Ectopic defense gene expression is associated with growth defects in Medicago truncatula lignin pathway mutants

First author: Chan Man Ha; Affiliations: The University of North Texas (北德克萨斯大学): Denton, USA

Corresponding author: Richard A. Dixon

Lignin provides essential mechanical support for plant cell walls but decreases the digestibility of forage crops and increases the recalcitrance of biofuel crops. Attempts to modify lignin content and/or composition by genetic modification often result in negative growth effects. Although several studies have attempted to address the basis for such effects in individual transgenic lines, no common mechanism linking lignin modification with perturbations in plant growth and development has yet been identified. To address whether a common mechanism exists, we have analyzed transposon insertion mutants resulting in independent loss of function of five enzymes of the monolignol pathway, as well as one double mutant, in the model legume Medicago truncatula. These plants exhibit growth phenotypes from essentially wild-type to severely retarded. Extensive phenotypic, transcriptomic, and metabolomics analyses, including structural characterization of differentially expressed compounds, revealed diverse phenotypic consequences of lignin pathway perturbation that were perceived early in plant development, but were not predicted by lignin content or composition alone. Notable phenotypes among the mutants with severe growth impairment were increased trichome numbers, accumulation of a variety of triterpene saponins, and extensive but differential ectopic expression of defense response genes. No currently proposed model explains the observed phenotypes across all lines. We propose that re-allocation of resources into defense pathways is linked to the severity of the final growth phenotype in monolignol pathway mutants of Medicago, although it remains unclear as to whether this is a cause or an effect of the growth impairment.

木质素为植物的细胞壁提供了必要的机械支撑，但也降低了饲料作物的可消化性，使得燃料作物不容易降解。通过遗传修饰来修改植物中的木质素含量或者组成通常会带来不利于生长的负效应。尽管已经有一些研究尝试在个别转基因株系中寻找这种负效应的潜在分子基础，但还未发现可以将木质素修饰与植物生长和发育关联起来的通用机制。为了验证这样一个通用机制是否存在，作者分析了模式豆科植物苜蓿的五个木质素单体通路上关键酶的转座子插入突变体和一个双突株系。这些突变体的表型变异很大，有的与野生型表型一致，有的却有着严重的发育迟缓。通过结合表型鉴定、转录组学及代谢组学分析，鉴定差异表达复合物的结构特征，作者发现木质素通路受到干扰所带来的各种表型缺陷在植株发育早期就会显现出来，但不能通过木质素含量或者组成单独进行预测。发育严重迟缓的突变体毛状体数量有所赠钱，并积累了多种三萜皂苷类化合物，同时伴随着防御响应基因的广泛但异位表达。目前，没有合适的模型能够解释所有突变体株系中所观察到的表型变异。作者提出苜蓿单木质素通路突变体将资源重新分配到防御通路是解释表型变异的可能原因，但至于这是否是导致植株生长缺陷的原因还不清楚。

通讯：Richard A. Dixon(https://biology.unt.edu/prof-richard-dixon)

个人简介：牛津大学，生物化学和生物学博士；剑桥大学，植物生物化学博士后。

研究方向：植物自然产物的生物化学、分子生物学及代谢研究。

doi: https://doi.org/10.1104/pp.19.00533

Journal: Plant Physiology

First Published: July 09, 2019