The perisperm-endosperm envelope in Cucumis: structure, proton diffusion and cell wall hydrolysing activity

Insights into the maternal pathway for Cucumber green mottle mosaic virus infection of cucurbit seeds

Cucumber green mottle mosaic virus (CGMMV), genus Tobamovirus, is a major pathogen of cucurbits that primarily affects cucumber, melon, and watermelon crops. The aim of this study was to reveal the contribution of CGMMV-infected female flowers to disease spread. Using a fluorescent in situ hybridization (FISH) technique, we show that ovaries and ovules of CGMMV-infected cucumber and melon plants showed a CGMMV-specific fluorescence signal prior to and following anthesis. The fluorescence signal was prominent but sporadic. Ripe fruits of infected melon plants showed strong signals in the funiculus, the seed stalk, which connects the developing seed to the interior ovary wall. Importantly, in seeds, a strong fluorescence signal was observed in the perisperm-endosperm (PE) envelope, which underlies the seed coat and surrounds the embryo. Interestingly, the fluorescence signal was not uniformly distributed in the PE envelope but was localized to a specific envelope layer. These results have important epidemiological implications for CGMMV management and commercial seed production, particularly regarding the improvement of seed disinfection methods that will contribute to limit the global distribution of the virus.

The mechanism behind lack-of-effect of lanthanum on seed germination of switchgrass

Switchgrass (Panicum virgatum L.) is a perennial warm-season C₄ grass identified as a model species for bioenergy feedstock production. Lanthanum (La) as a rare earth element can stimulate the physiological processes of plant growth. The purpose of this study was to investigate the effect of lanthanum on seed germination of switchgrass. However, no significant differences in seed germination were found. The energy dispersive X-ray analysis showed that abundant lanthanum deposits resided on the pericarp and testa of the seed while few lanthanum deposits were present on the aleurone and endosperm. This phenomenon demonstrates that a semi-permeable layer, which could restrict or impede solute exchange, while allowing the permeability of internal and external water and gas, may be located between the testa and aleurone. Light microscopy and histochemical analysis revealed that the main chemical composition of the semi-permeable layer would be expected to be suberin because the layer was stained yellow with aniline blue. The quantum chemical calculations predict that the intervals between adjacent carbon chains in suberin molecule are so small that lanthanum ([La(H₂O)₈]³⁺) cannot pass through the suberin molecule. In conclusion, the seed germination of switchgrass is not affected by lanthanum because the semi-permeable layer restricts the penetration of lanthanum into the embryo.

Peroxidase activity in scutella of maize in association with anatomical changes during germination and grain storage

The embryo of the maize grain (Zea mays L.) is separated from the starchy endosperm by a fibrous structure, which is called the fibrous layer (FL). Using histochemical staining, it was determined that the FL is composed of collapsed cellular layers that contain phenols, neutral lipids, and 1,3-β-glucan. Due to its composition, the FL prevents free diffusion and separates the embryo from the endosperm during germination. Twenty-four hours after imbibition, the scutellum epidermis initiated a series of asynchronous spatial modifications, including cell growth, the perforation of cell walls, increased peroxidase activity in the apoplastic space, and elevated levels of superoxide, phenols, and other components that interact with the fibrous layer, enabling its transformation in addition to the free flow between compartments. During storage at high relative humidity levels, which leads to fast or slow deterioration depending on the temperature, the activity of phenol peroxidase in the scutellum was associated with a loss of vigor and reduced germination capacity when compared with low temperature and low relative humidity conditions. Such deterioration is associated with alterations in autofluorescent emissions from endogenous compounds in the scutellum, indicating changes in the microenvironment or in the differential proportions of epidermal and FL components.

