The biochemistry underpinning industrial seed technology and mechanical processing of sugar beet

Aethionema arabicum dimorphic seed trait resetting during transition to seedlings

The transition from germinating seeds to emerging seedlings is one of the most vulnerable plant life cycle stages. Heteromorphic diaspores (seed and fruit dispersal units) are an adaptive bet-hedging strategy to cope with spatiotemporally variable environments. While the roles and mechanisms of seedling traits have been studied in monomorphic species, which produce one type of diaspore, very little is known about seedlings in heteromorphic species. Using the dimorphic diaspore model Aethionema arabicum (Brassicaceae), we identified contrasting mechanisms in the germination responses to different temperatures of the mucilaginous seeds (M+ seed morphs), the dispersed indehiscent fruits (IND fruit morphs), and the bare non-mucilaginous M- seeds obtained from IND fruits by pericarp (fruit coat) removal. What follows the completion of germination is the pre-emergence seedling growth phase, which we investigated by comparative growth assays of early seedlings derived from the M+ seeds, bare M- seeds, and IND fruits. The dimorphic seedlings derived from M+ and M- seeds did not differ in their responses to ambient temperature and water potential. The phenotype of seedlings derived from IND fruits differed in that they had bent hypocotyls and their shoot and root growth was slower, but the biomechanical hypocotyl properties of 15-day-old seedlings did not differ between seedlings derived from germinated M+ seeds, M- seeds, or IND fruits. Comparison of the transcriptomes of the natural dimorphic diaspores, M+ seeds and IND fruits, identified 2,682 differentially expressed genes (DEGs) during late germination. During the subsequent 3 days of seedling pre-emergence growth, the number of DEGs was reduced 10-fold to 277 root DEGs and 16-fold to 164 shoot DEGs. Among the DEGs in early seedlings were hormonal regulators, in particular for auxin, ethylene, and gibberellins. Furthermore, DEGs were identified for water and ion transporters, nitrate transporter and assimilation enzymes, and cell wall remodeling protein genes encoding enzymes targeting xyloglucan and pectin. We conclude that the transcriptomes of seedlings derived from the dimorphic diaspores, M+ seeds and IND fruits, undergo transcriptional resetting during the post-germination pre-emergence growth transition phase from germinated diaspores to growing seedlings.

Impact of climate perturbations on seeds and seed quality for global agriculture

In agriculture, seeds are the most basic and vital input on which croplands productivity depends. These implies a good starting material, good production lines and good storage options. High-quality seed lots must be free of pests and pathogens and contain a required degree of genetic purity. Seeds need also to be stored in good condition between harvest and later sowing, to insure later on the field a good plant density and higher crop yield. In general, these parameters are already widely accepted and considered in many countries where advanced technologies evaluate them. However, the more and more frequently devastating climate changes observed around the world has put seed quality under threat, and current seeds may not be adapted to hazardous and unpredictable conditions. Climate-related factors such as temperature and water availability directly affect seed development and later germination. For these reasons, investigating seed quality in response to climate changes is a step to propose new crop varieties and practices that will bring solutions for our future.

Seedborne Cercospora beticola Can Initiate Cercospora Leaf Spot from Sugar Beet (Beta vulgaris) Fruit Tissue

Cercospora leaf spot (CLS) is a globally important disease of sugar beet (Beta vulgaris) caused by the fungus Cercospora beticola. Long-distance movement of C. beticola has been indirectly evidenced in recent population genetic studies, suggesting potential dispersal via seed. Commercial sugar beet "seed" consists of the reproductive fruit (true seed surrounded by maternal pericarp tissue) coated in artificial pellet material. In this study, we confirmed the presence of viable C. beticola in sugar beet fruit for 10 of 37 tested seed lots. All isolates harbored the G143A mutation associated with quinone outside inhibitor resistance, and 32 of 38 isolates had reduced demethylation inhibitor sensitivity (EC₅₀ > 1 µg/ml). Planting of commercial sugar beet seed demonstrated the ability of seedborne inoculum to initiate CLS in sugar beet. C. beticola DNA was detected in DNA isolated from xylem sap, suggesting the vascular system is used to systemically colonize the host. We established nuclear ribosomal internal transcribed spacer region amplicon sequencing using the MinION platform to detect fungi in sugar beet fruit. Fungal sequences from 19 different genera were identified from 11 different sugar beet seed lots, but Fusarium, Alternaria, and Cercospora were consistently the three most dominant taxa, comprising an average of 93% relative read abundance over 11 seed lots. We also present evidence that C. beticola resides in the pericarp of sugar beet fruit rather than the true seed. The presence of seedborne inoculum should be considered when implementing integrated disease management strategies for CLS of sugar beet in the future.

Mechanism of Sugarbeet Seed Germination Enhanced by Hydrogen Peroxide

Seed germination is a critical first stage of plant development but can be arrested by factors including dormancy and environmental conditions. Strategies to enhance germination are of interest to plant breeders to ensure the ability to utilize the genetic potential residing inside a dormant seed. In this study, seed germination in two sugarbeet (Beta vulgaris ssp. vulgaris L.) lines F1004 and F1015 through incubating seeds in hydrogen peroxide (H₂O₂) solution was improved over 70% relative to germinating seeds through water incubation. It was further found that low germination from water incubation was caused by physical dormancy in F1015 seeds with initial seed imbibition blocked by the seed pericarp, and physiological dormancy in F1004 seeds with germination compromised due to the physiological condition of the embryo. To identify genes that are differentially expressed in response to cellular activities promoted by H₂O₂ during overcoming different type of dormancies, an RNA-Seq study was carried out and found H₂O₂ treatment during germination accelerated the degradation of seed stored mRNAs that were synthesized before or during seed storage to provide protections and maintain the dormant state. Comparison of transcripts in H₂O₂-treated seeds between the two sugarbeet lines identified differentially expressed genes (DEGs) that were higher in F1004 for alleviating physiological dormancy were known to relative to gene expression regulation. The research established that H₂O₂ overcomes both physical and physiological dormancies by hastening the transition of seeds from dormancy into germination. More DEGs related to gene expression regulation were involved in relieving physiological dormancy which provides new knowledge about the role of exogenous H₂O₂ as a signaling molecule for regulating gene activities during germination. Moreover, the protocol using H₂O₂ to promote germination will be useful for rescuing plant germplasms with poor germination.

