Michel Noirot

Unraveling the origin of Coffea arabica ‘Bourbon pointu’ from La Réunion: a historical and scientific perspective

Coffee is one of the main products on the international markets, in association with oil, corn, sugar, and paper pulp. The history of coffee-tree cultivation is incompletely documented, both regarding its domestication in Africa, and its assisted dispersal throughout the world. This review focuses on the coffee mutant, Coffea arabica ‘Laurina’ (Chevalier A in Encyclopedie Biologique. Vol 28, 1947), also named ‘Bourbon pointu’. This plant is generally acknowledged to have been selected in the island of ‘Bourbon’ (La Réunion) from a field at the beginning of the 19th century. Compared with the common ‘Bourbon’ variety, ‘Bourbon pointu’ trees are dwarf, with a characteristic Christmas-tree shape and the beans have an excellent cup quality. Although cited many times in literature, the origin of this variety is ambiguous and is largely discussed even today with increasing confusion, particularly in books and local newsletters. This article provides a thorough historical and bibliographical review of coffee cultivation in La Réunion, which leads to an understanding of the bottleneck responsible for the low genetic diversity of the ‘Bourbon’-type modern varieties. Complemented by a review of the scientific studies conducted on this subject, confirmation of the veracity of the various historical accounts becomes possible, and appropriate conclusions on the origin of the ‘Bourbon pointu’ are derived. Although historical texts provide important information and represent priceless resources that give direction to scientific research, it is clear that this same research makes it possible, in turn, to clarify and to interpret historical texts.

Expression profiles of key phenylpropanoid genes during Vanilla planifolia pod development reveal a positive correlation between PAL gene expression and vanillin biosynthesis

In Vanilla planifolia pods, development of flavor precursors is dependent on the phenylpropanoid pathway. The distinctive vanilla aroma is produced by numerous phenolic compounds of which vanillin is the most important. Because of the economic importance of vanilla, vanillin biosynthetic pathways have been extensively studied but agreement has not yet been reached on the processes leading to its accumulation. In order to explore the transcriptional control exerted on these pathways, five key phenylpropanoid genes expressed during pod development were identified and their mRNA accumulation profiles were evaluated during pod development and maturation using quantitative real-time PCR. As a prerequisite for expression analysis using qRT-PCR, five potential reference genes were tested, and two genes encoding Actin and EF1 were shown to be the most stable reference genes for accurate normalization during pod development. For the first time, genes encoding a phenylalanine ammonia-lyase (VpPAL1) and a cinnamate 4-hydroxylase (VpC4H1) were identified in vanilla pods and studied during maturation. Among phenylpropanoid genes, differential regulation was observed from 3 to 8 months after pollination. VpPAL1 was gradually up-regulated, reaching the maximum expression level at maturity. In contrast, genes encoding 4HBS, C4H, OMT2 and OMT3 did not show significant increase in expression levels after the fourth month post-pollination. Expression profiling of these key phenylpropanoid genes is also discussed in light of accumulation patterns for key phenolic compounds. Interestingly, VpPAL1 gene expression was shown to be positively correlated to maturation and vanillin accumulation.

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.

Spectral analysis combined with advanced linear unmixing allows for histolocalization of phenolics in leaves of coffee trees

An imaging method using spectral analysis combined with advanced linear unmixing was used to allow histolocalization of natural autofluorescent compounds such as hydroxycinnamic acid (chlorogenic acid) and xanthone (mangiferin) in living cells and tissues (mature coffee leaves). The tested method included three complementary steps: 1/ visualization of natural autofluorescence and spectrum acquisition with a multiphoton microscope; 2/ identification of some compounds using previous information on the chemical composition of the tissue, obtained from litterature; and 3/ localization of candidate compounds by spectral imaging. The second part of the study consisted of describing the histochemical structure of leaves during their development. This revealed very fast histochemical differentiation of leaves during the first week after their emergence. Lastly, young leaves of Coffea pseudozanguebariae (PSE), C. eugenioides (EUG), C. arabica (ARA) and C. canephora (CAN) were compared. This confirmed the presence of xanthone in PSE and EUG, but especially its precise tissue localization. This also highlighted the paternal CAN origin of the leaf structure in the allotetraploid species ARA. The limits and advantages of the method without staining are discussed relative to classical epifluorescence microscopy under UV light. This non-invasive optical technique does not require pretreatment and is an effective experimental tool to differentiate multiple naturally-occuring fluorochores in living tissues.

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.

Biochemical characterization of embryogenic calli of Vanilla planifolia in response to two years of thidiazuron treatment.

Vanilla planifolia embryogenic calli were cultured for two years on a medium containing thidiazuron (TDZ). Due to the presence of TDZ, these calli were under permanent chemical treatment and the differentiation of adventitious shoots from protocorm-like-bodies (PLBs) was blocked. When embryogenic calli were transferred onto a medium without TDZ, shoot organogenesis and plantlet regeneration occurred. To gain better knowledge about the biochemical and molecular processes involved in the morphoregulatory role of TDZ, hormonal and metabolomic analyses were performed. Our results indicate that in the presence of TDZ, embryogenic calli contained a high amount of abscisic acid (ABA) essentially metabolized into abscisic acid glucosyl ester (ABAGE) and phaseic acid (PA), which was the most abundant. When transferred onto a medium without TDZ, shoot regeneration and development take place in four stages that include: embryogenic calli growth, differentiation of PLBs from meristmatic cells zones (MCZ), shoot organogenesis from PLBs and the elongation of well-formed shoots. From a hormonal perspective, the significant reduction in ABA metabolism and its readjustment in the ABAGE pathway triggered PLBs formation. However, this first morphogenesis was stimulated by a strong reduction in IAA metabolism. The organogenesis of PLBs into shoots is associated with an increase in ABA catabolism and a gradual shift in cellular metabolism towards shoot differentiation. Thus, the initiation of the elongation process in shoots is correlated with an alteration in metabolite composition, including an increase in energy reserves (sucrose/starch) and a rapid decrease in alanine content. Our data highlighted the relationship between endogenous hormone signalling, carbohydrate metabolism and shoot organogenesis in Orchid plants.

