Gibberellic acid and abscisic acid modulate protein synthesis and mRNA levels in barley aleurone layers

Endophytes in Agriculture: Potential to Improve Yields and Tolerances of Agricultural Crops

Endophytic fungi and bacteria live asymptomatically within plant tissues. In recent decades, research on endophytes has revealed that their significant role in promoting plants as endophytes has been shown to enhance nutrient uptake, stress tolerance, and disease resistance in the host plants, resulting in improved crop yields. Evidence shows that endophytes can provide improved tolerances to salinity, moisture, and drought conditions, highlighting the capacity to farm them in marginal land with the use of endophyte-based strategies. Furthermore, endophytes offer a sustainable alternative to traditional agricultural practices, reducing the need for synthetic fertilizers and pesticides, and in turn reducing the risks associated with chemical treatments. In this review, we summarise the current knowledge on endophytes in agriculture, highlighting their potential as a sustainable solution for improving crop productivity and general plant health. This review outlines key nutrient, environmental, and biotic stressors, providing examples of endophytes mitigating the effects of stress. We also discuss the challenges associated with the use of endophytes in agriculture and the need for further research to fully realise their potential.

3D electron tomographic and biochemical analysis of ER, Golgi and trans Golgi network membrane systems in stimulated Venus flytrap (Dionaea muscipula) glandular cells

BACKGROUND

The insect-trapping leaves of Dionaea muscipula provide a model for studying the secretory pathway of an inducible plant secretory system. The leaf glands were induced with bovine serum albumin to secrete proteases that were characterized via zymogram activity gels over a 6-day period. The accompanying morphological changes of the endoplasmic reticulum (ER) and Golgi were analyzed using 3D electron tomography of glands preserved by high-pressure freezing/freeze substitution methods.

RESULTS

Secretion of multiple cysteine and aspartic proteases occurred biphasically. The majority of the Golgi was organized in clusters consisting of 3-6 stacks surrounded by a cage-like system of ER cisternae. In these clusters, all Golgi stacks were oriented with their cis-most C1 cisterna facing an ER export site. The C1 Golgi cisternae varied in size and shape consistent with the hypothesis that they form de novo. Following induction, the number of ER-bound polysomes doubled, but no increase in COPII vesicles was observed. Golgi changes included a reduction in the number of cisternae per stack and a doubling of cisternal volume without increased surface area. Polysaccharide molecules that form the sticky slime cause swelling of the trans and trans Golgi network (TGN) cisternae. Peeling of the trans-most cisternae gives rise to free TGN cisternae. One day after gland stimulation, the free TGNs were frequently associated with loose groups of oriented actin-like filaments which were not seen in any other samples.

CONCLUSIONS

These findings suggest that the secretory apparatus of resting gland cells is "overbuilt" to enable the cells to rapidly up-regulate lytic enzyme production and secretion in response to prey trapping.

Comparative and evolutionary analysis of α-amylase gene across monocots and dicots

α-amylase is an important enzyme involved in starch degradation to provide energy to the germinating seedling. The present study was conducted to reveal structural and functional evolution of this gene among higher plants. Discounting polyploidy, most plant species showed only a single copy of the gene making multiple isoforms in different tissues and developmental stages. Genomic length of the gene ranged from 1472 bp in wheat to 2369 bp in soybean, and the size variation was mainly due to differences in the number and size of introns. In spite of this variation, the intron phase distribution and insertion sites were mostly conserved. The predicted protein size ranged from 414 amino acid (aa) in soybean to 449aa in Brachypodium. Overall, the protein sequence similarity among orthologs ranged from 56.4 to 97.4 %. Key motifs and domains along with their relative distances were conserved among plants although several species, genera, and class specific motifs were identified. The glycosyl hydrolase superfamily domain length varied from 342aa in soybean to 384aa in maize and sorghum while length of the C-terminal β-sheet domain was highly conserved with 61aa in all monocots and Arabidopsis but was 59aa in soybean and Medicago. Compared to rice, 3D structure of the proteins showed 89.8 to 91.3 % similarity among the monocots and 72.7 to 75.8 % among the dicots. Sequence and relative location of the five key aa required for the ligand binding were highly conserved in all species except rice.

