Scutellar Cells Research Articles

Paternal imprinting of dosage-effect defective1 contributes to seed weight xenia in maize

Historically, xenia effects were hypothesized to be unique genetic contributions of pollen to seed phenotype, but most examples represent standard complementation of Mendelian traits. We identified the imprinted dosage-effect defective1 (ded1) locus in maize (Zea mays) as a paternal regulator of seed size and development. Hypomorphic alleles show a 5–10% seed weight reduction when ded1 is transmitted through the male, while homozygous mutants are defective with a 70–90% seed weight reduction. Ded1 encodes an R2R3-MYB transcription factor expressed specifically during early endosperm development with paternal allele bias. DED1 directly activates early endosperm genes and endosperm adjacent to scutellum cell layer genes, while directly repressing late grain-fill genes. These results demonstrate xenia as originally defined: Imprinting of Ded1 causes the paternal allele to set the pace of endosperm development thereby influencing grain set and size.

The location of iron and zinc in grain of conventional and biofortified lines of sorghum

Sorghum is an important source of dietary iron (Fe) and zinc (Zn) in parts of Africa and India, but there is a need to increase their concentrations to meet dietary requirements. Grains of a genetically biofortified sorghum line (Parbhani Shakti) had higher concentrations of Fe and Zn than a control line (M35-1). Analysis at the tissue level by histochemical staining and at the cellular level using NanoSIMS showed that both minerals are concentrated in the aleurone layer and in the scutellum of the embryo, with Zn also being concentrated in the embryonic axis. However, NanoSIMS showed that “hot spots” of 56Fe+ and 64Zn+ were also present in the sub-aleurone and starchy endosperm cells. Most of these hot spots also contained 31P16O+ indicating that the Fe and Zn are present as phytates, as in the aleurone and scutellum cells. Low concentrations of 56Fe+ and 64Zn+ were also observed in the protein matrix of these cells.

One-Week Scutellar Somatic Embryogenesis in the Monocot Brachypodium distachyon.

Plant somatic embryogenesis (SE) is a natural process of vegetative propagation. It can be induced in tissue cultures to investigate developmental transitions, to create transgenic or edited lines, or to multiply valuable crops. We studied the induction of SE in the scutellum of monocots with Brachypodium distachyon as a model system. Towards the in-depth analysis of SE initiation, we determined the earliest stages at which somatic scutellar cells acquired an embryogenic fate, then switched to a morphogenetic mode in a regeneration sequence involving treatments with exogenous hormones: first an auxin (2,4-D) then a cytokinin (kinetin). Our observations indicated that secondary somatic embryos could already develop in the proliferative calli derived from immature zygotic embryo tissues within one week from the start of in vitro culture. Cell states and tissue identity were deduced from detailed histological examination, and in situ hybridization was performed to map the expression of key developmental genes. The fast SE induction method we describe here facilitates the mechanistic study of the processes involved and may significantly shorten the production of transgenic or gene-edited plants.

Metabolic coordination of rice seed development to nighttime warming: In-situ determination of cellular redox states using picolitre pressure-probe electrospray-ionization mass spectrometry

High night temperature (HNT) at the ripening stage severely affects both rice yield and quality. HNT accelerates embryo growth and chalky formation in the developing grains, accompanying with a diminishment of endosperm cell size. Although these responses may be physiologically interacted each other in the grains, what signals are involved in the accelerated embryo development remains undetermined. In this work, we have used picolitre pressure-probe electrospray-ionization mass spectrometry (picoPPESI-MS) to conduct single-cell metabolomics at several regions in HNT-treated grains, embryonic scutellum and outer endosperms in the basal (‘chalky region’ at maturation) and middle (‘translucent region’ at maturation as a reference) positions. Microscopic observations showed that HNT promoted cell expansion rate in the scutellum. When embryonic cell expansion rate reached the maximum, spatial differences in several metabolisms including ascorbate-glutathione (ASC-GSH) pathway and purine were detected, together with considerable sugar and amino acid accumulations in embryonic scutellum cells. There was no treatment difference in GSH content during active cell expansion in HNT-treated embryos, although an increase in GSH/GSSG ratio due to a reduction in oxidized glutathione (GSSG) content has been contrastingly observed. In the endosperms, greater ASC accumulation with a difference in ASC/dehydroascobic acid ratio has been also detected under HNT conditions. Since dormancy is often correlated with GSSG concentration, it is concluded that spatial regulation of GSH redox homeostasis detected at cell-level might be essential for dormancy alleviation and embryo growth accelerated in HNT-treated seeds.

