Ectopic Defense Gene Expression Is Associated with Growth Defects in Medicago truncatula Lignin Pathway Mutants

Abstract

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 reallocation of resources into defense pathways is linked to the severity of the final growth phenotype in monolignol pathway mutants of M. truncatula, although it remains unclear whether this is a cause or an effect of the growth impairment.

Clavibacter michiganensis Downregulates Photosynthesis and Modifies Monolignols Metabolism Revealing a Crosstalk with Tomato Immune Responses

“…In addition to PAL, three other central genes were upregulated at 6 dpi, namely 4CL, HCT and CCoAOMT supporting that the monolignol pathway was turned on by Cmm infection. This conclusion is in agreement with previous genetic and physiological studies indicating that gene silencing of HCT caused severe loss of lignified structure integrity [54,55] and that of CCoAOMT compromised penetration resistance against pathogens [51]. Likewise, upregulation of HCT in infected maize plants showed increased lignin accumulation [56].…”

“…Similar to the fah1-2 mutant, the S/G ratio was strongly reduced in a mutant with COMT loss of function, as the result of a large reduction in S-units [62]. The near total loss of S-units in comt mutant was also not associated with growth defects [55].…”

Expression of a bacterial 3-dehydroshikimate dehydratase (QsuB) reduces lignin and improves biomass saccharification efficiency in switchgrass (Panicum virgatum L.)

“…The exact mechanism by which QsuB expression reduces lignin in switchgrass is still unresolved; in particular, whether the cytosolic pools of shikimate —required for HCT activity— and p -coumaroyl-shikimate are reduced remain to be demonstrated. Similarly, it would be interesting to determine the lignin monomeric composition in the different QsuB switchgrass lines, especially the relative amount of p -hydroxyphenyl (H) units, which is known to be higher in Arabidopsis QsuB plants and typically increases in HCT down-regulated dicot species [ 8 , 14 – 20 ]. Furthermore, the recent discovery in several plant species —including switchgrass— of genes encoding putative 3-hydroxylases (C3H) that convert p -coumarate to caffeate, as well as genetic evidence of their role in lignin formation in Brachypodium distachyon , question the exclusive role of HCT and the involvement of p -coumarate esters during lignin biosynthesis in monocots [ 21 ].…”

Hevea brasiliensis coniferaldehyde-5-hydroxylase (HbCAld5H) regulates xylogenesis, structure and lignin chemistry of xylem cell wall in Nicotiana tabacum

“…Genetic manipulation of lignin biosynthetic pathway resulted in high degree of changes in S/G monolignol composition which does not compromise plant fitness (Reddy et al 2005 ; Stewart et al 2009 ; Bonawitz and Chapple 2013 ; Anderson et al 2015 ). To find a mechanistic explanation for the growth phenotypes from normal to highly dwarfed, resulting from lignin modification, Ha et al ( 2019 ) performed a combined transcriptome and metabolome analysis in Medicago truncatula . Their data revealed that there were no altered transports or accumulation of toxic compounds in such plants where lignin modifications were noticed.…”