Proteomic analysis of seed dormancy in Arabidopsis

The mechanisms controlling seed dormancy in Arabidopsis (Arabidopsis thaliana) have been characterized by proteomics using the dormant (D) accession Cvi originating from the Cape Verde Islands. Comparative studies carried out with freshly harvested dormant and after-ripened non-dormant (ND) seeds revealed a specific differential accumulation of 32 proteins. The data suggested that proteins associated with metabolic functions potentially involved in germination can accumulate during after-ripening in the dry state leading to dormancy release. Exogenous application of abscisic acid (ABA) to ND seeds strongly impeded their germination, which physiologically mimicked the behavior of D imbibed seeds. This application resulted in an alteration of the accumulation pattern of 71 proteins. There was a strong down-accumulation of a major part (90%) of these proteins, which were involved mainly in energetic and protein metabolisms. This feature suggested that exogenous ABA triggers proteolytic mechanisms in imbibed seeds. An analysis of de novo protein synthesis by two-dimensional gel electrophoresis in the presence of [(35)S]-methionine disclosed that exogenous ABA does not impede protein biosynthesis during imbibition. Furthermore, imbibed D seeds proved competent for de novo protein synthesis, demonstrating that impediment of protein translation was not the cause of the observed block of seed germination. However, the two-dimensional protein profiles were markedly different from those obtained with the ND seeds imbibed in ABA. Altogether, the data showed that the mechanisms blocking germination of the ND seeds by ABA application are different from those preventing germination of the D seeds imbibed in basal medium.

Global patterns of gene expression in the aleurone of wild-type and dwarf1 mutant rice

The cereal aleurone layer is a model system for studying the regulation of transcription by gibberellin (GA) and abscisic acid (ABA). GA stimulates and ABA prevents the transcription of genes for alpha-amylases and other secreted hydrolytic enzymes, but how GA and ABA affect the transcription of other genes is largely unknown. We characterized gene expression in rice (Oryza sativa) aleurone using a half-genome rice microarray. Of the 23,000 probe sets on the chip, approximately 11,000 hybridized with RNA from rice aleurone treated with ABA, GA, or no hormone. As expected, GA regulated the expression of many genes, and 3 times as many genes were up-regulated by GA at 8 h than were down-regulated. Changes in gene expression resulting from ABA treatment were not consistent with the hypothesis that the role of ABA in this tissue is primarily to repress gene expression, and 10 times more genes were up-regulated by ABA at 8 h than were down-regulated by ABA. We also measured transcript abundance in aleurone of dwarf1 (d1) mutant rice. The d1 protein is the sole alpha-subunit of heterotrimeric G-proteins in rice. Genes up-regulated by GA or ABA had higher expression in wild type than in d1 aleurone, and genes down-regulated by GA had lower expression in wild type relative to d1 aleurone. The d1 mutation did not result in a decrease in sensitivity to GA at the level of transcription. Rather, changes in transcript abundance were smaller in the d1 mutant than in wild type.

Purification and characterization of a barley aleurone abscisic acid-binding protein

A protein designated ABAP1 and encoded by a novel gene (GenBank accession number AF127388) was purified and shown to specifically bind abscisic acid (ABA). ABAP1 protein is a 472-amino acid polypeptide containing a WW protein interaction domain and is induced by ABA in barley aleurone layers. Polyclonal antiidiotypic antibodies (AB2) cross-reacted with purified ABAP1 and with a corresponding 52-kDa protein associated with membrane fractions of ABA-treated barley aleurones. ABAP1 genes were detected in diverse monocot and dicot species, including wheat, tobacco, alfalfa, garden pea, and oilseed rape. The recombinant ABAP1 protein optimally bound (3)H-(+)-ABA at neutral pH. Denatured ABAP1 protein did not bind (3)H-(+)-ABA, nor did bovine serum albumin. The maximum specific binding as shown by Scatchard plot analysis was 0.8 mol of ABA mol(-1) protein with a linear function of r(2) = 0.94, an indication of one ABA-binding site with a dissociation constant (K(d)) of 28 x 10(-9) m. ABA binding in aleurone plasma membranes showed a maximum binding capacity of 330 nmol of ABA g(-1) protein with a K(d) of 26.5 x 10(-9) m. The similarities in the dissociation constants for ABA binding of the recombinant protein and that of the plasma membranes suggest that the protein within the plasma membrane fraction is the native form of ABAP1. The stereospecificity of ABAP1 was established by the incapability of ABA analogs and metabolites, including (-)-ABA, trans-ABA, phaseic acid, dihydrophaseic acid, and (+)-abscisic acid-glucose ester, to displace (3)H-(+)-ABA bound to ABAP1. However, two ABA precursors, (+)-ABA aldehyde and (+)-ABA alcohol, were able to displace (3)H-(+)-ABA, an indication that the structural requirement of ABAP1 at the C-1 position is not strict. Our data show that ABAP1 exerts high binding affinity for ABA. The interaction is reversible, follows saturation kinetics, and has stereospecificity, thus meeting the criteria for an ABA-binding protein.