Transcriptome Profiling Predicts New Genes to Promote Maize Callus Formation and Transformation.

Maize transformation is highly based on the formation of embryonic callus, which is mainly derived from scutellum cells of the immature maize embryo. However, only a few genes involved in callus induction have been identified in maize. To reveal the potential genes involved in the callus induction of maize, we carried out a high-throughput RNA sequencing on embryos that were cultured for 0, 1, 2, 4, 6, and 8 days, respectively, on a medium containing or lacking 2,4-dichlorophenoxyacetic acid. In total, 7,525 genes were found to be induced by 2,4-dichlorophenoxyacetic acid and categorized into eight clusters, with clusters 2 and 3 showing an increasing trend related to signal transmission, signal transduction, iron ion binding, and heme binding. Among the induced genes, 659 transcription factors belong to 51 families. An AP2 transcription factors, ZmBBM2, was dramatically and rapidly induced by auxin and further characterization showed that overexpression of ZmBBM2 can promote callus induction and proliferation in three inbred maize lines. Therefore, our comprehensive analyses provide some insight into the early molecular regulations during callus induction and are useful for further identification of the regulators governing callus formation.

The overexpression of rice ACYL-CoA-BINDING PROTEIN2 increases grain size and bran oil content in transgenic rice.

As Oryza sativa (rice) seeds represent food for over three billion people worldwide, the identification of genes that enhance grain size and composition is much desired. Past reports have indicated that Arabidopsis thaliana acyl-CoA-binding proteins (ACBPs) are important in seed development but did not affect seed size. Herein, rice OsACBP2 was demonstrated not only to play a role in seed development and germination, but also to influence grain size. OsACBP2 mRNA accumulated in embryos and endosperm of germinating seeds in qRT-PCR analysis, while β-glucuronidase (GUS) assays on OsACBP2pro::GUS rice transformants showed GUS expression in embryos, as well as the scutellum and aleurone layer of germinating seeds. Deletion analysis of the OsACBP2 5'-flanking region revealed five copies of the seed cis-element, Skn-I-like motif (-1486/-1482, -956/-952, -939/-935, -826/-822, and -766/-762), and the removal of any adversely affected expression in seeds, thereby providing a molecular basis for OsACBP2 expression in seeds. When OsACBP2 function was investigated using osacbp2 mutants and transgenic rice overexpressing OsACBP2 (OsACBP2-OE), osacbp2 was retarded in germination, while OsACBP2-OEs performed better than the wild-type and vector-transformed controls, in germination, seedling growth, grain size and grain weight. Transmission electron microscopy of OsACBP2-OE mature seeds revealed an accumulation of oil bodies in the scutellum cells, while confocal laser scanning microscopy indicated oil accumulation in OsACBP2-OE aleurone tissues. Correspondingly, OsACBP2-OE seeds showed gain in triacylglycerols and long-chain fatty acids over the vector-transformed control. As dietary rice bran contains beneficial bioactive components, OsACBP2 appears to be a promising candidate for enriching seed nutritional value.

Intra-Kernel Reallocation of Proteins in Maize Depends on VP1-Mediated Scutellum Development and Nutrient Assimilation.

