Microbes and their carbohydrate-active enzymes are central for depolymerization of complex lignocellulosic polysaccharides in the global carbon cycle. Their unique abilities to degrade and ferment carbohydrates are also utilized in many industrial processes such as baking, brewing and production of biofuels and drugs. Effective degradation and utilization of cellulose and hemicelluloses is important for the shift towards green bioeconomy, and requires microbes equipped with proper sets of carbohydrate-active enzymes (CAZymes). Knowledge of cellulolytic and xylanolytic CAZymes has mainly been generated from bacteria and filamentous fungi, while yeasts have been largely overlooked and may represent an untapped resource in natural CAZymes with industrial relevance. Cellulose and xylan-degrading yeasts with the ability to ferment saccharides are also promising candidates for consolidated bioprocesses (CBPs), as they can degrade lignocellulose and utilize its constituents to produce desired products at the same time. Cellulolytic yeasts able to utilize insoluble crystalline cellulose are rare while xylanolytic yeasts are rather widespread in nature. The lack of particular enzymes in yeasts can be remediated by introducing the missing enzymes into strains having outstanding product-forming attributes.

In this review, we provide a comprehensive overview of the cellulose- and xylan-degrading ascomycetous and basidiomycetous yeasts known to date. We describe how these yeasts can be identified through bioprospecting and bioinformatic approaches and summarize available growth and enzymatic assays for strain characterization. Known and predicted CAZymes are extensively analyzed, both in individual species and in a phylogenetic perspective. We also describe the strategies used for construction of recombinant cellulolytic and xylanolytic strains as well as current applications for polysaccharide-degrading yeasts. Finally, we discuss the great potential of these yeasts as industrial cell factories, identify open research questions and provide suggestions for future investigations.

微生物及其碳水化合物活性酶是全球碳循环中复杂木质纤维素多糖解聚的核心。它们降解和发酵碳水化合物的独特能力也被用于许多工业过程，例如烘焙、酿造以及生物燃料和药物的生产。纤维素和半纤维素的有效降解和利用对于向绿色生物经济的转变非常重要，并且需要配备适当的碳水化合物活性酶 (CAZymes) 组的微生物。纤维素分解和木聚糖分解 CAZymes 的知识主要来自细菌和丝状真菌，而酵母菌在很大程度上被忽视了，它们可能代表了具有工业相关性的天然 CAZymes 中尚未开发的资源。具有发酵糖类能力的纤维素和木聚糖降解酵母也是综合生物过程 (CBP) 的有希望的候选者，因为它们可以降解木质纤维素并同时利用其成分生产所需的产品。能够利用不溶性结晶纤维素的纤维素分解酵母很少见，而木聚糖分解酵母在自然界中相当普遍。酵母中特定酶的缺乏可以通过将缺失的酶引入具有突出产品形成属性的菌株来修复。

在这篇综述中，我们全面概述了迄今为止已知的降解纤维素和木聚糖的子囊菌和担子菌酵母。我们描述了如何通过生物勘探和生物信息学方法识别这些酵母，并总结了用于菌株表征的可用生长和酶测定。已知和预测的 CAZymes 在单个物种和系统发育的角度都得到了广泛的分析。我们还描述了用于构建重组纤维素分解和木聚糖分解菌株的策略以及多糖降解酵母的当前应用。最后，我们讨论了这些酵母作为工业细胞工厂的巨大潜力，确定开放的研究问题并为未来的研究提供建议。