Abscisic acid induced somatic embryogenesis in immature embryo explants of coconut (Cocos nucifera L.)

Somatic embryogenesis in coconut (Cocos nucifera L.) is generally induced by gradual reduction of auxin concentration in the culture medium and incorporation of cytokinins. Although plant regeneration through somatic embryogenesis is possible, the protocol is yet to be perfected. In this study, nodular callus was obtained from 7-9 months old immature zygotic embryos of coconut on a medium containing 24 µM 2,4-dichlorophenoxy acetic acid (2,4-D). As a novel approach, abscisic acid (ABA) at a concentration of 2.5-7.5 µM was incorporated into the culture medium for 3-7 weeks to induce somatic embryogenesis. Alternately, callus was subcultured at 5 weekly intervals on media containing gradually reducing concentrations of 2,4-D to induce somatic embryogenesis. Incorporation of ABA enhanced the production of somatic embryos. Application of 2.5-5 µM ABA for 5 weeks was found to be effective. A large number of somatic embryos developed on media containing ABA formed normal shoots and complete plants as compared to those produced in the media with low levels of 2,4-D.

Target genes and regulatory domains of the GAMYB transcriptional activator in cereal aleurone

GAMYB is an MYB transcription factor which is expressed in cereal aleurone cells in response to gibberellin (GA). HvGAMYB binds to the TAACAAA box of a high-pl alpha-amylase gene promoter and transcriptionally activates its expression. In this study, we examined the role of HvGAMYB in activating expression of other GA-regulated genes encoding hydrolytic enzymes. In transient expression experiments, HvGAMYB transactivated expression of reporter genes fused to a low-pl alpha-amylase gene promoter, an EII (1-3, 1-4)-beta-glucanase gene promoter and a cathepsin B-like protease promoter. HvGAMYB DNA binding specificity was determined using a PCR-based random site selection using HvGAMYB fusion protein isolated from E. coli. The deduced consensus closely resembled gibberellin response elements in alpha-amylase promoters. Functional analysis of HvGAMYB by transient expression of C terminal HvGAMYB deletions in barley aleurone cells identified two transcriptional activation domains (TADs) which function in transcriptional regulation of both high- and low-pl alpha-amylase promoters. The same TADs were identified using a heterologous yeast expression system. Together, these results indicate that HvGAMYB has two TADs. These domains are C-terminal to its DNA-binding domain.

Phytohormone-regulated beta-amylase gene expression in rice

The expression of beta-amylase genes in rice (Oryza sativa) and its regulation by phytohormones gibberellic acid (GA) and abscisic acid (ABA) were examined. Upon germination beta-amylase is synthesized de novo in aleurone cells and (GA) is not required. Exogenous addition of GA does not enhance the beta-amylase activity, while ABA inhibits the beta-amylase activity, mRNA accumulation, and the germination of rice seeds. GA can reverse ABA inhibition of beta-amylase expression, but not ABA inhibition of seed germination. Such regulation represents a new interaction of ABA and GA.

