Mechanisms of Maturation and Germination in Crop Seeds Exposed to Environmental Stresses with a Focus on Nutrients, Water Status, and Reactive Oxygen Species

Genotype-specific germination behavior induced by sustainable priming techniques in response to water deprivation stress in rice

Water stress brought about by climate change is among the major global concerns threatening food security. Rice is an important staple food which requires high water resources. Being a semi-aquatic plant, rice is particularly susceptible to drought. The aim of this work was to develop techniques directed to promote rice resilience to water deprivation stress during germination by implementing specific seed priming treatments. Five popular Italian rice varieties were subjected to priming treatments using novel, sustainable solutions, like poly-gamma-glutamic acid (γ-PGA), denatured γ-PGA (dPGA), and iron (Fe) pulsing, alone or in combination. The effect of the developed priming methods was tested under optimal conditions as well as under water deprivation stress imposed by polyethylene glycol (PEG) treatments. The priming efficacy was phenotypically determined in terms of germination behavior by measuring a series of parameters (germinability, germination index, mean germination time, seed vigor index, root and shoot length, germination stress tolerance index). Biochemical analyses were carried out to measure the levels of iron uptake and accumulation of reactive oxygen species (ROS). Integrative data analyses revealed that the rice varieties exhibited a strong genotype- and treatment-specific germination behavior. PEG strongly inhibited germination while most of the priming treatments were able to rescue it in all varieties tested except for Unico, which can be defined as highly stress sensitive. Molecular events (DNA repair, antioxidant response, iron homeostasis) associated with the transition from seed to seedling were monitored in terms of changes in gene expression profiles in two varieties sensitive to water deprivation stress with different responses to priming. The investigated genes appeared to be differentially expressed in a genotype-, priming treatment-, stress- and stage-dependent manner. The proposed seed priming treatments can be envisioned as sustainable and versatile agricultural practices that could help in addressing the impact of climate challenges on the agri-food system.

Noninvasive Methods to Detect Reactive Oxygen Species as a Proxy of Seed Quality

ROS homeostasis is crucial to maintain radical levels in a dynamic equilibrium within physiological ranges. Therefore, ROS quantification in seeds with different germination performance may represent a useful tool to predict the efficiency of common methods to enhance seed vigor, such as priming treatments, which are still largely empirical. In the present study, ROS levels were investigated in an experimental system composed of hydroprimed and heat-shocked seeds, thus comparing materials with improved or damaged germination potential. A preliminary phenotypic analysis of germination parameters and seedling growth allowed the selection of the best-per-forming priming protocols for species like soybean, tomato, and wheat, having relevant agroeconomic value. ROS levels were quantified by using two noninvasive assays, namely dichloro-dihydro-fluorescein diacetate (DCFH-DA) and ferrous oxidation-xylenol orange (FOX-1). qRT-PCR was used to assess the expression of genes encoding enzymes involved in ROS production (respiratory burst oxidase homolog family, RBOH) and scavenging (catalase, superoxide dismutase, and peroxidases). The correlation analyses between ROS levels and gene expression data suggest a possible use of these indicators as noninvasive approaches to evaluate seed quality. These findings are relevant given the centrality of seed quality for crop production and the potential of seed priming in sustainable agricultural practices.

Mepiquat chloride inhibits soybean growth but improves drought resistance

Soybeans are an important economic crop. As the most widely used growth regulator globally, the molecular mechanism of mepiquat chloride (DPC) in soybean remains unknown. In this study, RNA sequencing technology combined with ultra-performance liquid chromatography and tandem mass spectrometry were used to analyze the changes in the leaf transcriptome and metabolomics of soybean leaves at the seedling stage under DPC stress. The results showed that differentially expressed genes related to photosynthesis and cell wall synthesis were significantly downregulated at the transcriptional level. In addition, the syntheses of gibberellin, zeatin, brassinolide, and other plant hormones were inhibited in the signal transduction pathway of plant hormones, thereby inhibiting plant growth. In contrast, at the metabolic level, the expression levels of flavonoid differential metabolites were significantly increased, and the proportions of flavonoids in the two varieties were 61.5 and 66%, respectively. The combined analysis of transcriptome and metabolomics showed that the differential expressed genes and metabolites were mainly enriched in the isoflavonoid biosynthesis and flavonoid biosynthesis pathways. Principally, DPC inhibited plant growth but improved drought resistance. Our study is the first to report the molecular mechanism of DPC regulation in soybean, providing useful insights into the rational application of DPC in soybean.

RNA-Seq Analysis Demonstrates Different Strategies Employed by Tiger Nuts (Cyperus esculentus L.) in Response to Drought Stress

Drought stress, an important abiotic stress, has affected global agricultural production by limiting the yield and the quality of crops. Tiger nuts (Cyperus esculentus L.) are C4 crops in the Cyperaceae family, which have high-quality wholesome ingredients. However, data on mechanisms underlying the response of tiger nuts to drought stress are few. Here, the variety of Jisha 1 and 15% polyethylene glycol (PEG; a drought stress simulator) were used to study the mechanisms of stress response in tiger nuts. Our evaluation of the changes in physiological indicators such as electrolyte leakage (El), malondialdehyde (MDA), hydrogen peroxide (H₂O₂), superoxide anion (O₂⁻) and activities of reactive oxygen species (ROS) showed that 12 h was the most suitable time point to harvest and analyze the response to drought stress. Thereafter, we performed transcriptome (RNA-Seq) analysis in the control (CK) and stress treatment groups and showed that there was a total of 1760 differentially expressed genes (DEGs). Gene Ontology (GO) analysis showed that the DEGs were enriched in abscisic acid (ABA) terms, and pathways such as starch and sucrose metabolism (ko00500), phenylpropanoid biosynthesis (ko00940) and plant hormone signal transduction (ko04075) were significantly enriched in the Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis. In addition, quantitative real-time PCR (qRT-PCR) analysis of the DEGs demonstrated an upregulation of ABA and lignin content, as well as enzyme activities in enriched pathways, which validated the RNA-Seq data. These results revealed the pathways and mechanisms adopted by the tiger nuts in response to drought stress.

