Kai Eggert

VRS2 regulates hormone-mediated inflorescence patterning in barley

Plant architecture has clear agronomic and economic implications for crops such as wheat and barley, as it is a critical factor for determining grain yield. Despite this, only limited molecular information is available about how grain-bearing inflorescences, called spikes, are formed and maintain their regular, distichous pattern. Here we elucidate the molecular and hormonal role of Six-rowed spike 2 (Vrs2), which encodes a SHORT INTERNODES (SHI) transcriptional regulator during barley inflorescence and shoot development. We show that Vrs2 is specifically involved in floral organ patterning and phase duration by maintaining hormonal homeostasis and gradients during normal spike development and similarly influences plant stature traits. Furthermore, we establish a link between the SHI protein family and sucrose metabolism during organ growth and development that may have implications for deeper molecular insights into inflorescence and plant architecture in crops.

Dynamics and partitioning of the ionome in seeds and germinating seedlings of winter oilseed rape.

Germination and seedling establishment are among the most critical phases in the development of plants, and seed vigour has become an important trait for the selection of robust crop cultivars. Little is known about the potentially limiting role of mineral nutrients in early metabolic and developmental processes during germination. Therefore, we assessed the ionome and relative distribution of mineral elements in different seed and seedling tissues of oilseed rape (Brassica napus L.) and monitored the internal allocation of nutrients during germination. In seeds, cotyledons harboured the main pool of K, P, S, Mg, Fe, Mn and Zn, whereas the seed coat contained most of the Ca, Na, B, Cu and Mo. Although the early root and hypocotyl tissue expanded first, concentrations of most elements were initially low. Re-allocation of elements to the root/hypocotyl tissue from other pools set in two days after seed imbibition and was most rapid for K. Relative to the critical deficiency levels of vegetative tissues, seed tissues were particularly low in B, K and Fe. Further analyses of the ionome of seeds and seedlings, grouped according to their germination efficiency, indicated that in particular low S, Mg and Ca coincided with germination failure. This study documents highly dynamic changes in the ionome of seed and seedling tissues and provides evidence for potentially limiting elements during early germination and seedling establishment in rapeseed.

Cell Type-Specific Gene Expression Analyses by RNA Sequencing Reveal Local High Nitrate-Triggered Lateral Root Initiation in Shoot-Borne Roots of Maize by Modulating Auxin-Related Cell Cycle Regulation

Auxin regulates a progressive cell cycle during adult maize lateral root formation. Plants have evolved a unique plasticity of their root system architecture to flexibly exploit heterogeneously distributed mineral elements from soil. Local high concentrations of nitrate trigger lateral root initiation in adult shoot-borne roots of maize (Zea mays) by increasing the frequency of early divisions of phloem pole pericycle cells. Gene expression profiling revealed that, within 12 h of local high nitrate induction, cell cycle activators (cyclin-dependent kinases and cyclin B) were up-regulated, whereas repressors (Kip-related proteins) were down-regulated in the pericycle of shoot-borne roots. In parallel, a ubiquitin protein ligase S-Phase Kinase-Associated Protein1-cullin-F-box proteinS-Phase Kinase-Associated Protein 2B-related proteasome pathway participated in cell cycle control. The division of pericycle cells was preceded by increased levels of free indole-3-acetic acid in the stele, resulting in DR5-red fluorescent protein-marked auxin response maxima at the phloem poles. Moreover, laser-capture microdissection-based gene expression analyses indicated that, at the same time, a significant local high nitrate induction of the monocot-specific PIN-FORMED9 gene in phloem pole cells modulated auxin efflux to pericycle cells. Time-dependent gene expression analysis further indicated that local high nitrate availability resulted in PIN-FORMED9-mediated auxin efflux and subsequent cell cycle activation, which culminated in the initiation of lateral root primordia. This study provides unique insights into how adult maize roots translate information on heterogeneous nutrient availability into targeted root developmental responses.

The role of boron nutrition in seed vigour of oilseed rape (Brassica napus L.)