During maize (Zea mays) seed development, the endosperm functions as the major organ for storage of photoassimilate, serving to nourish the embryo. α-Zeins and globulins (GLBs) predominantly accumulate in the maize endosperm and embryo, respectively. Here, we show that suppression of α-zeins by RNA interference (αRNAi) in the endosperm results in more GLB1 being synthesized in the embryo, thereby markedly increasing the size and number of protein storage vacuoles. Glb genes are strongly expressed in the middle-to-upper section of the scutellum, cells of which are significantly enlarged by αRNAi induction. Elimination of GLBs caused an apparent reduction in embryo protein level, regardless of whether α-zeins were expressed or suppressed in the endosperm, indicating that GLBs represent the dominant capacity for storage of amino acids allocated from the endosperm. It appears that protein reallocation is mostly regulated at the transcriptional level. Genes differentially expressed between wild-type and αRNAi kernels are mainly involved in sulfur assimilation and nutrient metabolism, and many are transactivated by VIVIPAROUS1 (VP1). In vp1 embryos, misshapen scutellum cells contain notably less cellular content and are unable to respond to αRNAi induction. Our results demonstrate that VP1 is essential for scutellum development and protein reallocation from the endosperm to embryo.

Rice florigen gene Hd3a has conserved functions in callus development

The transition of meristems is an important developmental process for crop plants. Florigen is considered to be produced in leaves, then moves into the shoot apical meristem (SAM), triggers the transition from the vegetative to the reproductive phase. However, little is known whether Florigen functions in callus development or not. By fused reporter gene β-glucuronidase (GUS) to 1.7 kb promoter of Heading date 3a (Hd3a), GUS signals were detected in the scutellum cells, as well as in green point of the putative transgenic calli. Quantitative RT-PCR results demonstrated that the expression level of Hd3a was increased gradually over time along with the transition from scutellum-deprived callus to shoot. As reported that ectopic expression of FT-like genes caused earlier flowering, we also found that 80% constitutive expression of Hd3a transgenic callus showed formation floral-like organ structures. However, Hd3a RNA interference (RNAi) transgenic calli did not show any obvious phenotype, although AP1 or AP1-like genes—OsMADS14, OsMADS15, and OsMADS18- expression level is decreased during callus development. Both in Hd3a and RFT1 overexpression transgenic calli, Hd3a also modulated AP1 or AP1-like genes, as well AEPALLATA (SEP)-like gene, OsMADS34 during green point formation. Meanwhile, transgenic calli of RFT1and OsMADS50, but not OsEhd1, shared similar results as Hd3a. All of these findings suggested that florigen genes Hd3a and RFT1 have partial conserved functions in the transition of meristems during callus development.

The Associative Changes in Scutellum Nuclear Content and Morphology with Viability Loss of Naturally Aged and Accelerated Aging Wheat (Triticum aestivum) Seeds.

Timely prediction of seed viability loss over long-term storage represents a challenge in management and conservation of ex situ plant genetic resources. However, little attention has been paid to study the process of seed deterioration and seed aging signals under storage. An attempt was made here to investigate morphological and molecular changes in the scutellum and aleurone sections of naturally or artificially aged wheat seeds using TUNEL assay and DAPI staining. Twelve wheat genotypes or samples exposed to natural ageing (NA) or accelerated ageing (AA) were assayed and these samples had germination rates ranging from 11 to 93%. The assayed samples showed substantial changes in scutellum, but not aleurone. The nuclei observed in the majority of the scutellum cells of the NA seed samples of lower germination rates were longer in size and less visible, while the scutellum cell morphology or arrangement remained unchanged. In contrast, longer AA treatments resulted in the loss of scutellum cell structure, collapse of cell layers, and disappearance of honey comb arrangements. These nuclei and structural changes were consistent with the DNA assessments of nuclear alternations for the selected wheat samples. Interestingly, the sample seed germination loss was found to be associated with the reductions in the scutellum nuclear content and with the increases in the scutellum nuclei length to width ratio. These findings are significant for understanding the process of wheat seed deterioration and are also useful for searching for sensitive seed aging signals for developing tools to monitor seed viability under storage.

