Zeng Yining, Zhao Shuai, Wei Hui, Tucker Melvin P, Himmel Michael E, Mosier Nathan S, Meilan Richard, Ding Shi-You
Biosciences Center, National Renewable Energy Laboratory, Golden, CO 80401 USA.
National Bioenergy Center, National Renewable Energy Laboratory, Golden, CO 80401 USA.
Biotechnol Biofuels. 2015 Aug 27;8:126. doi: 10.1186/s13068-015-0312-1. eCollection 2015.
In higher plant cells, lignin provides necessary physical support for plant growth and resistance to attack by microorganisms. For the same reason, lignin is considered to be a major impediment to the process of deconstructing biomass to simple sugars by hydrolytic enzymes. The in situ variation of lignin in plant cell walls is important for better understanding of the roles lignin play in biomass recalcitrance.
A micro-spectroscopic approach combining stimulated Raman scattering microscopy and fluorescence lifetime imaging microscopy was employed to probe the physiochemical structure of lignin in poplar tracheid cell walls. Two forms of lignins were identified: loosely packed lignin, which had a long (4 ns) fluorescence lifetime and existed primarily in the secondary wall layers; and dense lignin, which had a short (0.5-1 ns) fluorescence lifetime and was present in all wall layers, including the cell corners, compound middle lamellae, and secondary wall. At low maleic acid concentration (0.025 and 0.05 M) pretreatment conditions, some of the dense lignin was modified to become more loosely packed. High acid concentration removed both dense and loosely packed lignins. These modified lignins reformed to make lignin-carbohydrate complex droplets containing either dense or loosely packed lignin (mostly from secondary walls) and were commonly observed on the cell wall surface.
We have identified dense and loosely packed lignins in plant cell walls. During maleic acid pretreatment, both dense lignin droplets and loosely packed lignin droplets were formed. Maleic acid pretreatment more effectively removes loosely packed lignin in secondary walls which increases enzyme accessibility for digestion.
在高等植物细胞中，木质素为植物生长提供必要的物理支撑，并抵御微生物的侵袭。出于同样的原因，木质素被认为是水解酶将生物质分解为单糖过程中的主要障碍。植物细胞壁中木质素的原位变化对于更好地理解木质素在生物质顽固性中所起的作用至关重要。
采用受激拉曼散射显微镜和荧光寿命成像显微镜相结合的显微光谱方法，探测杨树管胞细胞壁中木质素的物理化学结构。鉴定出两种形式的木质素：松散堆积的木质素，其荧光寿命长（4纳秒），主要存在于次生壁层；以及致密木质素，其荧光寿命短（0.5 - 1纳秒），存在于所有壁层，包括细胞角隅、复合中层和次生壁。在低马来酸浓度（0.025和0.05 M）预处理条件下，一些致密木质素被改性，变得更加松散堆积。高酸浓度会去除致密和松散堆积的木质素。这些改性木质素重新形成含有致密或松散堆积木质素（主要来自次生壁）的木质素 - 碳水化合物复合液滴，并且在细胞壁表面普遍观察到。
我们在植物细胞壁中鉴定出了致密和松散堆积的木质素。在马来酸预处理过程中，形成了致密木质素液滴和松散堆积木质素液滴。马来酸预处理能更有效地去除次生壁中松散堆积的木质素，从而增加酶消化的可及性。