Location of Acidovorax citrulli in infested watermelon seeds is influenced by the pathway of bacterial invasion

Watermelon seeds can become infested by Acidovorax citrulli, the causal agent of bacterial fruit blotch (BFB) of cucurbits via penetration of the ovary pericarp or by invasion of the pistil. This study investigated the effect of these invasion pathways on A. citrulli localization in seeds. Seed samples (n = 20 or 50 seeds/lot) from pistil- and pericarp-inoculated lots were dissected into testa, perisperm-endosperm (PE) layer, and embryo tissues and tested for A. citrulli by species-specific polymerase chain reaction (PCR) and by plating on semiselective media. Less than 8% of the testa samples were A. citrulli-positive regardless of the method of seed inoculation. Additionally, the difference in percentages of contaminated testae between the two seed lot types was not significant (P = 0.64). The percentage of A. citrulli-positive PE layer samples as determined by real-time PCR assay was significantly greater for seeds from pistil-inoculated lots (97%) than for seeds from pericarp-inoculated lots (80.3%). The mean percentage of A. citrulli-positive embryo samples was significantly greater for seeds from pistil-inoculated lots (94%) than for seeds from pericarp-inoculated lots (≈8.8%) (P = 0.0001). Removal of PE layers and testae resulted in a significant reduction in BFB seed-to-seedling transmission percentage for seeds from pericarp-inoculated lots (14.8%) relative to those from pistil-inoculated lots (72%). Additionally, using immunofluorescence microscopy, A. citrulli cells were observed in the PE layers and the cotyledons of pistil-inoculated seeds but only in the PE layers of pericarp-inoculated seeds. These results suggest that pericarp invasion results in superficial contamination of the testae and PE layers while pistil invasion results in the deposition of A. citrulli in seed embryos.

Alleviation of salt stress-induced inhibition of seed germination in cucumber (Cucumis sativus L.) by ethylene and glutamate

Ethylene is an important plant gas hormone, and the amino acid Glu is emerging as a messenger molecule in plants. To evaluate the role of ethylene and Glu in seed germination and radicle growth under salt stress, effects of 1-aminocyclopropane-1-carboxylic acid (ACC), Ethephon and Glu on germination and radicle growth of cucumber (Cucumis sativus L.) seeds in the absence and presence of 200 mM NaCl were investigated. Seed germination was markedly inhibited by salt stress, and this effect was alleviated by ACC and Ethephon. In contrast to seed germination, ACC and Ethephon had little effect on radicle growth under salt stress. In addition to ethylene, we found exogenous supply of Glu was effective in alleviating the salt stress-induced inhibition of seed germination and radicle growth. The effect of Glu on the seed germination and radicle growth was specific to L-Glu, whereas D-Glu and Gln had no effect. There was an increase in ethylene production during seed imbibition, and salt stress suppressed ethylene production. Exogenous L-Glu evoked ethylene evolution from the imbibed seeds and attenuated the reduction in ethylene evolution induced by salt stress. The alleviative effect of L-Glu on seed germination was diminished by antagonists of ethylene synthesis, aminoethoxyvinylglycine (AVG) and CoCl(2), suggesting that L-Glu is likely to exert its effect on seed germination by modulation of ethylene evolution. These findings demonstrate that ethylene is associated with suppression of seed germination under salt stress and that L-Glu interacts with ethylene in regulation of seed germination under salt stress.

Solute permeation across the apoplastic barrier in the perisperm-endosperm envelope in cucumber seeds

An apoplastic barrier consisting of callose and lipid layers in the perisperm-endosperm (PE) envelope is known to restrict inward and outward transport of solutes in cucurbit seeds. The present work examines permeability properties of the barrier using cucumber seed as a model system. Osmometrically determined osmotic potential of the apoplastic fluid was used as a basis for osmotic studies aimed at examining solute exclusion from the apoplastic barrier in the PE envelope. The assessment of apoplastic permeability involved measuring the amount of anionic and cationic organic dyes diffused into agarose gel discs through the PE envelope. Ionic/non-ionic solutes including polyethylene glycols having Stokes radii <or= 0.6 nm showed considerable permeation through the apoplastic barrier in the PE envelope as indicated by greater seed thickness/breadth ratios. Permeances of dyes across the PE envelope were in the order: 2,6-dichlorophenolindophenol (DCPIP) > 2,3,5-triphenyltetrazolium chloride (TTC) approximately methyl orange approximately methylene blue > Eosin Y >> Janus green approximately crystal violet approximately Evans Blue. Permeation time(0.5) for DCPIP and TTC was 9.71 and 9.96 h, respectively. Dyes having Stokes radii < 0.5 nm showed significant inward as well as outward diffusion across the PE envelope in contrast to restricted diffusion of dyes having Stokes radii > 0.5 nm. Size exclusion limit for apoplastic barrier in cucumber PE envelope was resolved to be about 0.5 nm by dye permeation and around 0.8 nm by osmotic studies. Dye permeances depended primarily on particle size as described by a quadratic polynomial function rather than on charge or log D.