Gibberellin-repressible gene expression in the barley aleurone layer

Gibberellins are noted for their ability to induce expression of genes, such as alpha-amylase, in the aleurone layers of cereals. However, a number of mRNA species in the mature imbibed aleurone cell of barley, such as a storage globulin (Heck et al., Mol Gen Genet 239: 209-218 1993), are simultaneously and specifically repressed by gibberellin. In a continuing effort to understand this effect, we report cloning and characterization of two additional cDNAs from barley designated pHvGS-1 and pcHth3 that have high corresponding mRNA levels in the mature imbibed aleurone but are repressed 10-fold or more within 24 h of treatment with gibberellic acid (GA3). The extent of repression was concentration dependent and maximally effective at 10(-6) M GA3. Repression was also noted in the constitutive gibberellin response mutant, slender, in the absence of exogenous GA3. The antagonistic phytohormone, abscisic acid, had no effect or was weakly inductive of the steady-state levels of these mRNAs. During development of the seed, repressible mRNAs are present to different degrees in the maturing aleurone layer and embryo, but not in the starchy endosperm. Some repressible mRNA persists in the mature dry aleurone layer, but is degraded during imbibition, replenished by de novo transcription, and maintained at high steady-state levels until GA3 is perceived. Preliminary investigation suggests that repression is at least partly due to destabilization of the mRNAs which have estimated half-lives of 12 h or greater in the absence of GA3. pcHth3 encodes a member of the gamma-thionin gene family located on chromosome 7. pHvGS-1 corresponds to a gene on chromosome 3 of unknown function.

Post-transcriptional regulation of bifunctional alpha-amylase/subtilisin inhibitor expression in barley embryos by abscisic acid

Changes in bifunctional alpha-amylase/subtilisin inhibitor (BASI) expression induced by abscisic acid (ABA) were studied using in vitro cultured barley (Hordeum vulgare cv. Bonanza) embryos. The steady-state levels of BASI mRNA and BASI protein were increased by exogenously applied ABA. Accumulation of BASI protein was preceded by an increase in message level. The results suggest that ABA does not affect BASI mRNA translation. Nuclear run-on assays demonstrated that ABA had no effect on transcriptional activity. BASI mRNA was not detectable in the embryos treated with a protein synthesis inhibitor, cycloheximide, which had no inhibitory effect on BASI transcription rate. We propose that ABA increases the stability of BASI mRNA through synthesis of a short-lived protein that protects the message.

Domain B protruding at the third beta strand of the alpha/beta barrel in barley alpha-amylase confers distinct isozyme-specific properties

alpha-Amylases belong to the alpha/beta-barrel protein family in which the active site is created by residues located at the C-terminus of the beta strands and in the helix-connecting loops extending from these ends. In the alpha-amylase family, a small separate domain B protrudes at the C-terminus of the third beta strand of the (beta/alpha)8-barrel framework. The 80% identical barley alpha-amylase isozymes 1 and 2 (AMY1 and AMY2, respectively) differ in substrate affinity and turnover rate, CaCl2 stimulation of activity, sensitivity to the endogenous 21-kDa alpha-amylase/subtilisin inhibitor, and stability at low pH. To identify regions that confer these isozyme-specific variations, AMY1-AMY2 hybrid cDNAs were generated by in vivo homologous recombination in yeast. The hybrids AMY1-(1-90)-AMY2-(90-403) and AMY1-(1-161)-AMY2-(161-403) characterized in this study contain the 90-residue and 161-residue N-terminal sequences, respectively, of AMY1 and complementary C-terminal regions of AMY2. AMY1-(1-90)-AMY2-(90-403) comprises the 60-amino-acid domain B of AMY2 and resembles this isozyme in sensitivity to alpha-amylase/subtilisin inhibitor and its low affinity for the substrates p-nitrophenyl alpha-D-maltoheptaoside, amylose and the inhibitor acarbose. Only AMY1-(1-161)-AMY2-(161-403) and AMY1, which both share domain B, are stable at low pH. However, AMY2 and both hybrid AMY species, but not AMY1, show maximum enzyme activity on insoluble blue starch at approximately 10 mM CaCl2. Domain B thus determines several functional and stability properties that distinguish the barley alpha-amylase isozymes.

