Jacques Escoute

Molecular genetics of rice root development

Plant roots have a large range of functions, including acquisition of water and nutrients, as well as structural support. Dissecting the genetic and molecular mechanisms controlling rice root development is critical for the development of new rice ideotypes that are better adapted to adverse conditions and for the production of sustainably achieved rice yield potential. Most knowledge regarding the gene networks involved in root development has been accumulated in the model dicotyledon plant species Arabidopsis thaliana. Rice, the model monocotyledon species, presents several singularities compared to A. thaliana, including a root architecture characterized by a fibrous root system comprising five types of embryonic and postembryonic roots. The anatomy and morphology of the rice root system, which is typical for a cereal, differs from that of A. thaliana, for instance, by the presence of a lysigenous cortex and additional cell layers compared to the dicotyledon model. Moreover, the structure and functions of the root apical meristem (RAM) of rice are distinct from those of A. thaliana. Recently, several rice root mutants have been identified via forward or reverse genetics, and these will aid in forming hypothesis to characterize either the divergence or conservation of genetic pathways relative to A. thaliana. Furthermore, these mutants will help to identify key genes in rice roots that may be missing in A. thaliana. This review summarizes both classical and recent data concerning the molecular genetics of rice root development, including root anatomy and morphology, RAM structure, RAM patterning, and root mutants.

Reproductive developmental complexity in the African oil palm (Elaeis guineensis, Arecaceae)

Species of the palm family (Arecaceae) are remarkably diverse in their inflorescence and floral morphologies, which make them a particularly interesting group for studies of reproductive development and its evolution. Using light and scanning electron microscopy, we describe inflorescence and flower development in the African oil palm Elaeis guineensis from the initiation of the inflorescence meristem to flower maturity. In mature palms, the inflorescence develops over 2-3 years and is characterized by individual stages within which differentiation may be either relatively slow, as in the case of early inflorescence meristem development, or rapid, as in the case of flower organogenesis. The female inflorescence bears floral triads composed of single pistillate flowers flanked by two abortive staminate flowers, whereas the male inflorescence contains single functional staminate flowers. This suggests a possible evolutionary movement from an ancestral hermaphrodite inflorescence form containing fully functional floral triads to the situation of temporal dioecy observed at present. Wild type flowers are compared to those bearing an epigenetic homeotic abnormality, known as mantled, involving an alteration of the identity of the organs in the fertile and sterile androecium.

In vitro somatic embryogenesis in two major rattan species: Calamus merrillii and Calamus subinermis

SummaryOccurrence of somatic embryogenesis in in vitro cultures of Calamus merrillii and Calamus subinermis, two major largecaned rattan species, was scientifically demonstrated for the first time. Tissue responsiveness varied markedly according to the species and the type of primary explants used when initiated on 10.4–31.2 μM picloram-enriched Murashige and Skoog callus induction media. In C. merrillii, within 6 wk after inoculation, 84% of the leaf and 90% of the zygotic embryo explants produced friable embryogenic calluses, by contrast with those formed by 74% of the root explants. In C. subinermis, callogenesis was observed only 6 mo. after inoculation in 68% of root and 48% of zygotic explants. Leaf explants did not respond at all. Only root-derived calluses developed into nodular embryogenic structures. Irrespective of these initial differences, the further steps of the somatic embryogenesis developmental pattern was similar for both species. Histological analyses established that callus formation took place in the perivascular zones, and could give rise to embryogenic isolated cells from which the proembryos were derived. Reducing the picloram concentration stimulated the maturation process resulting ultimately in the germination of somatic embryos that exhibited bipolar development, despite an apparent lack of starch and protein reserves. The somatic embryo-derived plantlets of C. merrillii, overall more prone to somatic embryogenesis than C. subinermis in the given conditions, were successfully acclimatized to outdoor conditions.

Homoeologous chromosome pairing between the A and B genomes of Musa spp. revealed by genomic in situ hybridization.