Control of Rice Embryo Development, Shoot Apical Meristem Maintenance, and Grain Yield by a Novel Cytochrome P450

Angiosperm seeds usually consist of two major parts: the embryo and the endosperm. However, the molecular mechanism(s) underlying embryo and endosperm development remains largely unknown, particularly in rice, the model cereal. Here, we report the identification and functional characterization of the rice GIANT EMBRYO (GE) gene. Mutation of GE resulted in a large embryo in the seed, which was caused by excessive expansion of scutellum cells. Post-embryonic growth of ge seedling was severely inhibited due to defective shoot apical meristem (SAM) maintenance. Map-based cloning revealed that GE encodes a CYP78A subfamily P450 monooxygenase that is localized to the endoplasmic reticulum. GE is expressed predominantly in the scutellar epithelium, the interface region between embryo and endosperm. Overexpression of GE promoted cell proliferation and enhanced rice plant growth and grain yield, but reduced embryo size, suggesting that GE is critical for coordinating rice embryo and endosperm development. Moreover, transgenic Arabidopsis plants overexpressing AtCYP78A10, a GE homolog, also produced bigger seeds, implying a conserved role for the CYP78A subfamily of P450s in regulating seed development. Taken together, our results indicate that GE plays critical roles in regulating embryo development and SAM maintenance.

The scutellum of germinated wheat grains undergoes programmed cell death: identification of an acidic nuclease involved in nucleus dismantling.

Programmed cell death (PCD) is a crucial phenomenon in the life cycle of cereal grains. In germinating grains, the scutellum allows the transport of nutrients from the starchy endosperm to the growing embryo, and therefore it may be the last grain tissue to undergo PCD. Thus, the aim of this work was to analyse whether the scutellum of wheat grains undergoes PCD and to perform a morphological and biochemical analysis of this process. Scutellum cells of grains following germination showed a progressive increase of DNA fragmentation, and the TUNEL assay showed that PCD extended in an apical-to-basal gradient along the scutellum affecting epidermal and parenchymal cells. Electron-transmission microscopy revealed high cytoplasm vacuolation, altered mitochondria, and the presence of double-membrane structures, which might constitute symptoms of vacuolar cell death, whereas the nucleus appeared lobed and had an increased heterochromatin content as the most distinctive features. An acid- and Zn2+-dependent nucleolytic activity was identified in nuclear extracts of scutellum cells undergoing PCD. This nuclease was not detected in grains imbibed in the presence of abscisic acid, which inhibited germination. This nucleolytic activity promoted DNA fragmentation in vitro on nuclei isolated from healthy cells, thus suggesting a main role in nucleus dismantling during PCD.

Control of the scutellar epithelial cell elongation during germination in maize (Zea mays L.)

Summary The scutellum is the single cotyledon of grass embryos. The cells in direct contact with the endosperm form the scutellar epithelium which plays a major role in controlling the mobilization of the endosperm nutrient reserves. After imbibition, the epithelial cells elongate which leads to an increase in the contact area between the endosperm and the scutellum. We studied which factors regulate epithelial cell elongation. Elongation was inhibited by ABA and salicylic acid, basic and acid pH and high concentrations of sorbitol. Exogenous gibberellins stimulated elongation, but a reduction in gibberellin synthesis or perception did not inhibit it. Elongation was increased by sucrose, but not glucose. The application of transcription and translation inhibitors reduced elongation. Experimental and discussion The scutellum is the cotyledon of the grass embryo and the part of the embryo in contact with the starchy endosperm. The scutellar cells in direct contact with the endosperm form the scutellar epithelium (also known as glandular layer or secretory layer). This is formed by a continuous layer of cylindrical cells and constitutes a secretory and absorptive organ that plays a major role in controlling the mobilization of the endosperm nutrient reserves. Once the seed starts imbibition, scutellar epithelial cells undergo a series of morphological changes. In the mature seeds, epithelial cells are slightly elongated towards the outer layer and are densely packed without inter-cellular spaces. During imbibition the epithelial cells lose their adhesion and elongate perpendicularly to the scutellar surface and form finger-like extensions that project into the starchy endosperm (Bewley and Black, 1994). This process contributes to an increase in the contact area between the endosperm and scutellum, thus providing the exposure of a greater surface for secretion and absorption. Little is known about the physiological mechanisms that control epithelial cell elongation.