The oxygen status of the developing seed

Recent applications of oxygen-sensitive microsensors have demonstrated steep oxygen gradients in developing seeds of various crops. Here, we present an overview on oxygen distribution, major determinants of the oxygen status in the developing seed and implications for seed physiology. The steady-state oxygen concentration in different seed tissues depends on developmental parameters, and is determined to a large extent by environmental factors. Photosynthetic activity of the seed significantly diminishes hypoxic constraints, and can even cause transient, local hyperoxia. Changes in oxygen availability cause rapid adjustments in mitochondrial respiration and global metabolism. We argue that nitric oxide (NO) is a key player in the oxygen balancing process in seeds, avoiding fermentation and anoxia in vivo. Molecular approaches aiming to increase oxygen availability within the seed are discussed.

Transcriptional profiling of cell wall protein genes in chickpea embryonic axes during germination and growth

Cell wall hydrolases have been assumed to be involved in the regulation of seed germination, mostly through their contribution to the cell wall disassembly associated with endosperm cap weakening. In Cicer arietinum (a non-endospermic leguminosae seed), we have focused our research directly on the elongation process of the embryonic axes themselves during germination. The genes encoding cell wall proteins, previously implicated in the elongation of chickpea epicotyls, might also be involved in the expansion of embryonic axis cells, and the modulation of their expression could be part of the control of the germinative process. Thus, chickpea alpha-expansins and xyloglucan endotransglycosylase/hydrolase (XTH) acting on the cellulose/xyloglucan network seem to be involved in the elongation of both chickpea epicotyls and embryonic axes, although the products of different genes perform their actions on each organ. Among the four known cDNAs encoding chickpea alpha-expansins, Ca-EXPA1 was the only isoform highly expressed in embryonic axes during germination. In contrast to epicotyl elongation, the genes encoding cell wall beta-galactosidases, involved in pectin degradation, were not expressed during germination, suggesting no role in embryonic axis elongation, mainly due to the different metabolism of pectins during cell wall loosening in embryonic axis or epicotyl cells. The results concerning CanST-1 and -2, encoding two growth-related cell wall proteins, suggested that these genes were not involved in elongation of embryonic axes during germination. The transcription pattern of Cap28, which encodes a glutamic acid rich cell wall protein of unknown function, indicated a role in the development of the embryonic axes during germination.

Seed dormancy and the control of germination

Seed dormancy is an innate seed property that defines the environmental conditions in which the seed is able to germinate. It is determined by genetics with a substantial environmental influence which is mediated, at least in part, by the plant hormones abscisic acid and gibberellins. Not only is the dormancy status influenced by the seed maturation environment, it is also continuously changing with time following shedding in a manner determined by the ambient environment. As dormancy is present throughout the higher plants in all major climatic regions, adaptation has resulted in divergent responses to the environment. Through this adaptation, germination is timed to avoid unfavourable weather for subsequent plant establishment and reproductive growth. In this review, we present an integrated view of the evolution, molecular genetics, physiology, biochemistry, ecology and modelling of seed dormancy mechanisms and their control of germination. We argue that adaptation has taken place on a theme rather than via fundamentally different paths and identify similarities underlying the extensive diversity in the dormancy response to the environment that controls germination.