WOOD ForMS: Wood Formation Mechanisms under Stress

Trees reacting to greenhouse gases associated with climate change (elevated concentrations of carbon dioxide and ozone).

PI: Dany Afif (UMR 1137 « Écologie et Ecophysiologie Forestières » – EEF)

Co-applicants:  Mireille Cabané, Pascale Maillard (UMR 1137 – EEF)

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Context –  Wood is an important economic resource valued by numerous industries for its varied uses, particularly as feedstock for bio-energy fuel.  Biosynthesis of cellulose and lignin biosynthesis, both important components of wood, are highly controlled during the trees development phase and affected by environmental stress factors.  Since biomass production is increased by elevated CO2 levels and decreased by high ozone levels, it is particularly important to gauge the impact these two gases have on the mechanisms of wood formation.

Objective — The project aims to understand the mechanisms that drive cellular mechanisms that in turn control the biosynthesis of cellulose and lignin in poplar wood subject to stress conditions. As the formation of wood depends on the regulation of genes involved in biosynthesis, of cellulose and lignin polymers and the supply of carbon for each, distribution between these two polymers can be altered by stress.

Approach — Our analysis is focused on (i) regulation of cell wall-related NAC/MYB transcription factors involved in the regulation of secondary wall formation and target genes involved in cellulose and lignin biosynthesis; (ii) metabolic control analysis by ¹³C incorporated into cellulose and lignin as well as carbon distribution among these two polymers.

Key results —

• Cellulose biosynthesis in poplar wood subjected to ozone stress was modified. The ¹³C incorporation into cellulose was reduced which indicated a decrease in the synthesis of this polymer. The gene expression of 18 cellulose synthase (CesA), UGP and SUSY supplying substrate for cellulose synthesis, and Korrigan and COBRA for cellulose deposition and crystallinity showed two expression profiles. The transcript abundance of CesA genes involved in the formation of the secondary cell wall, UGP, SUSY genes and members of Korrigan and COBRA gene families were strongly reduced. On the other hand, the gene expression of CesA involved in cellulose synthesis of primary cell wall, and other members of Korrigan and COBRA gene families were strongly induced. These observations suggested a reorganization of the cell wall architecture. Five transcription factors, WND / MYB / KNAT7, showed a similar expression profile to that of CesA of the secondary cell wall. These transcription factors were regulated in response to stress.

• Under conditions of elevated CO₂, the ¹³C incorporation into cellulose increased, which suggested a stimulation of cellulose synthesis; however, the transcript abundance of genes, cited above, was unchanged or was slightly reduced.

• The two gases combined had an intermediate effect. Elevated CO₂ seemed to mitigate the ozone effect.

• The reduction of cellulose content must be connected to the content of lignin. Cellulose / lignin ratio was decreased whatever the stress, in spite of the differences in ¹³C incorporation between ozone and elevated CO₂

Main findings —Wood formation was altered under conditions of ozone stress. Modifications observed in secondary and primary cell wall biosynthesis suggested a reorganization of the cell wall architecture. Transcription factors which control wood formation were regulated in response to ozone stress. Cellulose / lignin balance under stress conditions will determine new wood characteristics.

Future perspectives — From these results we plan to: (i) establish a network for gene regulation under stress conditions (RNAseq strategy); (ii) search for genes controlling cellulose / lignin balance (iii) assess the impact of carbon allocation to the stem and on the control of cellulose / lignin balance.