Pollen source effects on growth of kernel structures and embryo chemical compounds in maize

Previous studies have reported effects of pollen source on the oil concentration of maize (Zea mays) kernels through modifications to both the embryo/kernel ratio and embryo oil concentration. The present study expands upon previous analyses by addressing pollen source effects on the growth of kernel structures (i.e. pericarp, endosperm and embryo), allocation of embryo chemical constituents (i.e. oil, protein, starch and soluble sugars), and the anatomy and histology of the embryos. Maize kernels with different oil concentration were obtained from pollinations with two parental genotypes of contrasting oil concentration. The dynamics of the growth of kernel structures and allocation of embryo chemical constituents were analysed during the post-flowering period. Mature kernels were dissected to study the anatomy (embryonic axis and scutellum) and histology [cell number and cell size of the scutellums, presence of sub-cellular structures in scutellum tissue (starch granules, oil and protein bodies)] of the embryos. Plants of all crosses exhibited a similar kernel number and kernel weight. Pollen source modified neither the growth period of kernel structures, nor pericarp growth rate. By contrast, pollen source determined a trade-off between embryo and endosperm growth rates, which impacted on the embryo/kernel ratio of mature kernels. Modifications to the embryo size were mediated by scutellum cell number. Pollen source also affected (P < 0.01) allocation of embryo chemical compounds. Negative correlations among embryo oil concentration and those of starch (r = 0.98, P < 0.01) and soluble sugars (r = 0.95, P < 0.05) were found. Coincidently, embryos with low oil concentration had an increased (P < 0.05-0.10) scutellum cell area occupied by starch granules and fewer oil bodies. The effects of pollen source on both embryo/kernel ratio and allocation of embryo chemicals seems to be related to the early established sink strength (i.e. sink size and sink activity) of the embryos.

A hydrogen peroxide detoxification system in the nucleus of wheat seed cells

Aerobic metabolism inevitably produces reactive oxygen species (ROS), including hydrogen peroxide, which may cause damage to the cell. Besides this toxic effect, hydrogen peroxide has an important signalling function in plant development and response to environmental stimuli. So, the balance of toxic and signaling effects of hydrogen peroxide is highly dependent on mechanisms to adjust its level in the different cell compartments. We recently described a redox system, formed by NADPH thioredoxin reductase (NTR) and 1-Cys peroxiredoxin (1-Cys Prx), able to use the reducing power of NADPH to reduce hydrogen peroxide. This system is localized in the nucleus of wheat seed cells and probably has an important antioxidant function in aleurone and scutellum cells, which suffer oxidative stress during seed development and germination. We discuss here the possibility that the control of the level of hydrogen peroxide in the nucleus may be important to balance redox regulation of gene expression and cell death in cereal seed cells.