Suppression of alpha-amylase gene expression by antisense oligodeoxynucleotide in barley cultured aleurone layers

Antisense oligodeoxynucleotides (ODNs) have been applied to regulate gene expression using cell-free media or animal cells. Here we demonstrate the specific inhibition of barley alpha-amylase gene expression by synthetic antisense ODNs. In a cell free system using wheat-germ extracts, 5 microM of a 20-mer antisense ODN prevented the synthesis of the polypeptide corresponding to the predetermined length of alpha-amylase translated in vitro, whereas there was no effect on other protein synthesis. Furthermore, in cultured aleurone cells, alpha-amylase activity was efficiently decreased by addition of ODNs. At the concentrations higher than 5 microM, antisense ODN inhibited alpha-amylase gene expression almost completely. These results imply that ODN could transport into the cultured aleurone cells crossing the cell membrane, and regulate specific gene expression. This simple model system could be applicable not only for the analysis of the alpha-amylase multigene family in barley but also for studying functions of cryptic genes in higher plant.

Control of transient expression of chimaeric genes by gibberellic acid and abscisic acid in protoplasts prepared from mature barley aleurone layers

Gibberellic acid (GA3) and abscisic acid (ABA) control the transcription of alpha-amylase genes in barley aleurone cells. This control is likely to be exerted through cis-acting hormone-responsive elements in the promoter region of the gene. In order to further define these elements, we have developed procedures for obtaining transient expression of chimaeric genes in protoplasts prepared from mature barley aleurone layers. Constructs with heterologous constitutive promoters and with heterologous and homologous GA3- and ABA-regulated promoters were expressed specifically by these cells. This system would appear to offer great potential in gene regulation studies especially for hormonally regulated homologous genes. Functional analysis of a barley alpha-amylase gene has been performed using this system. A 2050 bp fragment from a high-pI alpha-amylase gene was fused to a reporter gene (GUS) and control of its expression was examined. Deletion analysis of this promoter fragment showed that major GA- and ABA-responsive elements occurred between 174 and 41 bp upstream from the transcription initiation site.

Hormonal regulation of gene expression in barley aleurone layers : Induction and suppression of specific genes

As part of our investigation of the mode of action of plant hormones in barley (Hordeum vulgare L.) aleurone layers, we have studied the expression of five identified and three unidentified mRNA species in the presence of exogenous gibberellic acid (GA3) and abscisic acid. Three of the mRNAs are GA3-inducible, three are suppressed by GA3, and two are constitutive. The extent of the GA3 effect differs considerably for both inducible and suppressible mRNAs. For example, a ten-fold higher concentration of GA3 (10(-8) M) is required for full induction of the high-pl group α-amylase mRNA than is required for the low-pI α-amylase mRNA (10(-9) M). Temporal regulation of mRNA abundance also varies between the two α-amylase isoenzyme groups. The three GA3-suppressible mRNA species studied, alcohol dehydrogenase (ADH1), a probable amylase and protease inhibitor, and an unidentified barley mRNA species also varied in response to GA3. The ADH1 mRNA decreased drastically within 8 h of GA3 treatment, whereas the other two began to decrease in abundance only after 12-16 h of GA3 treatment. Abscisic-acid treatment counteracted the GA3 effects for both the inducible and suppressible mRNA species. Comparison of α-amylase-mRNA levels and α-amylase-synthesis rates showed a strong correlation between the two parameters, the only exception being a lack of α-amylase synthesis in the presence of α-amylase mRNA at low GA3 concentrations. Therefore, the expression of α-amylase seems to be regulated primarily by its mRNA levels.

Cloning and characterization of a cDNA encoding a mRNA rapidly-induced by ABA in barley aleurone layers

Abscisic acid (ABA) inhibits the gibberellic acid induced synthesis of α-amylase in barley aleurone layers, yet ABA itself induces more than a dozen polypeptides (Lin & Ho, Plant Physiol 82: 289-297, 1986). As part of our effort to elucidate the molecular action of ABA in barley aleurone layers, we have isolated and characterized an ABA-induced cDNA clone, pHV A1. This cDNA clone hybridizes to an RNA species of approximately 1.1 kb from ABA-treated barley aleurone layers. The level of this mRNA is tripled within 40 minutes after ABA treatment, reaches a peak at 8-12 h, and is present up to 48 h. The induction of this mRNA responds to concentrations of ABA as low as 10(-9) M, but higher ABA concentrations induce higher expression of this mRNA. The products of hybrid-select translation and in vitro transcription/translation with pHV A1 comigrate on SDS gel as a 27 kDa polypeptide. However, the sequence of pHV A1 indicates that it has an open reading frame encoding a 22 kDa protein. This size discrepancy is probably due to the high content of the basic amino acid, lysine. This notion has been confirmed by two-dimensional gel electrophoresis showing that this polypeptide is one of the most basic proteins in ABA-treated barley aleurone layers. The deduced amino acid sequence of pHV A1 contains nine imperfect repeats 11 amino acids long which share homology with cotton Lea 7 protein (Baker, Steele & Dure, Plant Mol Biol, in press). The identity and function of the encoded product of pHV A1 is under investigation.

