Variations in cell wall monosaccharide composition during seed development in Coffea arabica L. Comparison between Coffea arabica var. Bourbon and Coffea arabica var. Laurina

Abstract

Key message

Cell wall polysaccharide composition changed over seed development. Differences between B and BP only concerned the fruit growth period.

Abstract

Coffea arabica var. Laurina, also known as ‘Bourbon Pointu’ (BP), is a natural mutant of Coffea arabica var. Bourbon (B). BP is known for its high organoleptic quality, which is today compared with the well-known ‘Blue Mountain’. Cell wall composition of seeds could explain the better cup quality of BP. To test this hypothesis, we investigated possible impacts of the laurina mutation on the cell wall composition of seeds over a time course, and more precisely on polysaccharides. The identification of cell wall polysaccharides (CWP) was deduced from permethylation analysis, whereas cell wall monosaccharide (CWM) composition was estimated using trifluoroacetic acid (TFA) and H2SO4. The observation of CWM over time allowed defining three phases. The φ 1 phase, from the 8th to 14th week after flowering (WAF), was characterized by the predominance of arabinogalactans, arabinoxylans and arabinans. The φ 2 phase, from the 14th to 20th WAF, was mainly characterized by the increased importance of highly branched galactomannans at the expense of arabinoxylans. Lastly, galactomannans constituted the main CWP present in the third phase (from the 20th to the 32nd WAF), associated with arabinogalactans and arabinans. Variations in CWP composition were connected to fruit and seed development. The end of the φ 1 phase coincides with the end of the fruit growth, i.e., with the end of the endosperm development. During the φ 2 phase, endosperm becomes milky and then hard due to the galactomannan deposition. Moreover, the φ 3 phase corresponded to the fruit maturation stage in which CWM composition did not change over time. Galactomannans were less substituted and constitute the main seed CWP. Lastly, the evidence of laurina mutation impact on cell wall polysaccharides of seeds was only observed during the fruit growth period. Consequently, the difference of cup quality between B and BP would not be due to CWP composition at the end of the φ 3 phase, i.e., when coffee beans are harvested.