| Rice is an important food crop over the world and provides large amounts of lignocellulose-rich straw convertible for cellulosic ethanol and other biochemicals.In principle,lignocellulose conversion for bioethanol production involves in three major steps:lignocellulose pretreatment,enzymatic saccharification and yeast fermentation.However,the natural plant cell wall recalcitrance basically decides a costly bioethanol conversion with potential secondary pollution to the environment.As genetic engineering of plant cell walls could improve lignocellulose recalcitrance,it remains to explore a cost-effective and green-like pretreatment for complete enzymatic saccharification towards high bioethanol production.Using previously-identified site-mutant（cesa9）of OsCESA9 essential for cellulose biosynthesis of secondary cell walls in rice,this study established optimal biomass pretreatment to enhance enzymatic saccharification for high bioethanol production,and then elucidated why the genetic-modified lignocellulose is of remarkably enhanced biomass enzymatic digestibility.The major results were described below:1.Compared with wild type（NPB）,the cesa9 mutant contained much higher soluble sugars by 15%in the mature straws,which is directly fermentable for bioethanol production.The mutant also showed either reduced cellulose level by 36%or reduced cellulose DP by 33%and CrI by 21%for raised cellulose accessibility by 17%,leading to integrated enhancement on lignocellulose enzymatic hydrolysis.2.The acid（2%H2SO4）pretreatment led to biomass loss by 50%and cellulose loss by 23%and also caused incomplete lignocellulose degradation;whereas the ultrasonic pretreatment led to relatively high lignin content in the pretreated lignocellulose residue with slightly enhanced biomass enzymatic saccharification.By comparison,either the0.7%NaOH pretreatment or the 0.5%NaOH pretreatment assisted with ultrasonic（1:18/w:v,400 W,20 min）could lead to almost complete biomass enzymatic saccharification in the mutant.3.By integrating XRD,SEM,FTIR and other techniques,this study examined the major lignocellulose factors on biomass enzymatic saccharification;In particular,the ultrasonic-assisted 0.5%Na OH pretreatment could reduce cellulose DP by 14%-17%,extract large amounts of hemicellulose and lignin,and raise cellulose accessibility by55%-75%with slight cellulose loss.4.Under the optimal ultrasonic-assisted alkali pretreatment,the mutant was of high sugar-ethanol conversion rate at 93%,being higher than that of the wild type by 7%.In addition,the mutant could produce bioethanol yield at 55%（%cellulose）,which was higher than that of the wild type by 51%.In summary,this study has proposed a mechanistic model that highlights how genetic engineering of plant cell walls could reduce lignocellulose recalcitrance for much enhanced biomass enzymatic saccharification and bioethanol production under optimal ultrasonic-assisted alkali pretreatment by effectively extracting lignin and hemicellulose and largely reducing cellulose DP for remarkably raised cellulose accessibility,providing a cost-effective and green-like strategy for potential full utilization of crop straw biomass. |
