenesis-derived individuals and is morphologically similar to mutants defective in the B-class MADS-box genes. This somaclonal variation has been previously demonstrated to be associated to a significant deficit in genome-wide DNA methylation. In order to elucidate the possible role of DNA methylation in the transcriptional regulation of EgDEF1, the APETALA3 ortholog of oil palm, we studied this epigenetic mark within the gene in parallel with transcript accumulation in both normal and mantled developing inflorescences. We also examined the methylation and expression of two neighboring retrotransposons that might interfere with EgDEF1 regulation. We show that the EgDEF1 gene is essentially unmethylated and that its methylation pattern does not change with the floral phenotype whereas expression is dramatically different, ruling out a direct implication of DNA methylation in the regulation of this gene. Also, we find that both the gypsy element inserted within an intron of the EgDEF1 gene and the copia element located upstream from the promoter are heavily methylated and show little or no expression. Interestingly, we identify a shorter, alternative transcript produced by EgDEF1 and characterize its accumulation with respect to its full-length counterpart. We demonstrate that, depending on the floral phenotype, the respective proportions of these two transcripts change differently during inflorescence development. We discuss the possible phenotypical consequences of this alternative splicing and the new questions it raises in the search for the molecular mechanisms underlying the mantled phenotype in the oil palm.

Effect of the successive steps of a cryopreservation protocol on the structural integrity of Rubia akane Nakai hairy roots.

In this work, we studied the impact of the successive steps of the droplet-vitrification protocol technique employed for cryopreservation of Rubia akane hairy roots on the features of cortical, pericycle and endoderm cells of apical and central root segments, using histology techniques and combining qualitative and quantitative observations. In apical segments, plasmolysis (22-71 %, depending on cell type) was observed only after the loading treatment and did not increase after the following steps of the protocol. By contrast, in central segments, plasmolysis (39-45 %) was already observed after the sucrose pretreatment; it increased to 54-68 %, depending on cell type, after the loading treatment, but no further changes were noted after treatment with the vitrification solution. After liquid nitrogen exposure and unloading treatment, deplasmolysis was more rapid in apical segments, with cortical and pericycle cells having retrieved their original features. In central segments, only cortical cells had retrieved their original features and endoderm and pericycle cells were still highly plasmolysed. Nuclei were more strongly impacted by the cryopreservation protocol in central segments, where they displayed a highly condensed nucleoplasm from the loading treatment onwards and had not retrieved their original aspect after the unloading treatment. By contrast, nuclei had a much less condensed nucleoplasm in cells of apical segments, and they had retrieved their original aspect after the unloading treatment.

Histocytological analysis of yam (Dioscorea alata) shoot tips cryopreserved by encapsulation-dehydration.

In this work, we performed qualitative and quantitative observations of the cytological changes occurring in cells of yam (Dioscorea alata) in vitro shoot tips cryopreserved using the encapsulation-dehydration (E-D) technique. Shoot tip osmoprotection for 24 h in 1.25 M sucrose medium induced drastic changes in cellular cytological features, including high plasmolysis in all three cellular areas studied, the external cell layer (L1), one to three (L1-3) and seven to nine (L7-9) cell layers from the surface of the meristematic dome, pyknotic nuclei in meristematic area cells and disappearance of nucleoli. Nucleus size decreased significantly in all cellular areas studied. Nucleocytoplasmic ratio decreased significantly in L1-3 and L7-9 cells. Nuclear protein content increased, particularly in L1 and L1-3 cells. After physical dehydration, plasma membrane of numerous basal part cells was broken and intracellular soluble protein leakage was observed. Nucleus area and nucleocytoplasmic ratio decreased significantly in L7-9 cells. One week after cryopreservation, shoot tips showed regrowth and living cells had recovered their original morphology. In all cellular areas studied, nuclei had retrieved their original staining and nucleoli were visible. Original nucleus area values were recovered in L1-3 and L1 cells. The nucleocytoplasmic ratio retrieved its initial value in L1 cells but remained at levels observed after osmoprotection for L1-3 and L7-9 cells. The nuclear protein content had retrieved its original level. This investigation provided new insights in changes occurring in D. alata apices throughout an E-D protocol.

Temporal and spatial expression of polygalacturonase gene family members reveals divergent regulation during fleshy fruit ripening and abscission in the monocot species oil palm.

BACKGROUND: Cell separation that occurs during fleshy fruit abscission and dry fruit dehiscence facilitates seed dispersal, the final stage of plant reproductive development. While our understanding of the evolutionary context of cell separation is limited mainly to the eudicot model systems tomato and Arabidopsis, less is known about the mechanisms underlying fruit abscission in crop species, monocots in particular. The polygalacturonase (PG) multigene family encodes enzymes involved in the depolymerisation of pectin homogalacturonan within the primary cell wall and middle lamella. PG activity is commonly found in the separation layers during organ abscission and dehiscence, however, little is known about how this gene family has diverged since the separation of monocot and eudicots and the consequence of this divergence on the abscission process. RESULTS: The objective of the current study was to identify PGs responsible for the high activity previously observed in the abscission zone (AZ) during fruit shedding of the tropical monocot oil palm, and to analyze PG gene expression during oil palm fruit ripening and abscission. We identified 14 transcripts that encode PGs, all of which are expressed in the base of the oil palm fruit. The accumulation of five PG transcripts increase, four decrease and five do not change during ethylene treatments that induce cell separation. One PG transcript (EgPG4) is the most highly induced in the fruit base, with a 700-5000 fold increase during the ethylene treatment. In situ hybridization experiments indicate that the EgPG4 transcript increases preferentially in the AZ cell layers in the base of the fruit in response to ethylene prior to cell separation. CONCLUSIONS: The expression pattern of EgPG4 is consistent with the temporal and spatial requirements for cell separation to occur during oil palm fruit shedding. The sequence diversity of PGs and the complexity of their expression in the oil palm fruit tissues contrast with data from tomato, suggesting functional divergence underlying the ripening and abscission processes has occurred between these two fruit species. Furthermore, phylogenetic analysis of EgPG4 with PGs from other species suggests some conservation, but also diversification has occurred between monocots and eudicots, in particular between dry and fleshy fruit species.

