Long cold exposure induces transcriptional and biochemical remodelling of xylem secondary cell wall in Eucalyptus

Abstract

Although eucalypts are the most planted hardwood trees worldwide, the majority of them are frost sensitive. The recent creation of frost-tolerant hybrids such as Eucalyptus gundal plants (E. gunnii × E. dalrympleana hybrids), now enables the development of industrial plantations in northern countries. Our objective was to evaluate the impact of cold on the wood structure and composition of these hybrids, and on the biosynthetic and regulatory processes controlling their secondary cell-wall (SCW) formation. We used an integrated approach combining histology, biochemical characterization and transcriptomic profiling as well as gene co-expression analyses to investigate xylem tissues from Eucalyptus hybrids exposed to cold conditions. Chilling temperatures triggered the deposition of thicker and more lignified xylem cell walls as well as regulation at the transcriptional level of SCW genes. Most genes involved in lignin biosynthesis, except those specifically dedicated to syringyl unit biosynthesis, were up-regulated. The construction of a co-expression network enabled the identification of both known and potential new SCW transcription factors, induced by cold stress. These regulators at the crossroads between cold signalling and SCW formation are promising candidates for functional studies since they may contribute to the tolerance of E. gunnii × E. dalrympleana hybrids to cold.

A systems biology view of wood formation in Eucalyptus grandis trees submitted to different potassium and water regimes

“…RNA extraction and integrity controls were performed as previously described (Ployet et al ., ). Three sequencing libraries per treatment were generated and sequenced (Illumina, San Diego, CA, USA; 150 bp paired‐end reads) by the Genotoul GeT platform (https://get.genotoul.fr/en/; France) using the HiSeq3000 (Illumina).…”

“…Biochemical analyses of SCWs were done in triplicate from extractive‐free xylem residue (EXR) obtained as described by Ployet et al . () and detailed in Method S1. Saccharification was estimated with or without alkali pretreatment, mainly as described by Van Acker et al .…”

“…For xylem structure analysis, transverse sections were made from micro‐core samples (three biological replicates per condition) and transgenic roots (six p35S:EgMYB137 lines and four controls), and embedded in LR White resin (London Resin Company Ltd, Reading, UK). Semi‐automatic image analyses of semi‐thin sections (1 μm) were performed as previously described (Ployet et al ., ) to estimate SCW thickness (Method S1). Average vessel diameter was measured by image analyses on > 400 vessels (diameter cut‐off > 35 μm) using ImageJ software (v.1.5).…”

“…Recent studies have highlighted the importance of a dynamic cross‐talk between the regulation of SCWs during development and in response to stresses, which is probably necessary to promote adaptation to environmental changes (Zinkgraf et al ., ). For instance, changes in the thickness, composition and/or structure of SCWs have been observed in response to several stresses such as nitrogen excess or depletion (Camargo et al ., ; Euring et al ., ), mechanical stress (Mellerowicz & Gorshkova, ) or low temperature (Ployet et al ., ). Transcription factors (TFs) involved in the regulation of SCW formation as well as SCW biosynthesis genes were reported to be induced by environmental constraints such as high salinity or iron deprivation in Arabidopsis (Taylor‐Teeples et al ., ) or cold stress in Eucalyptus (Ployet et al ., ).…”

“…For instance, changes in the thickness, composition and/or structure of SCWs have been observed in response to several stresses such as nitrogen excess or depletion (Camargo et al ., ; Euring et al ., ), mechanical stress (Mellerowicz & Gorshkova, ) or low temperature (Ployet et al ., ). Transcription factors (TFs) involved in the regulation of SCW formation as well as SCW biosynthesis genes were reported to be induced by environmental constraints such as high salinity or iron deprivation in Arabidopsis (Taylor‐Teeples et al ., ) or cold stress in Eucalyptus (Ployet et al ., ). Some of these TFs were shown to be involved in stress tolerance (Ramírez et al ., ; Guo et al ., ).…”