An antioxidant redox system in the nucleus of wheat seed cells suffering oxidative stress

Cereal seed cells contain different mechanisms for protection against the oxidative stress that occurs during maturation and germination. One such mechanism is based on the antioxidant activity of a 1-Cys peroxiredoxin (1-Cys Prx) localized in the nuclei of aleurone and scutellum cells. However, nothing is known about the mechanism of activation of this enzyme. Here, we describe the pattern of localization of NADPH thioredoxin reductase (NTR) in developing and germinating wheat seeds using an immunocytochemical analysis. The presence of NTR in transfer cells, vascular tissue, developing embryo and root meristematic cells, agrees with the localization of thioredoxin h (Trx h), and supports the important function of the NTR/Trx system in cell proliferation and communication. Interestingly, NTR is found in the nuclei of seed cells suffering oxidative stress, thus showing co-localization with Trx h and 1-Cys Prx. To test whether the NTR/Trx system serves as a reductant of the 1-Cys Prx, we cloned a full-length cDNA encoding 1-Cys Prx from wheat, and expressed the recombinant protein in Escherichia coli. Using the purified components, we show NTR-dependent activity of the 1-Cys Prx. Mutants of the 1-Cys Prx allowed us to demonstrate that the peroxidatic residue of the wheat enzyme is Cys46, which is overoxidized in vitro under oxidant conditions. Analysis of extracts from developing and germinating seeds confirmed 1-Cys Prx overoxidation in vivo. Based on these results, we propose that NADPH is the source of the reducing power to regenerate 1-Cys Prx in the nuclei of seed cells suffering oxidative stress, in a process that is catalyzed by NTR.

Artificial chromosome formation in maize (Zea mays L.)

We report on the construction of maize minichromosomes using shuttle vectors harboring native centromeric segments, origins of replication, selectable marker genes, and telomeric repeats. These vectors were introduced into scutellar cells of maize immature embryos by microprojectile bombardment. Several independent transformation events were identified containing minichromosomes in addition to the normal diploid complement of 20 maize chromosomes. Immunostaining indicated that the minichromosomes recruited centromeric protein C, which is a specific component of the centromere/kinetochore complex. Minichromosomes were estimated to be 15-30 Mb in size based on cytological measurements. Fluorescent in situ hybridization (FISH) showed that minichromosomes contain the centromeric, telomeric, and exogenous unique marker sequences interspersed with maize retrotransposons. Minichromosomes were detected for at least a year in actively dividing callus cultures, providing evidence for their stability through numerous cell cycles. Plants were regenerated and minichromosomes were detected in root tips, providing confirmation of their normal replication and transmission during mitosis and through organogenesis. Assembly of maize artificial chromosomes may provide a tool to study centromere function and a foundation for developing new high capacity vectors for plant functional genomics and breeding.

Characteristic and Expression Analysis of a Metallothionein Gene, OsMT2b, Down-Regulated by Cytokinin Suggests Functions in Root Development and Seed Embryo Germination of Rice

Metallothioneins (MTs) are low molecular mass and cysteine-rich metal-binding proteins known to be mainly involved in maintaining metal homeostasis and stress responses. But, their functions in higher plant development are scarcely studied. Here, we characterized rice (Oryza sativa) METALLOTHIONEIN2b (OsMT2b) molecularly and found that its expression was down-regulated by cytokinins. OsMT2b was preferentially expressed in rice immature panicles, scutellum of germinating embryos, and primordium of lateral roots. In contrast with wild-type plants, OsMT2b-RNA interference (RNAi) transgenic plants had serious handicap in plant growth and root formation, whereas OsMT2b-overexpressing transformants were dwarfed and presented more adventitious roots and big lateral roots. The increased cytokinin levels in RNAi plants and decreased cytokinin levels in overexpressing plants were confirmed by high-performance liquid chromatography quantitative analysis in the roots of wild-type and transgenic plants. In RNAi plants, localization of isopentenyladenosine, a kind of endogenous cytokinin, in roots and germinating embryos expanded to the whole tissues, whereas in overexpressing plants, the isopentenyladenosine signals were very faint in the vascular tissues of roots and scutellum cells of germinating embryos. In vitro culture of embryos could largely resume the reduced germination frequency in RNAi plants but had no obvious change in overexpressing plants. Taken together, these results indicate a possible feedback regulation mechanism of OsMT2b to the level of endogenous cytokinins that is involved in root development and seed embryo germination of rice.