Morphological and histological impacts of the laurina mutation on fructification and seed characteristics in Coffea arabica L.

Key messageThe comparison between the cultivar Bourbon and its mutant, the Bourbon pointu, ofCoffea arabicaled to five novel findings on fruit development and three main impacts of the mutation.AbstractCoffea arabica ‘Laurina’ (Bourbon pointu) is a natural mutant of Coffea arabica ‘Bourbon’. Relative to the ‘Bourbon’ cultivar, it is characterized by internode dwarfism, a Christmas tree shape, and lower caffeine content. The effects of the laurina mutation on fructification over time, the fruit structure and seed characteristics were studied here. Fruits of ‘Bourbon’ and ‘Bourbon pointu’ were monitored. The trees were grown in the same plot and flowered on the same day. Harvesting was done every 2xa0weeks from the 6th to the 26th week after flowering. Histological observations were carried out using multiphoton and conventional microscopes. The measurements concerned the fruit, parchment and seed. Five novel findings on fructification development were obtained: (1) a sigmoid model and non-linear regression efficiently described the phenomenon; (2) a precise relationship was defined between the qualitative stages of fructification and quantitative observations, thus revealing key weeks in this process; (3) the parchment had a mesocarpic origin; (4) a meristematic zone was present close to the parchment; and (5) an endocarp with three cell layers was visible in young fruits. Three effects of the laurina mutation were highlighted: (1) fruit growth ended 1xa0week earlier in ‘Bourbon’, but without difference in fruit length. In contrast, fruits were wider on average in ‘Bourbon’; (2) the parchment of narrow seeds in ‘Bourbon pointu’ was thicker than in other ‘Bourbon pointu’ and ‘Bourbon’ seeds; and (3) the narrow seed frequency in ‘Bourbon pointu’ depended on environmental conditions.

Variation in intron length in caffeic acid O-methyltransferase (COMT) in Vanilla species (Orchidaceae)

Variation in intron length in caffeic acid O-methyltransferase (COMT) in Vanilla was studied and demonstrated that COMT genes in Vanilla are organized with four exons and three introns. At least two to four different versions (either allelic or paralogous) of the COMT multigenic family in the genus Vanilla (in terms of intron sizes) were detected. The three introns were differentially variable, with intron-1 being the most length-polymorphic. Patterns of variations were in accordance with known phylogenetic relationships in the genus obtained with neutral markers. In particular, the genus displayed a strong Old World versus New World differentiation with American fragrant species being characterized by a specific 99bp intron-1 size-variant and a unique 226bp intron-3 variant. Conversely, leafless species of the genus displayed unexpected variations in intron lengths. Due to their role in primary (lignin) and secondary (phenolics, e.g., vanillin, alkaloids) metabolisms, COMT genes might not be neutral markers, and represent candidate functional markers for resistance, aromatic or medicinal properties of Vanilla species. Investigating the orthologous/paralogous status of the different genes revealed (in terms of intron size) will allow the evolution of the COMT genes to be studied.

Flowering synchronisation between male and female West African cultivated yams (Dioscorea cayenensis-rotundata complex)

It has long been admitted that male and female cultivars of Dioscorea cayenensis-rotundata complex do not flower at the same period and consequently natural outcrossing rarely occurs. Flowering in thirteen (7 males and 6 females) traditional cultivars were studied. Four main results were obtained: i) the first bud emergence occurred 25 days earlier in males; ii) the male and female first flower opening overlapped for 10 days (85 to 95 days after sprouting); iii) for both sexes, highest frequency of flower opening occurred at mid-day; and iv) the female cultivar 1800 could flower for 8 consecutive days, with a peak between day 4 and day 8. These results are discussed in relation to intercrossing success.

Reproductive isolation, gene flow and speciation in the former Coffea subgenus: a review

Key messageThe formerCoffeasubgenus is a species complex showing qualitative gene flow and reproductive barriers between species. Such qualitative gene flow allowed its evolution over time, particularly during the successive forest expansion-regression cycles in relation with glaciation periods.AbstractThe present paper reviews the main botanical, geographical and genetic characteristics of the Coffea genus and then focuses on the former Coffea subgenus. Its broad distribution in Africa, Madagascar and Mascarene Islands is related to the high diversity of ecological situations. The importance of sympatry and parapatry cases and their role on gene flow possibilities between species is then underlined in the paper. Such gene flow is nevertheless partially limited by reproductive barriers: flowering date, frequency of hybrid F1 emergence, as well as the vigor and fertility of such hybrids. When hybridization occurs, distortion of segregation and disruptive selection would allow qualitative flow of non-adaptative genes, thus limiting the effect of genetic drift in small populations. The last part of the paper defines the notion of metaspecies in the case of the former Coffea by extension of the concept of metapopulation to species. The evolution over time of a metaspecies is finally discussed in relation with sympatry situations, gene flow possibilities and forest fragmentation.