BACKGROUND AND AIMS Most cooking banana and several desert bananas are interspecific triploid hybrids between Musa acuminata (A genome) and Musa balbisiana (B genome). In addition, M. balbisiana has agronomical characteristics such as resistance to biotic and abiotic stresses that could be useful to improve monospecific acuminata cultivars. To develop efficient breeding strategies for improving Musa cultivars, it is therefore important to understand the possibility of chromosome exchange between these two species. METHODS A protocol was developed to prepare chromosome at meiosis metaphase I suitable for genomic in situ hybridization. A series of technical challenges were encountered, the main ones being the hardness of the cell wall and the density of the microsporocytes cytoplasm, which hampers accessibility of the probes to the chromosomes. Key parameters in solving these problems were addition of macerozyme in the enzyme mix, the duration of digestion and temperature during the spreading phase. RESULTS AND CONCLUSIONS This method was applied to analyse chromosome pairing in metaphase from triploid interspecific cultivars, and it was clearly demonstrated that interspecific recombinations between M. acuminata and M. balbisiana chromosomes do occur and may be frequent in triploid hybrids. These results provide new insight into Musa cultivar evolution and have important implications for breeding.

Reactive Oxygen Species and Cellular Interactions Between Mycosphaerella fijiensis and Banana

Globally, the banana plant (Musa spp) is the fourth most important crop after rice, wheat and corn (based on production in tons). It is cultivated in more than 100 tropical and subtropical countries, mainly by small producers and is a fundamental food source for millions of people. Black leaf streak disease (BLSD), caused by Mycosphaerella fijiensis Morelet (sexual phase) or Paracercospora fijiensis (Morelet) Deighton (asexual phase), is the main disease affecting the world’s banana culture. This disease has a wide geographical distribution accounting for losses exceeding 50% of global banana production. We conducted a comparative histocytological study on the kinetics of the infection process using three banana genotypes with phenotypes that differ in resistance to BLSD: Grand Naine (Susceptible), Pisang Madu (Partially Resistant) and Calcutta 4 (Resistant). Experiments were conducted under controlled conditions with the objective of characterizing the cellular interaction processes between M. fijiensis and Musa acuminata. Conidia germination occurred 24 hours after inoculation. Germination rates were high (97%) and there were no significant differences between the three genotypes (P > 0.147). The Peroxidase enzyme and H2O2 were associated with a hypersensitivity-like reaction in the resistant genotype Calcutta 4, indicating a possible role of the enzyme or its product as defense mechanisms against M. fijiensis in banana plants.

Laurina mutation affected Coffea arabica tree size and shape mainly through internode dwarfism

The two varieties—Bourbon (B) and its natural mutant Bourbon pointu (BP)—of Arabica coffee (Coffea arabica L.) differ by an epigenetic, monolocus, and recessive laurina mutation that results in pleiotropic effects, such as tree dwarfism and tree-shape modification. The objective of the study was to search for the origin of the differences in size and shape of the tree both at the macroscopic (length and number of internodes, branching angle) and at the microscopic levels (size and number of cells within the internode pith). At the macroscopic level, the laurina mutation acted only by decreasing the internode size. Neither the angle of branching nor the number of internodes was influenced by the mutation. At the microscopic level, the mutation lowered mainly the number of cells present along the longitudinal axis of the internode, and, at a lesser extent, the cell height. Especially, the internode size decreasing explained both the tree dwarfism and the tree-shape modification. In fact, the laurina mutation strengthened the dwarfism of plagiotropic internodes when compared to that of the orthotropic ones, and such an impact was mainly due to a strong cell number decrease. To summarize, two major pleiotropic effects of the laurina mutation can be explained only by a modification of the trade-off between meresis and auxesis during the internode growth. This opens new perspectives for the characterization of the other effects at the hormonal level, and then, for the identification of the gene at the molecular level.

Uvitex2B: a rapid and efficient stain for detection of arbuscular mycorrhizal fungi within plant roots

The study of arbuscular mycorrhiza often requires the staining of fungal structures using specific dyes. Fluorescent dyes such as acid fuchsin and wheat germ agglutinin conjugates give excellent results, but these compounds are either hazardous or very expensive. Here, we show that a safer and inexpensive dye, Uvitex2B, can be efficiently used to stain intraradical fungal structures formed by the arbuscular mycorrhizal fungus Glomus intraradices in three plant species: carrot, Casuarina equisetifolia, and Medicago truncatula. The intensity and stability of Uvitex2B allow the acquisition of high-quality images using not only confocal laser scanning microscopy but also epifluorescence microscopy coupled with image deconvolution. Furthermore, we demonstrate that Uvitex2B and β-glucuronidase staining are compatible and can thus be used to reveal arbuscular mycorrhizal structures in the context of promoter activation analysis.