Environmental regulation of sex determination in oil palm: current knowledge and insights from other species.

BACKGROUND: The African oil palm (Elaeis guineensis) is a monoecious species of the palm subfamily Arecoideae. It may be qualified as 'temporally dioecious' in that it produces functionally unisexual male and female inflorescences in an alternating cycle on the same plant, resulting in an allogamous mode of reproduction. The 'sex ratio' of an oil palm stand is influenced by both genetic and environmental factors. In particular, the enhancement of male inflorescence production in response to water stress has been well documented. SCOPE: This paper presents a review of our current understanding of the sex determination process in oil palm and discusses possible insights that can be gained from other species. Although some informative phenological studies have been carried out, nothing is as yet known about the genetic basis of sex determination in oil palm, nor the mechanisms by which this process is regulated. Nevertheless new genomics-based techniques, when combined with field studies and biochemical and molecular cytological-based approaches, should provide a new understanding of the complex processes governing oil palm sex determination in the foreseeable future. Current hypotheses and strategies for future research are discussed.

The shoot apical meristem of oil palm (Elaeis guineensis; Arecaceae): developmental progression and dynamics.

BACKGROUND AND AIMS: Oil palm, an unbranched perennial monocotyledon, possesses a single shoot apical meristem (SAM), which is responsible for the initiation of the entire above-ground structure of the plant. To compare the palm SAM structure with those of other monocots and to study variations in its structure throughout the life of the plant, its organization was characterized from the embryonic stage to that of the reproductive plant. METHODS: SAM structure was studied by a combination of stained histological sections, light and confocal microscopy, and serial section-based three-dimensional reconstructions. KEY RESULTS: The oil palm SAM is characterized by two developmental phases: a juvenile phase with a single tunica-corpus structure displaying a gradual increase in size; and a mature phase characterized by a stable size, a modified shape and an established histological zonation pattern. In mature plants, fluctuations in SAM shape and volume occur, mainly as a consequence of changes in the central zone, possibly in relation to leaf initiation. CONCLUSIONS: Development of the oil palm SAM is characterized by a juvenile to mature phase transition accompanied by establishment of a zonal pattern and modified shape. SAM zonation is dynamic during the plastochron period and displays distinct features compared with other monocots.

Divergent expression patterns of miR164 and CUP-SHAPED COTYLEDON genes in palms and other monocots: implication for the evolution of meristem function in angiosperms.

In order to understand how the morphology of plant species has diversified over time, it is necessary to decipher how the underlying developmental programs have evolved. The regulatory network controlling shoot meristem activity is likely to have played an important role in morphological diversification and useful insights can be gained by comparing monocots and eudicots. These two distinct monophyletic groups of angiosperms diverged 130 Ma and are characterized by important differences in their morphology. Several studies of eudicot species have revealed a conserved role for NAM and CUC3 genes in meristem functioning and pattern formation through the definition of morphogenetic boundaries during development. In this study, we show that NAM- and CUC3-related genes are conserved in palms and grasses, their diversification having predated the radiation of monocots and eudicots. Moreover, the NAM-miR164 posttranscriptional regulatory module is also conserved in palm species. However, in contrast to the CUC3-related genes, which share a similar expression pattern between the two angiosperm groups, the expression domain of the NAM-miR164 module differs between monocot and eudicot species. In our studies of spatial expression patterns, we compared existing eudicot data with novel results from our work using two palm species (date palm and oil palm) and two members of the Poaceae (rice and millet). In addition to contrasting results obtained at the gene expression level, major differences were also observed between eudicot and monocot NAM-related genes in the occurrence of putative cis-regulatory elements in their promoter sequences. Overall, our results suggest that although NAM- and CUC3-related proteins are functionally equivalent between monocots and eudicots, evolutionary radiation has resulted in heterotopy through alterations in the expression domain of the NAM-miR164 regulatory module.

Cell cycle arrest characterizes the transition from a bisexual floral bud to a unisexual flower in Phoenix dactylifera.

BACKGROUND AND AIMS: Phoenix dactylifera (date palm) is a dioecious species displaying strong dimorphism between pistillate and staminate flowers. The mechanisms involved in the development of unisexual flowers are as yet unknown. METHODS: This paper describes the results of inflorescence and flower development studies using different histological and molecular cytological approaches. Nuclear integrity and cell division patterns in reproductive organs were investigated through DAPI staining and in situ hybridization using a histone H4 gene probe. KEY RESULTS: The earliest sex-related difference in flower buds is observed at an otherwise 'bisexual' stage, at which the number of cells in the gynoecium of pistillate flowers is higher than in their staminate counterparts. In the pistillate flower, staminodes (sterile stamens) display precocious arrest of development followed by cell differentiation. In the staminate flower, pistillodes (sterile gynoecium) undergo some degree of differentiation and their development ceases shortly after the ovule has been initiated. Staminode and pistillode cells exhibit nuclear integrity although they did not show any accumulation of histone H4 gene transcripts. CONCLUSIONS: These results strongly suggest that the developmental arrest of sterile sex organs and the subsequent unisexuality of date palm flowers result from a cessation of cell division and precocious cell differentiation rather than from cell death.