Seed Priming with Polyamines Improves the Germination and Early Seedling Growth in Fine Rice

Comparative Metabolomics Profiling Reveals Key Metabolites and Associated Pathways Regulating Tuber Dormancy in White Yam (Dioscorea rotundata Poir.)

Yams are economic and medicinal crops with a long growth cycle, spanning between 9-11 months due to their prolonged tuber dormancy. Tuber dormancy has constituted a major constraint in yam production and genetic improvement. In this study, we performed non-targeted comparative metabolomic profiling of tubers of two white yam genotypes, (Obiaoturugo and TDr1100873), to identify metabolites and associated pathways that regulate yam tuber dormancy using gas chromatography-mass spectrometry (GC-MS). Yam tubers were sampled between 42 days after physiological maturity (DAPM) till tuber sprouting. The sampling points include 42-DAPM, 56-DAPM, 87DAPM, 101-DAPM, 115-DAPM, and 143-DAPM. A total of 949 metabolites were annotated, 559 in TDr1100873 and 390 in Obiaoturugo. A total of 39 differentially accumulated metabolites (DAMs) were identified across the studied tuber dormancy stages in the two genotypes. A total of 27 DAMs were conserved between the two genotypes, whereas 5 DAMs were unique in the tubers of TDr1100873 and 7 DAMs were in the tubers of Obiaoturugo. The differentially accumulated metabolites (DAMs) spread across 14 major functional chemical groups. Amines and biogenic polyamines, amino acids and derivatives, alcohols, flavonoids, alkaloids, phenols, esters, coumarins, and phytohormone positively regulated yam tuber dormancy induction and maintenance, whereas fatty acids, lipids, nucleotides, carboxylic acids, sugars, terpenoids, benzoquinones, and benzene derivatives positively regulated dormancy breaking and sprouting in tubers of both yam genotypes. Metabolite set enrichment analysis (MSEA) revealed that 12 metabolisms were significantly enriched during yam tuber dormancy stages. Metabolic pathway topology analysis further revealed that six metabolic pathways (linoleic acid metabolic pathway, phenylalanine metabolic pathway, galactose metabolic pathway, starch and sucrose metabolic pathway, alanine-aspartate-glutamine metabolic pathways, and purine metabolic pathway) exerted significant impact on yam tuber dormancy regulation. This result provides vital insights into molecular mechanisms regulating yam tuber dormancy.

Changes in polyamine contents during Fusarium graminearum and Fusarium verticillioides inoculation in maize seedlings with or without seed-priming

Maize (Zea mays L.) is the most produced field crop all over the world. One of its most critical diseases that results in economic loss is ear rot caused by various Fusarium species. Previous researches have shown that polyamines, found in all living cells, play crucial role in biotic stress responses. At the same time, biosynthesis of polyamines is of paramount importance not only for plants but also for their pathogens to promote stress tolerance and pathogenicity. In our study, we investigated the polyamine content changes induced in the seedlings of two maize genotypes of different susceptibility by isolates of Fusarium verticillioides and Fusarium graminearum, two Fusarium species of different lifestyles. Apart from that, it was examined how infection efficiency and changes in polyamine contents were modified by salicylic acid or putrescine seed soaking pre-treatments. Our observations confirmed that initial and stress-induced changes in the polyamine contents are not directly related to tolerance in either coleoptile or radicle. However, the two pathogens with different lifestyles induced remarkably distinct changes in the polyamine contents. The effect of the seed soaking pre-treatments depended on the pathogens and plant resistance as well: both salicylic acid and putrescine seed soaking had positive results against F. verticillioides, while in the case of infection with F. graminearum, seed soaking with distilled water alone affected biomass parameters positively in the tolerant genotype.

The chances in the redox priming of nondormant recalcitrant seeds by spermidine

The problems posed by seed sensitivity to desiccation and aging have motivated the development of various techniques for mitigating their detrimental effects. The redox priming of seeds in antioxidant solution to improve their postharvest performance is one of the approaches. Spermidine (Spd) was tested as an invigorating solution on nondormant recalcitrant (desiccation sensitive) seeds of the silver maple (Acer saccharinum L.). The treatment resulted in an 8-10% increase in germination capacity in seeds subjected to mild and severe desiccation, while in aged seeds stored for six months, no significant change was observed. The cellular redox milieu, genetic stability, mitochondrial structure and function were investigated to provide information about the cellular targets of Spd activity. Spd improved the antioxidative capacity, especially the activity of catalase, and cellular membrane stability, protected genome integrity from oxidative damage and increased the efficiency of mitochondria. However, it also elicited a hydrogen peroxide burst. Therefore, it seems that redox priming in nondormant seeds that are highly sensitive to desiccation, although positively affected desiccated seed performance, may not be a simple solution to reinvigorate stored seeds with a low-efficiency antioxidant system.