Cold-induced secondary dormancy and its regulatory mechanisms in Beta vulgaris

The dynamic behaviour of seeds in soil seed banks depends on their ability to act as sophisticated environmental sensors to adjust their sensitivity thresholds for germination by dormancy mechanisms. Here we show that prolonged incubation of sugar beet fruits at low temperature (chilling at 5°C, generally known to release seed dormancy of many species) can induce secondary nondeep physiological dormancy of an apparently nondormant crop species. The physiological and biophysical mechanisms underpinning this cold-induced secondary dormancy include the chilling-induced accumulation of abscisic acid in the seeds, a reduction in the embryo growth potential and a block in weakening of the endosperm covering the embryonic root. Transcriptome analysis revealed distinct gene expression patterns in the different temperature regimes and upon secondary dormancy induction and maintenance. The chilling caused reduced expression of cell wall remodelling protein genes required for embryo cell elongation growth and endosperm weakening, as well as increased expression of seed maturation genes, such as for late embryogenesis abundant proteins. A model integrating the hormonal signalling and master regulator expression with the temperature-control of seed dormancy and maturation programmes is proposed. The revealed mechanisms of the cold-induced secondary dormancy are important for climate-smart agriculture and food security.

Influence of Plasma Activated Water Generated in a Gliding Arc Discharge Reactor on Germination of Beetroot and Carrot Seeds

One of the new methods of protecting and supporting plant growth is the use of low-temperature plasma. The aim of this study is to evaluate the feasibility of using plasma activated water produced in an atmospheric pressure gliding arc reactor for germination of beetroot (Beta vulgaris) and carrot (Daucus carota) seeds. The study was carried out for different plasma treatment times of water (5, 10 and 20 min) and with fixed geometry and power of the discharge system, using air as the working gas. The effect on germination was evaluated based on the fraction of germinated seeds and their length at 7 and 14 days after treatment. Analysis of fungi present on the seed surface and imaging of the seed surface using scanning electron microscopy (SEM) were auxiliary methods to evaluate the type of treatment effect. In the case of beetroot, a positive effect on the number and length of germinated seeds was observed, which increased with increasing treatment time. This effect can be attributed, among other things, to the surface changes observed on microscopic photographs. In the case of carrot seeds, a more significant positive effect on germination was observed. Fungal decontamination effect was relatively weaker than with the use of the chemical method with sodium hypochlorite.

The effects of high oxygen partial pressure on vegetable Allium seeds with a short shelf-life

Storage at an elevated partial pressure of oxygen and classical artificial ageing cause a rapid loss of seed viability of short-lived vegetable seeds. Prolonging seed longevity during storage is of major importance for gene banks and the horticultural industry. Slowing down biochemical deterioration, including oxygen-dependent deterioration caused by oxidative processes can boost longevity. This can be affected by the seed structure and the oxygen permeability of seed coat layers. Classical artificial seed ageing assays are used to estimate seed 'shelf-life' by mimicking seed ageing via incubating seeds at elevated temperature and elevated relative humidity (causing elevated equilibrium seed moisture content). In this study, we show that seed lots of vegetable Allium species are short-lived both during dry storage for several months and in seed ageing assays at elevated seed moisture levels. Micromorphological analysis of the Allium cepa x Allium fistulosum salad onion seed identified intact seed coat and endosperm layers. Allium seeds equilibrated at 70% relative humidity were used to investigate seed ageing at tenfold elevated partial pressure of oxygen (high pO₂) at room temperature (22 ºC) in comparison to classical artificial ageing at elevated temperature (42 ºC). Our results reveal that 30 days high pO₂ treatment causes a rapid loss of seed viability which quantitatively corresponded to the seed viability loss observed by ~ 7 days classical artificial ageing. A similar number of normal seedlings develop from the germinating (viable) proportion of seeds in the population. Many long-lived seeds first exhibit a seed vigour loss, evident from a reduced germination speed, preceding the loss in seed viability. In contrast to this, seed ageing of our short-lived Allium vegetable seems to be characterised by a rapid loss in seed viability.

Impact of sugar beet seed priming using the SMP method on the properties of the pericarp

BACKGROUND

This study determined the effects of two solid matrix priming methods on changes in the characteristics of two lots of the same variety of sugar beet fruits that differ in the level of vigour.

RESULTS

Seed treatment within each level of vigour did not significantly affect helium and apparent density, total pore volume and total porosity. However, there was a tendency to increase porosity due to priming. This is probably why seed priming significantly increased mesopore diameter in both high and low vigour seeds. These changes increased the water content in the pericarp and the seeds and increased the water potential during germination. The high level of electrical conductivity of the fruit extracts was associated with low seed vigour. Low vigour resulted in higher humidity of the pericarp and decreased seed moisture and was also associated with lower water potential of the pericarp and seeds.

CONCLUSIONS

A significant difference in the water content in the pericarp and seeds was indicative of imbibition and problems with water flow between these centres, which resulted in a low water diffusion coefficient of the pericarp. This low water diffusion coefficient was correlated with the prolongation of the seed germination time.