Opposite Root Growth Phenotypes of hy5 versus hy5 hyh Mutants Correlate with Increased Constitutive Auxin Signaling

The Arabidopsis transcription factor HY5 controls light-induced gene expression downstream of photoreceptors and plays an important role in the switch of seedling shoots from dark-adapted to light-adapted development. In addition, HY5 has been implicated in plant hormone signaling, accounting for the accelerated root system growth phenotype of hy5 mutants. Mutants in the close HY5 homolog HYH resemble wild-type, despite the largely similar expression patterns and levels of HY5 and HYH, and the functional equivalence of the respective proteins. Moreover, the relative contribution of HYH to the overall activity of the gene pair is increased by an alternative HYH transcript, which encodes a stabilized protein. Consistent with the enhanced root system growth observed in hy5 loss-of-function mutants, constitutively overexpressed alternative HYH inhibits root system growth. Paradoxically, however, in double mutants carrying hy5 and hyh null alleles, the hy5 root growth phenotype is suppressed rather than enhanced. Even more surprisingly, compared to wild-type, root system growth is diminished in hy5 hyh double mutants. In addition, the double mutants display novel shoot phenotypes that are absent from either single mutant. These include cotyledon fusions and defective vasculature, which are typical for mutants in genes involved in the transcriptional response to the plant hormone auxin. Indeed, many auxin-responsive and auxin signaling genes are misexpressed in hy5 mutants, and at a higher number and magnitude in hy5 hyh mutants. Therefore, auxin-induced transcription is constitutively activated at different levels in the two mutant backgrounds. Our data support the hypothesis that the opposite root system phenotypes of hy5 single and hy5 hyh double mutants represent the morphological response to a quantitative gradient in the same molecular process, that is gradually increased constitutive auxin signaling. The data also suggest that HY5 and HYH are important negative regulators of auxin signaling amplitude in embryogenesis and seedling development

Lignin and Hydroxycinnamic Acids: Biosynthesis and the Buildup of the Cell Wall. Advances in Botanical Research. Elsevier. Book series

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Crop breeding: Turning a lawn into a field

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Lignification: different mechanisms for a versatile polymer

Lignins are cell wall phenolic polymers resulting from monolignol radical coupling. They have characteristically high diversity in their structures which is a direct consequence of the versatile character of the lignification mechanisms discussed in this review. We will relate the latest discoveries regarding the main participants involved in lignin deposition in various tissues. Lignification is often described as a cell autonomous event occurring progressively in all cell wall layers during lignifying cell life and stopping with the cell death. However, recent data combined to old data from studies of tree lignification and zinnia cultures challenged these entrenched views and showed that the lignification process is cell-type dependent and can involve neighboring cells. Therefore, we consider recent data on cell-autonomous and non-cell autonomous lignification processes. We conclude that the role of lignins still need to be assessed during plant development and that control of polymerization/lignin deposition remains elusive and need to be investigated

Grass secondary cell walls, Brachypodium distachyon as a model for discovery

International audienceA key aspect of plant growth is the synthesis and deposition of cell walls. In specific tissues and cell types including xylem and fibre, a thick secondary wall comprised of cellulose, hemicellulose and lignin is deposited. Secondary cell walls provide a physical barrier that protects plants from pathogens, promotes tolerance to abiotic stresses and fortifies cells to withstand the forces associated with water transport and the physical weight of plant structures. Grasses have numerous cell wall features that are distinct from eudicots and other plants. Study of the model species Brachypodium distachyon as well as other grasses has revealed numerous features of the grass cell wall. These include the characterisation of xylosyl and arabinosyltransferases, a mixed-linkage glucan synthase and hydroxycinnamate acyltransferases. Perhaps the most fertile area for discovery has been the formation of lignins, including the identification of novel substrates and enzyme activities towards the synthesis of monolignols. Other enzymes function as polymerising agents or transferases that modify lignins and facilitate interactions with polysaccharides. The regulatory aspects of cell wall biosynthesis are largely overlapping with those of eudicots, but salient differences among species have been resolved that begin to identify the determinants that define grass cell walls

Distribution, mobility, and anchoring of lignin-related oxidative enzymes in Arabidopsis secondary cell walls

Lignin is an important phenolic biopolymer that provides strength and rigidity to the secondary cell walls of tracheary elements, sclereids, and fibers in vascular plants. Lignin precursors, called monolignols, are synthesized in the cell and exported to the cell wall where they are polymerized into lignin by oxidative enzymes such as laccases and peroxidases. In Arabidopsis thaliana, a peroxidase (PRX64) and laccase (LAC4) are shown to localize differently within cell wall domains in interfascicular fibers: PRX64 localizes to the middle lamella whereas LAC4 localizes throughout the secondary cell wall layers. Similarly, laccases localized to, and are responsible for, the helical depositions of lignin in protoxylem tracheary elements. In addition, we tested the mobility of laccases in the cell wall using fluorescence recovery after photobleaching. mCHERRY-tagged LAC4 was immobile in secondary cell wall domains, but mobile in the primary cell wall when ectopically expressed. A small secreted red fluorescent protein (sec-mCHERRY) was engineered as a control and was found to be mobile in both the primary and secondary cell walls. Unlike sec-mCHERRY, the tight anchoring of LAC4 to secondary cell wall domains indicated that it cannot be remobilized once secreted, and this anchoring underlies the spatial control of lignification

Brachypodium distachyon UNICULME4 and LAXATUM-A are redundantly required for development

International audienceAbstract In cultivated grasses, tillering, leaf and inflorescence architecture, as well as abscission ability, are major agronomical traits. In barley (Hordeum vulgare), maize (Zea mays), rice (Oryza sativa), and brachypodium (Brachypodium distachyon), NOOT-BOP-COCH-LIKE (NBCL) genes are essential regulators of vegetative and reproductive development. Grass species usually possess two-to-four NBCL copies and until now a single study in O. sativa showed that the disruption of all NBCL genes strongly altered O. sativa leaf development. To improve our understanding of the role of NBCL genes in grasses, we extended the study of the two NBCL paralogs BdUNICULME4 (CUL4) and BdLAXATUM-A (LAXA) in the non-domesticated grass B. distachyon. For this, we applied reversed genetics and generated original B. distachyon single and double nbcl mutants by CRISPR-Cas9 approaches and genetic crossing between nbcl TILLING mutants. Through the study of original single laxa CRISPR-Cas9 null alleles, we validated functions previously proposed for LAXA in tillering, leaf patterning, inflorescence and flower development and also unveiled roles for these genes in seed yield. Furthermore, the characterization of cul4laxa double mutants revealed essential functions for nbcl genes in B. distachyon development, especially in the regulation of tillering, stem cell elongation and secondary cell wall composition as well as for the transition towards the reproductive phase. Our results also highlight recurrent antagonist interactions between NBCLs occurring in multiple aspects of B. distachyon development