Oligonucleotide-directed gene repair in wheat using a transient plasmid gene repair assay system

Oligonucleotide-directed gene repair is a potential technique for agricultural trait modification in economically important crops. However, large variation in the repair frequencies among the scientific reports indicates that there are many factors influencing the repair process. We report here a transient assay system using GFP as a reporter for testing the efficiency of plasmid DNA repair in cultured wheat cells. This assay showed that osmotic medium supplemented with 2,4-D increased the oligo-targeting frequency, and that the repair of a point mutation was more efficient than repair of a single base deletion mutation in cultured scutellum cells of immature wheat embryos. This study provides the first evidence that oligonucleotide-directed mutagenesis is applicable to regenerable cultured wheat scutellum cells.

The concentrations and distributions of phytic acid phosphorus and other mineral nutrients in wild-type and low phytic acid Js-12-LPA wheat (Triticum aestivum) grain parts

Concentrations of P, phytic acid (myo-inositol hexakisphosphate, IP6), and other mineral storage elements were studied in wild-type and low phytic acid (lpa) genotype Js-12-LPA wheat (Triticum aestivum L.) embryos and rest-of-grain fractions. Environmental scanning electron microscopy images revealed a decreased average size and an increased number of aleurone layer globoids in lpa grains compared with the wild type. Energy-dispersive X-ray analyses of unfixed aleurone layer and scutellum cell cytoplasm revealed mainly C, O, P, K, and Mg in both grain types. The starchy endosperm contained virtually no P, K, or Mg, demonstrating no shift of mineral nutrients to that compartment. Scanning transmission electron microscopy – energy-dispersive X-ray analyses of scutellum and aleurone layer globoids in both genotypes revealed that P, K, and Mg were the main mineral nutrients in globoids with low amounts of Ca, Fe, and Zn. Traces of Mn were only in scutellum globoids. Total P was similar between genotypes for the rest-of-grain fractions, which are 97% of grain mass. The main inositol phosphate was IP6, but a small amount of IP5 was present. Both lpa grain fractions exhibited major reductions in IP6 compared with the wild type and a threefold increase in inorganic P. The concentration of K decreased in both fractions, while Ca increased 25% in the Js-12-LPA rest-of-grain compared with the wild type. The lack of large differences in mineral concentration and distribution between the wild type and Js-12-LPA indicates that there is no direct role of localization of IP6 synthesis in mineral distribution.

Distribution of Cadmium, Lead, Nickel, and Strontium in Imbibing Maize Caryopses

Seed germination is tolerant to heavy metals apparently because the seed coat is impermeable to metal ions. However, it is not clear whether the seed coat is a universal barrier for all metals. In addition, depending on their physical and chemical properties, a distribution of various metals may differ within an imbibing caryopsis, and therefore they produce dissimilar effects on seed germination. The toxic effects of Cd(NO3)2, Pb(NO3)2, Ni(NO3)2, and Sr(NO3)2 were estimated from the germination rates of maize (Zea mays L.) caryopses following two-day incubation with these salts. The distribution of heavy metals and Sr was studied by histochemical methods based on the formation of colored complexes with dithizone (Cd and Pb), dimethylglyoxyme (Ni), and sodium rhodizonate (Sr). Although the metals under study did not affect maize radicle protrusion, they inhibited seed germination in the following order: Cd > Ni ≈ Pb > Sr. Cd and Pb accumulated mainly in the seed coat cells, but Sr and Ni in the embryo cells and in the cells of endosperm (Sr) and scutellum (Ni). Although Cd was found only in the seed coat, it was the strongest inhibitor of seed germination. Apparently, due to high toxicity, Cd exerted its inhibitory effect at the concentrations too low for histochemical assay. In spite of easy translocation across the seed coat of imbibing caryopses, Sr did not considerably inhibit radicle protrusion and seed germination, apparently because of its low toxicity and predominant localization in the apoplast of embryo and endosperm cells.