Marek Marzec

Induced Variations in Brassinosteroid Genes Define Barley Height and Sturdiness, and Expand the “Green Revolution” Genetic Toolkit

Historic barley short-culm mutants deficient in brassinosteroid genes are attractive targets for development of lodging-resistant crop plants. Reduced plant height and culm robustness are quantitative characteristics important for assuring cereal crop yield and quality under adverse weather conditions. A very limited number of short-culm mutant alleles were introduced into commercial crop cultivars during the Green Revolution. We identified phenotypic traits, including sturdy culm, specific for deficiencies in brassinosteroid biosynthesis and signaling in semidwarf mutants of barley (Hordeum vulgare). This set of characteristic traits was explored to perform a phenotypic screen of near-isogenic short-culm mutant lines from the brachytic, breviaristatum, dense spike, erectoides, semibrachytic, semidwarf, and slender dwarf mutant groups. In silico mapping of brassinosteroid-related genes in the barley genome in combination with sequencing of barley mutant lines assigned more than 20 historic mutants to three brassinosteroid-biosynthesis genes (BRASSINOSTEROID-6-OXIDASE, CONSTITUTIVE PHOTOMORPHOGENIC DWARF, and DIMINUTO) and one brassinosteroid-signaling gene (BRASSINOSTEROID-INSENSITIVE1 [HvBRI1]). Analyses of F2 and M2 populations, allelic crosses, and modeling of nonsynonymous amino acid exchanges in protein crystal structures gave a further understanding of the control of barley plant architecture and sturdiness by brassinosteroid-related genes. Alternatives to the widely used but highly temperature-sensitive uzu1.a allele of HvBRI1 represent potential genetic building blocks for breeding strategies with sturdy and climate-tolerant barley cultivars.

Asymmetric growth of root epidermal cells is related to the differentiation of root hair cells in Hordeum vulgare (L.)

The root epidermis of most vascular plants harbours two cell types, namely trichoblasts (capable of producing a root hair) and atrichoblasts. Here, in vivo analysis, confocal laser-scanning microscopy, transmission electron microscopy, histological analysis, and three-dimensional reconstruction were used to characterize the cell types present in the barley root epidermis and their distribution in the tissue. Both trichoblasts and atrichoblasts were present in the wild-type cultivars and could be distinguished from one another at an early stage. Trichoblast/atrichoblast differentiation depended on asymmetric cell expansion after a period of symmetrical cell division. After asymmetric growth, only the shorter epidermal cells could produce root hairs, whereas the longer cells became atrichoblasts. Moreover, the root epidermis did not develop root hairs at all if the epidermal cells did not differentiate into two asymmetric cell types. The root hairless phenotype of bald root barley (brb) and root hairless 1.b (rhl1.b) mutants was caused by a mutation in a gene related to the asymmetric expansion of the root epidermal cells. Additionally, the results showed that the mechanism of trichoblast/atrichoblast differentiation is not evolutionally conserved across the subfamilies of the Poaceae; in the Pooideae subfamily, both asymmetric division and asymmetric cell expansion have been observed.

The role of strigolactones in nutrient-stress responses in plants.

Strigolactones (SLs) are a new group of plant hormones, which have been intensively investigated during the last few years. The wide spectrum of SLs actions, including the regulation of shoot/root architecture, and the stimulation of the interactions between roots and fungi or bacteria, as well as the stimulation of germination of parasitic plants, indicates that this group of hormones may play an important role in the mechanisms that control soil exploration, and the root-mediated uptake of nutrients. Current studies have shown that SLs might be factors that have an influence on the plant response to a deficiency of macronutrients. Experimental data from the last four years have confirmed that the biosynthesis and exudation of SLs are increased under phosphorus and nitrogen deficiency. All these data suggest that SLs may regulate the complex response to nutrient stress, which include not only the modification of the plant developmental process, but also the cooperation with other organisms in order to minimize the effects of threats. In this paper the results of studies that indicate that SLs play an important role in the response to nutrient stress are reviewed and the consequences of the higher biosynthesis and exudation of SLs in response to phosphorus and nitrogen deficiency are discussed.

In Silico Analysis of the Genes Encoding Proteins that Are Involved in the Biosynthesis of the RMS/MAX/D Pathway Revealed New Roles of Strigolactones in Plants

Strigolactones were described as a new group of phytohormones in 2008 and since then notable large number of their functions has been uncovered, including the regulation of plant growth and development, interactions with other organisms and a plant’s response to different abiotic stresses. In the last year, investigations of the strigolactone biosynthesis pathway in two model species, Arabidopsis thaliana and Oryza sativa, resulted in great progress in understanding the functions of four enzymes that are involved in this process. We performed in silico analyses, including the identification of the cis-regulatory elements in the promoters of genes encoding proteins of the strigolactone biosynthesis pathway and the identification of the miRNAs that are able to regulate their posttranscriptional level. We also searched the databases that contain the microarray data for the genes that were analyzed from both species in order to check their expression level under different growth conditions. The results that were obtained indicate that there are universal regulations of expression of all of the genes that are involved in the strigolactone biosynthesis in Arabidopsis and rice, but on the other hand each stage of strigolactone production may be additionally regulated independently. This work indicates the presence of crosstalk between strigolactones and almost all of the other phytohormones and suggests the role of strigolactones in the response to abiotic stresses, such as wounding, cold or flooding, as well as in the response to biotic stresses.

