Functional expression in S. cerevisiae of a highly active fungal XI has paved the way for metabolic engineering of this yeast towards high-yield, rapid production of ethanol from D-xylose under fully anaerobic conditions. On theoretical grounds, this XI-based approach is superior to the extensively studied xylose reductase/xylitol dehydrogenase strategy. While considerable experimental proof to substantiate this statement has been obtained under “academic” conditions, a next important challenge is to do the same under industrial conditions. While the first experiments in real-life plant biomass hydrolysates are quite promising, there remains plenty of scope for integrat­ing the D-xylose-fermentation genotype with other metabolic and process­engineering strategies for further increased robustness under process condi­tions.

In addition to D-xylose, plant biomass hydrolysates contain several other potentially fermentable substrates that cannot be converted by wild-type S. cerevisiae strains [69]. While these compounds often represent only a few percent of the potentially fermentable carbon, they can have a decisive impact on economical competitiveness and sustainability of high-yield, high-volume processes such as fuel ethanol production. Functional integration of a highly efficient D-xylose fermentation pathway with pathways that are under devel­opment (e. g. arabinose [9,36]) or under consideration (e. g. rhamnose [69]) therefore presents an additional challenge in metabolic engineering for ef­ficient fermentation of plant biomass hydrolysates. We are convinced that creative integration of metabolic engineering, evolutionary engineering and process design can result in rapid breakthroughs in these areas.