Background and aimsSeed vigour is of great importance for seedling establishment and plant productivity. Despite the high boron (B) demand of oilseed rape and the potential benefit of B for improved seed vigour, it is still unclear whether B fertilization to mother plants is effective to enrich B in seeds to an extent that improves seed vigour. We addressed this question and investigated first the dynamics of B enrichment in mother plants and their seeds and then the influence of B on seed vigour using seeds that either were B-enriched from their mother plants or received B from soil fertilization.MethodsOilseed rape was grown in a pot experiment and supplied with different B levels. Developing and mature seeds were analysed for biomass and mineral element profiles. In mature seeds, embryos and seed coats were analyzed separately. Seeds were subsequently germinated under no, low, sufficient or excessive B supplies to assess germination rates and seedling establishment.ResultsDuring seedling establishment external B supply strongly promoted water and nutrient uptake as well as biomass formation. When maternal plants were supplied with elevated levels of B, seeds became enriched with B mainly in the seed coat. Sowing these seeds in low B substrate did not provide any advantage for germination or seedling establishment. By contrast, nutrient uptake and especially tissue water content strongly increased with external B supply but irrespective of seed B levels, except when excess B was supplied.ConclusionsAdditional B supply to mother plants allows for an increase in seed B levels. However, additional B is mainly the seed coat and ineffective in improving seed germination or seedling establishment. In contrast, B fertilization to the soil effectively improves seedling establishment. We conclude that adequate B supply via the soil is more effective to improve seed vigour than fertilization of mother plants for B enrichment in seeds.

Phloem-Specific Methionine Recycling Fuels Polyamine Biosynthesis in a Sulfur-Dependent Manner and Promotes Flower and Seed Development

Phloem-specific methionine recycling is sufficient to overcome sulfur-deficient growth conditions that would otherwise result in polyamine shortage and impair flower and seed development. The Yang or Met Cycle is a series of reactions catalyzing the recycling of the sulfur (S) compound 5′-methylthioadenosine (MTA) to Met. MTA is produced as a by-product in ethylene, nicotianamine, and polyamine biosynthesis. Whether the Met Cycle preferentially fuels one of these pathways in a S-dependent manner remained unclear so far. We analyzed Arabidopsis (Arabidopsis thaliana) mutants with defects in the Met Cycle enzymes 5-METHYLTHIORIBOSE-1-PHOSPHATE-ISOMERASE1 (MTI1) and DEHYDRATASE-ENOLASE-PHOSPHATASE-COMPLEX1 (DEP1) under different S conditions and assayed the contribution of the Met Cycle to the regeneration of S for these pathways. Neither mti1 nor dep1 mutants could recycle MTA but showed S-dependent reproductive failure, which was accompanied by reduced levels of the polyamines putrescine, spermidine, and spermine in mutant inflorescences. Complementation experiments with external application of these three polyamines showed that only the triamine spermine could specifically rescue the S-dependent reproductive defects of the mutant plants. Furthermore, expressing gene-reporter fusions in Arabidopsis showed that MTI1 and DEP1 were mainly expressed in the vasculature of all plant parts. Phloem-specific reconstitution of Met Cycle activity in mti1 and dep1 mutant plants was sufficient to rescue their S-dependent mutant phenotypes. We conclude from these analyses that phloem-specific S recycling during periods of S starvation is essential for the biosynthesis of polyamines required for flowering and seed development.

Response of the plant hormone network to boron deficiency

Plant hormones (PH) adjust plant growth to environmental conditions such as nutrient availability. Although responses of individual PHs to growth-determining nutrient supplies have been reported, little is known about simultaneous dynamics in the metabolism of different PH species. Brassica napus seedlings were grown under increasing supply of B, and LC-MS/MS was used to characterize bioactive forms of different PH species together with several of their precursors, storage and inactivated forms. Increasing shoot B concentrations in response to B supply were accompanied by decreasing concentrations of abscisic acid (ABA) and indole-3-acetic acid (IAA), which appeared to be synthesized under B deficiency mainly via indole-3-acetonitrile (IAN). By contrast, shoot B concentrations correlated closely with cytokinins, and the B-dependent growth response appeared to be triggered primarily by de-novo synthesis of cytokinins and by re-routing less active towards highly active forms of cytokinin. Also gibberellin biosynthesis strongly increased with B supply, in particular gibberellin species from the non-13-hydroxylation pathway. The brassinosteroid castasterone appeared to support shoot growth primarily at suboptimal B nutrition. These results indicate that a variable B nutritional status causes coordinated changes in PH metabolism as prerequisite for an adjusted growth response.