Genetic mapping of maize metabolites using high-throughput mass profiling

Plant metabolites are the basis of human nutrition and have biological relevance in ecology. Farmers selected plants with favorable characteristics since prehistoric times and improved the cultivars, but without knowledge of underlying mechanisms. Understanding the genetic basis of metabolite production can facilitate the successful breeding of plants with augmented nutritional value. To identify genetic factors related to the metabolic composition in maize, we generated mass profiles of 198 recombinant inbred lines (RILs) and their parents (B73 and Mo17) using direct-injection electrospray ionization mass spectrometry (DLI-ESI MS). Mass profiling allowed the correct clustering of samples according to genotype. We quantified 71 mass features from grains and 236 mass features from leaf extracts. For the corresponding ions, we identified tissue-specific metabolic 'Quantitative Trait Loci' (mQTLs) distributed across the maize genome. These genetic regions could regulate multiple metabolite biosynthesis pathways. Our findings demonstrate that DLI-ESI MS has sufficient analytical resolution to map mQTLs. These identified genetic loci will be helpful in metabolite-focused maize breeding. Mass profiling is a powerful tool for detecting mQTLs in maize and enables the high-throughput screening of loci responsible for metabolite biosynthesis.

Putrescine: A Key Metabolite Involved in Plant Development, Tolerance and Resistance Responses to Stress

Putrescine (Put) is the starting point of the polyamines (PAs) pathway and the most common PA in higher plants. It is synthesized by two main pathways (from ornithine and arginine), but recently a third pathway from citrulline was reported in sesame plants. There is strong evidence that Put may play a crucial role not only in plant growth and development but also in the tolerance responses to the major stresses affecting crop production. The main strategies to investigate the involvement of PA in plant systems are based on the application of competitive inhibitors, exogenous PAs treatments, and the most efficient approaches based on mutant and transgenic plants. Thus, in this article, the recent advances in understanding the role of this metabolite in plant growth promotion and protection against abiotic and biotic stresses will be discussed to provide an overview for future research.

Comparative Metabolomics Reveals Two Metabolic Modules Affecting Seed Germination in Rice (Oryza sativa)

The process of seed germination is crucial not only for the completion of the plant life cycle but also for agricultural production and food chemistry; however, the underlying metabolic regulation mechanism involved in this process is still far from being clearly revealed. In this study, one indica variety (Zhenshan 97, with rapid germination) and one japonica variety (Nipponbare, with slow germination) in rice were used for in-depth analysis of the metabolome at different germination stages (0, 3, 6, 9, 12, 24, 36, and 48 h after imbibition, HAI) and exploration of key metabolites/metabolic pathways. In total, 380 annotated metabolites were analyzed by using a high-performance liquid chromatography (HPLC)-based targeted method combined with a nontargeted metabolic profiling method. By using bioinformatics and statistical methods, the dynamic changes in metabolites during germination in the two varieties were compared. Through correlation analysis, coefficient of variation analysis and differential accumulation analysis, 74 candidate metabolites that may be closely related to seed germination were finally screened. Among these candidates, 29 members belong to the ornithine–asparagine–polyamine module and the shikimic acid–tyrosine–tryptamine–phenylalanine–flavonoid module. As the core member of the second module, shikimic acid’s function in the promotion of seed germination was confirmed by exogenous treatment. These results told that nitrogen flow and antioxidation/defense responses are potentially crucial for germinating seeds and seedlings. It deepens our understanding of the metabolic regulation mechanism of seed germination and points out the direction for our future research.

Enhancement of Brassica napus Tolerance to High Saline Conditions by Seed Priming

Plants grown in saline soils undergo osmotic and oxidative stresses, affecting growth and photosynthesis and, consequently, the yield. Therefore, the increase in soil salinity is a major threat to crop productivity worldwide. Plant's tolerance can be ameliorated by applying simple methods that induce them to adopt morphological and physiological adjustments to counteract stress. In this work, we evaluated the effects of seed priming on salt stress response in three cultivars of rapeseed (Brassica napus L.) that had different tolerance levels. Seed chemical priming was performed with 2.5 mM spermine (SPM), 5 mM spermidine (SPD), 40 mM NaCl and 2.5 mM Ca (NO₃)₂. Primed and not primed seeds were sown on saline and not saline (controls) media, and morphological and physiological parameters were determined. Since SPD treatment was effective in reducing salinity negative effects on growth, membrane integrity and photosynthetic pigments, we selected this priming to further investigate plant salt stress response. The positive effects of this seed treatment on growth and physiological responses were evident when primed plants were compared to not primed ones, grown under the same saline conditions. SPD priming ameliorated the tolerance towards saline stress, in a genotype-independent manner, by increasing photosynthetic pigments, proline amounts and antioxidant responses in all cultivars exposed to salt. These results may open new perspectives for crop productivity in the struggle against soil salinization.

