Water Deficit-Responsive QTLs for Cell Wall Degradability and Composition in Maize at Silage Stage.

The use of lignocellulosic biomass for animal feed or biorefinery requires the optimization of its degradability. Moreover, biomass crops need to be better adapted to the changing climate and in particular to periods of drought. Although the negative impact of water deficit on biomass yield has often been mentioned, its impact on biomass quality has only been recently reported in a few species. In the present study, we combined the mapping power of a maize recombinant inbred line population with robust near infrared spectroscopy predictive equations to track the response to water deficit of traits associated with biomass quality. The population was cultivated under two contrasted water regimes over 3 consecutive years in the south of France and harvested at silage stage. We showed that cell wall degradability and ß-O-4-linked H lignin subunits were increased in response to water deficit, while lignin and p-coumaric acid contents were reduced. A mixed linear model was fitted to map quantitative trait loci (QTLs) for agronomical and cell wall-related traits. These QTLs were categorized as "constitutive" (QTL with an effect whatever the irrigation condition) or "responsive" (QTL involved in the response to water deficit) QTLs. Fifteen clusters of QTLs encompassed more than two third of the 213 constitutive QTLs and 13 clusters encompassed more than 60% of the 149 responsive QTLs. Interestingly, we showed that only half of the responsive QTLs co-localized with constitutive and yield QTLs, suggesting that specific genetic factors support biomass quality response to water deficit. Overall, our results demonstrate that water deficit favors cell wall degradability and that breeding of varieties that reconcile improved drought-tolerance and biomass degradability is possible.

A systems biology approach uncovers a gene co-expression network associated with cell wall degradability in maize.

Understanding the mechanisms triggering variation of cell wall degradability is a prerequisite to improving the energy value of lignocellulosic biomass for animal feed or biorefinery. Here, we implemented a multiscale systems approach to shed light on the genetic basis of cell wall degradability in maize. We demonstrated that allele replacement in two pairs of near-isogenic lines at a region encompassing a major quantitative trait locus (QTL) for cell wall degradability led to phenotypic variation of a similar magnitude and sign to that expected from a QTL analysis of cell wall degradability in the F271 × F288 recombinant inbred line progeny. Using DNA sequences within the QTL interval of both F271 and F288 inbred lines and Illumina RNA sequencing datasets from internodes of the selected near-isogenic lines, we annotated the genes present in the QTL interval and provided evidence that allelic variation at the introgressed QTL region gives rise to coordinated changes in gene expression. The identification of a gene co-expression network associated with cell wall-related trait variation revealed that the favorable F288 alleles exploit biological processes related to oxidation-reduction, regulation of hydrogen peroxide metabolism, protein folding and hormone responses. Nested in modules of co-expressed genes, potential new cell-wall regulators were identified, including two transcription factors of the group VII ethylene response factor family, that could be exploited to fine-tune cell wall degradability. Overall, these findings provide new insights into the regulatory mechanisms by which a major locus influences cell wall degradability, paving the way for its map-based cloning in maize.

Suppression of Dwarf and irregular xylem Phenotypes Generates Low-Acetylated Biomass Lines in Arabidopsis.

eskimo1-5 (esk1-5) is a dwarf Arabidopsis (Arabidopsis thaliana) mutant that has a constitutive drought syndrome and collapsed xylem vessels, along with low acetylation levels in xylan and mannan. ESK1 has xylan O-acetyltransferase activity in vitro. We used a suppressor strategy on esk1-5 to screen for variants with wild-type growth and low acetylation levels, a favorable combination for ethanol production. We found a recessive mutation in the KAKTUS (KAK) gene that suppressed dwarfism and the collapsed xylem character, the cause of decreased hydraulic conductivity in the esk1-5 mutant. Backcrosses between esk1-5 and two independent knockout kak mutants confirmed suppression of the esk1-5 effect. kak single mutants showed larger stem diameters than the wild type. The KAK promoter fused with a reporter gene showed activity in the vascular cambium, phloem, and primary xylem in the stem and hypocotyl. However, suppression of the collapsed xylem phenotype in esk1 kak double mutants was not associated with the recovery of cell wall O-acetylation or any major cell wall modifications. Therefore, our results indicate that, in addition to its described activity as a repressor of endoreduplication, KAK may play a role in vascular development. Furthermore, orthologous esk1 kak double mutants may hold promise for ethanol production in crop plants.

Interactions between Nosema microspores and a neonicotinoid weaken honeybees (Apis mellifera).

Global pollinators, like honeybees, are declining in abundance and diversity, which can adversely affect natural ecosystems and agriculture. Therefore, we tested the current hypotheses describing honeybee losses as a multifactorial syndrome, by investigating integrative effects of an infectious organism and an insecticide on honeybee health. We demonstrated that the interaction between the microsporidia Nosema and a neonicotinoid (imidacloprid) significantly weakened honeybees. In the short term, the combination of both agents caused the highest individual mortality rates and energetic stress. By quantifying the strength of immunity at both the individual and social levels, we showed that neither the haemocyte number nor the phenoloxidase activity of individuals was affected by the different treatments. However, the activity of glucose oxidase, enabling bees to sterilize colony and brood food, was significantly decreased only by the combination of both factors compared with control, Nosema or imidacloprid groups, suggesting a synergistic interaction and in the long term a higher susceptibility of the colony to pathogens. This provides the first evidences that interaction between an infectious organism and a chemical can also threaten pollinators, interactions that are widely used to eliminate insect pests in integrative pest management.