Early effects of the mutation laurina on the functioning and size of the shoot apex in coffee tree and analysis of the plastochron phases : Relationships with the dwarfism of leaves

The current article presents the investigations into the effect of the laurina mutation on the functioning and size of the shoot apical meristem (SAM) in Coffea arabica. This monolocus and Mendelian mutation is known to have pleiotropic effects on tree shape and dwarfism. A comparison between the wild type C. arabica var. Bourbon and its natural dwarf mutant C. arabica var. laurina, also called Bourbon pointu, was carried out leading to three main results: (1) the effects appeared immediately after the emergence of the buttress but did not affect the dome-shaped SAM (size and shape); (2) the effects were located at the peripheral zone and maintained subsequently within the leaf primordia; (3) the effects consisted of reduction in both the size of primordia and the height of incipient internode, consequently resulting in dwarfism of mature leaves and internodes. By contrast, the laurina mutation had no effect on the relationship between the phyllochron and the plastochron, the decussate and opposite phyllotaxis, and the relative timing of SAM functioning within the plastochron.

Chromosome number variations in micropropagated true-to-type and off-type banana plants (Musa AAA Grande Naine cv.)

SummaryThe objective of the present study on banana plants (Musa AAA Grande Naine cv.), obtained byin vitro shoot tip culture, was to determine whether modifications in chromosome number could account for the appearance of the off-types with mosaiclike leaf defects or dwarf stature, the most frequent off-types observed after micropropagation. Chromosome counts were conducted on shoot tip samples treated with 8-hydroxyquinoline, digested in pectinase and stained with Schiffs reagent. On average, 160 counts were made for each treatment. Four types of plant material were studied: phenotypically true-to-type plants, dwarf off types, mosaiclike off-types obtained by micropropagation, as well as true-to-type plants obtained by standard propagation techniques of suckers with no micropropagation history. Some cells from all four types of plant material were found to have an abnormal chromosome number (i.e., 2n = 3x = 33), characteristic of triploid banarias. The percentages of aneuploid cells were 14%, 22%, 35%, and 5%, respectively. Descending aneuploidy was noted in micropropagated plants derived from true-to-type and dwarf off-type suckers. The statistical analysis revealed that the two latter types of plant material had the same percentage of aneuploid cells. Thus, the dwarfism could not be correlated with a change in the chromosome number. Conversely, ascending aneuploidy was observed in the mosaiclike material, with 34 or 35 chromosomes in almost 28% of the cells. This percentage was significantly higher than in true-to-type plants and highlight the genetic origin of the mosaiclike variation.

Morphogenetic trends in the morphological, optical and biochemical features of phyllodes in Acacia mangium Willd (Mimosaceae).

Endogenous variations in the annual growth of trees suggest that similar trends would occur in phyllodes. In comparison to leaves, the characteristics of phyllodes are less well known, hence this study examines the effects of architectural position and age of tree on the phyllodes of Acacia mangium. Phyllodes were investigated on 1-, 2-, and 3-year-old trees from three axis positions within the crown. We focused on the morphological, optical and biochemical traits of the phyllodes. The increase in phyllode area and lamina thickness is more pronounced in the older trees. Leaf mass area (LMA), stomatal density, nitrogen and chlorophyll content increase with tree age. The values of these characteristics decrease from the main stem to the lower branches for the older trees. Phyllode light absorptance increased with tree age whereas reflectance was higher for the upper position compared to the lower position within the crown. Carotenoid content and chlorophyll a/b ratio were higher for the younger phyllodes of younger trees. Increasing tree size induced modifications in the phyllode characteristics which are influenced by both morphogenetic and light gradients within the crown. This study demonstrated pronounced changes in terms of morphological and functional indicators of photosynthetic capacity in relation to phyllode position within the crown and to tree age. These morphogenetic effects on the phyllode characteristics should be taken into account in studies on phenotypic plasticity.