Effects of altered lignin biosynthesis on phenylpropanoid metabolism and plant stress

635 ISSN 1759-7269 10.4155/BFS.13.56 © 2013 Future Science Ltd While lignocellulosic feedstocks represent a promising renewable and sustainable alternative to petroleumbased fuels, high production costs associated with conversion processes currently prevent them from being economically viable for large-scale implementation [1]. The production of biofuels from lignocellulosic feedstocks requires the depolymerization of cell wall carbohydrates into simple sugars that can be utilized during fermentation. However, the desired cellulose microfibrils are surrounded by a matrix of lignin and hemicellulose, which greatly inhibits their accessibility to hydrolytic enzymes [1,2]. Lignin is a phenolic polymer that reinforces the secondary cell wall, confers structural integrity to the plant, aids in water transport and also plays an important role in plant responses to various environmental stresses. The presence of lignin in plant cells has been identified as a major contributor to the resistance of converting cell walls into fuel precursors [3]. Expensive and energy-intensive thermochemical pretreatments are generally required to disrupt the lignin–polysaccharide barrier and allow better access of the cellulose to hydrolysis prior to fermentation. An alternative approach for improving the accessibility of cell wall sugars and reducing the need for pretreatment is through genetic engineering of the lignin biosynthetic pathway. Reducing lignin content or modifying its composition can be achieved by downregulating or overexpressing genes involved in either lignin biosynthesis or its regulation [3–5]. Early studies from a decade or more ago that contributed to our current understanding of lignin biosynthesis and its manipulation focused on poplar (Populus spp.) to improve pulping performance [6,7], as well as forage species, such as alfalfa (Medicago sativa L.) and tall fescue (Festuca arundinacea), for improving digestibility [8–10]. A great deal of insight into lignin engineering has also been achieved through studies with the model species Arabidopsis thaliana [11] and tobacco (Nicotiana tabacum) [12]. Because the lignin biosynthetic pathway appears to be highly conserved among plant species, many of the genetic engineering strategies that have proven to be successful in model species can also be applied to lignocellulosic feedstocks [4]. Of particular importance to biofuel-related studies, A. thaliana has been identified as Effects of altered lignin biosynthesis on phenylpropanoid metabolism and plant stress