Root Engineering in Barley: Increasing Cytokinin Degradation Produces a Larger Root System, Mineral Enrichment in the Shoot and Improved Drought Tolerance

Root-specific expression of a cytokinin-degrading CKX gene in barley roots causes formation of a larger root system leading to higher element content in shoot organs and improved drought tolerance. Root size and architecture are important crop plant traits, as they determine access to water and soil nutrients. The plant hormone cytokinin is a negative regulator of root growth and branching. Here, we generated transgenic barley (Hordeum vulgare) plants with an enlarged root system by enhancing cytokinin degradation in roots to explore the potential of cytokinin modulations in improving root functions. This was achieved through root-specific expression of a CYTOKININ OXIDASE/DEHYDROGENASE gene. Enhanced biomass allocation to roots did not penalize shoot growth or seed yield, indicating that these plants were not source limited. In leaves of transgenic lines, the concentrations of several macroelements and microelements were increased, particularly those with low soil mobility (phosphorus, manganese, and zinc). Importantly, seeds contained up to 44% more zinc, which is beneficial for human nutrition. Transgenic lines also demonstrated dampened stress responses to long-term drought conditions, indicating lower drought sensitivity. Taken together, this work demonstrates that root engineering of cereals is a promising strategy to improve nutrient efficiency, biofortification, and drought tolerance.

Genome-Wide Association Study of Calcium Accumulation in Grains of European Wheat Cultivars

Mineral concentrations in cereals are important for human health, especially for people who depend mainly on consuming cereal diet. In this study, we carried out a genome-wide association study (GWAS) of calcium concentrations in wheat (Triticum aestivum L.) grains using a European wheat diversity panel of 353 varieties [339 winter wheat (WW) plus 14 of spring wheat (SW)] and phenotypic data based on two field seasons. High genotyping densities of single-nucleotide polymorphism (SNP) markers were obtained from the application of the 90k iSELECT ILLUMINA chip and a 35k Affymetrix chip. Inductively coupled plasma optical emission spectrometry (ICP-OES) was used to measure the calcium concentrations of the wheat grains. Best linear unbiased estimates (BLUEs) for calcium were calculated across the seasons and ranged from 288.20 to 647.50 among the varieties (μg g-1 DW) with a mean equaling 438.102 (μg g-1 DW), and the heritability was 0.73. A total of 485 SNP marker–trait associations (MTAs) were detected in data obtained from grains cultivated in both of the two seasons and BLUE values by considering associations with a -log10 (P-value) ≥3.0. Among these SNP markers, we detected 276 markers with a positive allele effect and 209 markers with a negative allele effect. These MTAs were found on all chromosomes except chromosomes 3D, 4B, and 4D. The most significant association was located on chromosome 5A (114.5 cM) and was linked to a gene encoding cation/sugar symporter activity as a potential candidate gene. Additionally, a number of candidate genes for the uptake or transport of calcium were located near significantly associated SNPs. This analysis highlights a number of genomic regions and candidate genes for further analysis as well as the challenges faced when mapping environmentally variable traits in genetically highly diverse variety panels. The research demonstrates the feasibility of the GWAS approach for illuminating the genetic architecture of calcium-concentration in wheat grains and for identifying putative candidate genes underlying this trait.

Grain yield and quality responses of wheat expressing a barley sucrose transporter to combined climate change factors

HOSUT wheat, enhanced in grain sucrose transport, has a superior performance for yield-related traits, partially phenocopies effects from CO2 enrichment, and benefits more from high N fertilization.

Abscisic acid influences tillering by modulation of strigolactones in barley

Repression of ABA degradation in transgenic barley alters the expression of genes involved in strigolactone biosynthesis, revealing cross-talk between ABA and strigolactone that affects the development of tillers.