Root hair development in the grasses: what we already know and what we still need to know?

A combination of histological and molecular studies uncovers mechanisms of root hair development in monocots. A priority in many crop improvement programs for a long time has been to enhance the tolerance level of plants to both abiotic and biotic stress. Recognition that the root system is the prime determinant of a plant’s ability to extract both water and minerals from the soil implies that its architecture is an important variable underlying a cultivar’s adaptation. The density and/or length of the root hairs (RHs) that are formed are thought to have a major bearing on the plant’s performance under stressful conditions. Any attempt to improve a crop’s root system will require a detailed understanding of the processes of RH differentiation. Recent progress in uncovering the molecular basis of root epidermis specialization has been recorded in the grasses. This review seeks to present the current view of RH differentiation in grass species. It combines what has been learned from molecular-based analyses, histological studies, and observation of the phenotypes of both laboratory- and field-grown plants.

Arabinogalactan proteins are involved in root hair development in barley

Highlight Arabinogalactan proteins (AGPs) recognized by the LM2, LM14, and MAC207 antibodies are diversely deposited in rhizodermis of barley root hair-producing genotypes, but evenly deposited in rhizodermal cells of the root hairless mutant.

The evolutionary context of root epidermis cell patterning in grasses (Poaceae).

In the last century, the mechanism for establishing the root epidermal pattern in grasses was proposed as a differentiating trait that can be used in taxonomic studies and as a useful tool to indicate the relationships between genera. However, knowledge about root hair differentiation in monocots is still scarce. During the last few years, this process has been studied intensively, mainly based on genetics and histological studies. A histological analysis of the root epidermis pattern composed from root hairs (trichoblasts) and non-root hair cells (atrichoblasts), as well as observations of the mechanism of the establishment of this pattern allowed 2 different methods of epidermal cell specialization in monocots to be precisely described. Additionally, a recently published paper describing root hair development in barley shed new light on the evolutionary context of the mechanism of root epidermis cell specialization, which is discussed in the presented work.

Strigolactones as Part of the Plant Defence System

Strigolactones (SLs) are plant hormones, described as regulators of plant growth and development. Recently, it was proposed that these hormones might also be involved in the biotic stress response. However, SLs do not have a universal role in plant protection, instead only playing a part in resistance to specific pathogens.

Identification and functional analysis of the HvD14 gene involved in strigolactone signaling in Hordeum vulgare.

In this study, the barley HvD14 gene encoding α/β hydrolase, which is involved in strigolactone (SL) signaling, was identified. Bioinformatics analysis revealed that the identified gene is an orthologue of the D14, AtD14 and PhDAD2 genes that have been described in rice, Arabidopsis thaliana and petunia, respectively. Using TILLING strategy, an hvd14.d mutant that carried the G725A transition, located in the second exon, was identified. This mutation led to the substitution of a highly conserved glycine-193 to glutamic acid in the conserved fragment of the α/β hydrolase domain of the HvD14 protein. The plants that carry the hvd14.d allele were semi-dwarf and produced a higher number of tillers in comparison to the wild-type (WT) parent cultivar. Additionally, the root architecture of mutant plants was affected: the total length of the seminal roots was significantly reduced, and the density of the lateral roots was higher than in the WT. Plants with the hvd14.d allele were insensitive to treatment with GR24, which is the synthetic analogue of SL. Analysis of the indole-3-acetic acid (IAA) concentration in the lateral buds showed no differences between the WT and mutant plants. By contrast, the WT seedlings treated with GR24 developed a lower number of tillers, longer primary roots with a reduced number of lateral roots and had an increased concentration of IAA in lateral buds. This paper describes the first barley SL mutant and shows the potential functions of SLs in barley growth and development.

Perception and Signaling of Strigolactones.

Strigolactones (SLs), a recently discovered class of phytohormones, are important regulators of plant growth and development. While the biosynthetic pathway of these molecules is well documented, until recently there was not much known about the molecular mechanisms underlying SL perception and signal transduction in plants. Certain aspects of their perception and signaling, including the hormone-mediated interaction between receptor and F-box protein, degradation of suppressor proteins and activation of transcription factors, are also found in other phytohormones. However, some of SL signaling features seem to be specific for the SL signaling pathway. These include the enzymatic activity of the SL receptor and its destabilization caused by SLs. This review summarizes the current knowledge about SL signaling pathway in plants.