Sucrose synthase gene expression analysis in the fibre nettle (Urtica dioica L.) cultivar “clone 13”

The use and valorisation of fibre crops are sustainable solutions to reduce the world's dependence on petroleum-derived products and fossil energy. Fibre crops have a relatively short growth cycle and provide high amounts of biomass used in different industrial sectors. Among fibre crops there is stinging nettle (Urtica dioica L.), a perennial herbaceous plant growing in temperate regions. Nettle phloem fibres (a.k.a. bast fibres) have a high cellulose content (ca. 80%) and low amount of lignin (ca. 4%); additionally, they are silky and have a high tensile strength. The gelatinous cell walls of bast fibres are primarily composed of cellulose. The biosynthesis of cellulose is dependent on the provision of uridine diphosphate glucose, which, besides being formed from glucose-1-phosphate through uridine diphosphate glucose pyrophosphorylase, can also be produced via the reaction catalysed by sucrose synthase (SUS). A regulation of SUS gene expression accompanying the developmental stages of the bast fibres is therefore likely to exist along the stem of nettle plants. The objectives of this study were: 1) to identify the SUS genes in nettle and 2) to analyse their differential expression in tissues of stem internodes sampled at different heights (i.e. top, middle and bottom). The gene expression analysis is accompanied by optical and confocal microscopy observations concerning cellulose and lignin distribution. The results here presented identify 6 SUS genes in nettle belonging to the three Angiosperm groups previously reported (groups I–III). Their gene expression analysis shows a differential regulation in the stem tissues sampled at different heights, which reflects the increase in cell wall thickness of bast fibres along the stem of nettle. In particular, 3 expression patterns of genes either more expressed in young/old stem regions or peaking at the middle internode are identified with the heat map hierarchical clustering. This is the first study on the expression of SUS genes in a nettle fibre variety and on the immunohistochemical analysis of U. dioica internodes sampled at different stem heights. This work will serve as a basis for future molecular studies on a neglected, yet potential multi-purpose plant.