Polyamines modulate growth, antioxidant activity, and genomic DNA in heavy metal-stressed wheat plant

A pot experiment was performed to assess the useful effects of seed soaking or seedling foliar spray using 0.25 mM spermine (Spm), 0.50 mM spermidine (Spd), or 1 mM putrescine (Put) on heavy metal tolerance in wheat plants irrigated with water contaminated by cadmium (2 mM Cd²⁺ in CdCl₂) or lead (2 mM Pb²⁺ in PbCl₂). Cd²⁺ or Pb²⁺ presence in the growth medium resulted in significant reductions in growth and yield characteristics and activities of leaf peroxidase (POD), glutathione reductase (GR), ascorbic acid oxidase (AAO), and polyphenol oxidase (PPO) of wheat plants. In contrast, significant increases were observed for Cd²⁺ content in roots, leaves and grains, superoxide dismutase (SOD) and catalase (CAT) activities, radical scavenging activity (DPPH), reducing power capacity, and fragmentation in DNA in comparison to controls (without Cd²⁺ or Pb²⁺ addition). However, treating the Cd²⁺- or Pb²⁺-stressed wheat plants with Spm, Spd, or Put, either by seed soaking or foliar spray, significantly improved growth and yield characteristics and activities of POD, GR, AAO, PPO, SOD, and CAT, DPPH, and reducing power capacity in wheat plants. In contrast, Cd²⁺ levels in roots, leaves, and yielded grains, and fragmentation in DNA were significantly reduced compared with the stressed (with Cd²⁺ or Pb²⁺) controls. Generally, seed soaking treatments were more effective than foliar spray treatments. More specifically, seed priming in Put was the best treatment under heavy metal stress. Results of this study recommend using polyamines, especially Put, as seed soaking to relieve the adverse effects of heavy metals in wheat plants.

Spermidine Enhances Heat Tolerance of Rice Seeds by Modulating Endogenous Starch and Polyamine Metabolism

Polyamines have been reported to be involved in grain filling and they might contribute to the construction of heat resistance of some cereals. In this study, the hybrid rice 'YLY 689' was used to explore the possible effects of exogenous spermidine (Spd) on seed quality under high temperature during the filling stage. Rice spikes were treated with Spd or its synthesis inhibitor cyclohexylamine (CHA) after pollination, and then the rice plants were transferred to 40 °C for 5-day heat treatment. The results showed that, compared with the control under high temperature, Spd pretreatment significantly improved the germination percentage, germination index, vigor index, seedling shoot height, and dry weight of seeds harvested at 35 days after pollination, while the CHA significantly decreased the seed germination and seedling growth. Meanwhile, Spd significantly increased the peroxidase (POD) activity and decreased the malondialdehyde (MDA) content in seeds. In addition, after spraying with Spd, the endogenous content of spermidine and spermine and the expression of their synthetic genes, spermidine synthase (SPDSYN) and spermine synthase (SPMS1 and SPMS2), significantly increased, whereas the accumulation of amylose and total starch and the expression of their related synthase genes, soluble starch synthase II-3 (SS II-3) and granules bound starch synthase I (GBSSI), also increased to some extent. The data suggests that exogenous Spd pretreatment could alleviate the negative impacts of high temperature stress on rice seed grain filling and improve the rice seed quality to some extent, which might be partly caused by up-regulating endogenous polyamines and starch metabolism.

Potential Applications of Polyamines in Agriculture and Plant Biotechnology

The polyamines putrescine, spermidine and spermine have been implicated in a myriad of biological functions in many organisms. Research done during the last decades has accumulated a large body of evidence demonstrating that polyamines are key modulators of plant growth and development. Different experimental approaches have been employed including the measurement of endogenous polyamine levels and the activities of polyamine metabolic enzymes, the study of the effects resulting from exogenous polyamine applications and chemical or genetic manipulation of endogenous polyamine titers. This chapter reviews the role of PAs in seed germination, root development, plant architecture, in vitro plant regeneration, flowering and plant senescence. Evidence presented here indicates that polyamines should be regarded as plant growth regulators with potential applications in agriculture and plant biotechnology.

Exogenous spermidine improves seed germination of white clover under water stress via involvement in starch metabolism, antioxidant defenses and relevant gene expression

This study was designed to determine the effect of exogenous spermidine (Spd) (30 μM) on white clover seed germination under water stress induced by polyethylene glycol 6000. Use of seed priming with Spd improved seed germination percentage, germination vigor, germination index, root viability and length, and shortened mean germination time under different water stress conditions. Seedling fresh weight and dry weight also increased significantly in Spd-treated seeds compared with control (seeds primed with distilled water). Improved starch metabolism was considered a possible reason for this seed invigoration, since seeds primed with Spd had significantly increased α-amylase/β-amylase activities, reducing sugar, fructose and glucose content and transcript level of β-amylase gene but not transcript level of α-amylase gene. In addition, the physiological effects of exogenous Spd on improving seeds’ tolerance to water deficit during germination were reflected by lower lipid peroxidation levels, better cell membrane stability and significant higher seed vigour index in seedlings. Enhanced antioxidant enzyme activities (superoxide dismutase, peroxidase, catalase and ascorbate peroxidase), ascorbate-glutathione cycle (ASC-GSH cycle) and transcript level of genes encoding antioxidant enzymes induced by exogenous Spd may be one of the critical reasons behind acquired drought tolerance through scavenging of reactive oxygen species (ROS) in water-stressed white clover seeds. The results indicate that Spd plays an important function as a stress-protective compound or physiological activator.