Biosynthesis of Nano-Selenium and Its Impact on Germination of Wheat under Salt Stress for Sustainable Production

Selenium and its derivatives have been found capable of excellent biological responses. However, the element in its bulk form has low bioavailability and increased toxicity, meaning the production of effective forms with sustainable methods has become urgent. Several microorganisms, including fungi, bacteria and yeast, as well as higher plants, are capable of biosynthesizing nanoparticles such as nano-selenium (nano-Se), which has wide applications in medicine, agriculture and industry. Thus, the biosynthesis of nano-Se using some bacterial species was the main target of this study. The production of nano-Se and the monitoring of its impact on the wheat germination of seeds under salt stress (i.e., 50, 100, and 150 mM NaCl) was also evaluated in the current study. The ameliorative role of nano-Se doses (i.e., 50, 75, and 100 mg L−1) in the germination of wheat seeds under salt stress was also investigated. Based on sodium selenite tolerance and reducing selenite to elemental Se-NPs, the most effective isolate (TAH) was selected for identification using the 16S rRNA gene sequence, which belonged to Bacillus cereus TAH. The final germination percent, mean germination time, vigor index and germination rate index were improved by 25, 25, 39.4 and 11%, respectively, under 15 mM sodium chloride concentration when 100 mg L−1 nano-selenium was used. On the other hand, the results obtained from a gnotobiotic sand system reveal that with treatment with 100 mg L−1 nano-selenium under high Ec values of 14 ds m−1, the vegetative growth parameters of shoot length, root length, fresh weight and dry weight were improved by 22.8, 24.9, 19.2 and 20%, respectively, over untreated controls. The data obtained from this study reveal that the use of nano-selenium produced by Bacillus cereus offers improved wheat seed germination under a salt-affected environment.

Elucidation of the miR164c-Guided Gene/Protein Interaction Network Controlling Seed Vigor in Rice

MicroRNAs (miRNAs) play important roles in various aspects of plant physiology and metabolism. The expression level of miR164c is negatively correlated with seed vigor in rice (Oryza sativa L.); however, the mechanism of seed vigor regulation by miR164c remains unknown. Anti-aging capacity is an important indicator of seed vigor. Here, we report an miR164c-guided gene/protein interaction network that regulates the anti-aging ability of rice seeds. Seeds of the wild-type (WT) rice cultivar "Kasalath" and its transgenic derivatives, miR164c-silenced line (MIM164c) and miR164c overexpression line (OE164c), with significant differences in anti-aging capacity, showed significant differences in gene and protein expression levels. The differentially expressed genes (DEGs) or proteins were significantly enriched in six metabolic functional categories related to seed vigor, including "stress response," "protein processing in endoplasmic reticulum (ER)," "embryo development," "serine-type endopeptidase inhibitor," "energy metabolism," and "other." Differences in the expression levels of genes or proteins related to energy metabolism, serine endopeptidase, and stress response in seeds under normal storage conditions may be associated with anti-aging capacity. The results of gene/protein interaction analyses suggest that miR164c first targets PSK5, and the PSK5 protein then interacts with the ubiquitin-associated gene RPS27AA, which simultaneously impacts the genes/proteins in the six above-mentioned functional categories. Expression levels of some of the key genes and proteins in the interaction network were verified by real-time fluorescence quantitative PCR (RT-qPCR) and multiple reaction monitoring mass spectrometry (MRM-MS), respectively. Thus, the present study provides new insights into the miRNA-mediated gene and protein interaction network that regulates seed vigor.

miR164c and miR168a regulate seed vigor in rice

MicroRNAs (miRNAs) are key regulators of gene expression in many important biological processes of plants. However, few miRNAs have been shown to regulate seed vigor. Here, we conducted microarray assays to analyze miRNA expression levels in seeds of the rice (Oryza sativa L.) cultivar ZR02. Results showed significant differences in the expression of 11 miRNAs between artificially aged and untreated control seeds. Among these, osa-miR164c was transcriptionally upregulated, while osa-miR168a was downregulated in artificially aged seeds; this was verified by quantitative real-time PCR analysis. Under the same aging condition, osa-miR164c overexpression in OE164c transgenic seeds and osa-miR168a silencing in MIM168a transgenic seeds of the rice cultivar Kasalath led to lower germination rates, whereas osa-miR164c silencing in MIM164c and osa-miR168a overexpression in OE168a resulted in higher seed germination rates compared with wild-type seeds. Meanwhile, changes in cytomembrane permeability of seeds and in the expression level of osa-miR164c target genes (OsPM27 and OsPSK5) and osa-miR168a target genes (OsAGO1 and OsPTR2) under aging conditions coincided with changes in seed vigor induced by osa-miR164c and osa-miR168a. Thus, genetic manipulation of miRNAs has important implications in the development of crop cultivars with high vigor and extended life span of seeds.