RNA complementary to α-amylase mRNA in barley

Two experimental approaches demonstrate that different types of RNA complementary to α-amylase mRNA are present in barley. S1 nuclease assays identify an RNA that is complementary to essentially the full length of both the type A and type B α-amylase mRNAs. Complementarity, however, is imperfect: the S1 nuclease-resistant products can only be identified if they are electrophoresed as RNA-DNA hybrids. This RNA is present in developing endosperm + aleurone tissue and in mature aleurone tissue cultured in the absence of hormonal treatment or in the presence of abscisic acid, but not in shoot or root tissue. In mature aleurone tissue treated with abscisic acid, its steady-state abundance is similar to that of α-amylase mRNA. Northern blot analysis indicated the presence of a second type of antisense RNA. Under conditions of moderate stringency, antisense-specific probes detect discrete hybridizing species of 1.6, 1.4, and 1.0 kilobases in mature aleurone and shoot tissues that do not represent spurious "hybridization" to rRNA, α-amylase mRNA, or the abundant, G+C-rich mRNA for a probable amylase/protease inhibitor. The different results are consistent with the fact that the hybridization assay can tolerate relatively short regions of complementarity separated by large, nonhomologous sequences, while the nuclease protection assay cannot.

The release of α-amylase through gibberellin-treated barley aleurone cell walls : An immunocytochemical study with Lowicryl K4M

Localisation of α-amylase (EC 3.2.1.1.) in low-temperature-embedded isolated barley (Hordeum vulgare L.) aleurone has been achieved using rhodamine-labelled secondary antibodies and the protein A-gold technique. Treatment with gibberellic acid (GA3) resulted in an increase of immunofluorescence in the cytoplasm of aleurone cells and also its appearance in specific regions of the cell walls. Cytoplasmic label was neither perinuclear nor associated specifically with aleurone grains as had been found in earlier work, but was present throughout the cytoplasm of all cells. A relatively high level of labelling occurred in hydrolysed wall regions. Label was also associated with plasmodesmata in both hydrolysed and unhydrolysed wall regions. The pattern of labelling indicates that α-amylase is released from aleurone via digested wall channels and that, except for the inner wall layer, unhydrolysed regions are impermeable to the enzyme. It is suggested that the resistant wall tubes around plasmodesmata may facilitate enzyme release by providing a pathway for transfer, especially of wall hydrolases, into the more impermeable parts of the wall.

Structure and organization of two divergent α-amylase genes from barley

We have isolated several α-amylase genomic clones from an Eco RI library of barley DNA in λ-Charon 32. Five of these clones exhibit unique restriction maps and differences in their abilities to hybridize with two previously characterized α-amylase cDNA probes representing two different loci, α-Amy 1 (high pI) and α-Amy 2 (low pI) on barley chromosomes 6 and 1, respectively. Stringent hybridizations indicate that four of the five genomic clones contain α-Amy 1 sequences and one contains α-Amy 2 sequences. The regions containing α-amylase genes from one representative genomic clone of each group have been sub-cloned, mapped and sequenced. S1-nuclease protection experiments indicate that the two α-amylase genes contained in these clones are functional in aleurone tissue. Transcription start sites in these genes were determined by primer extension using specific synthetic oligonucleotide primers.The DNA sequences of the two α-amylase genes, including promoter regions, are divergent, as are the predicted amino acid sequences of the mature proteins and the N-terminal "leader" peptides. The α-Amy 1 gene contains two introns while the α-Amy 2 gene has three introns. In the coding region, each gene shows 7-10% sequence divergence with respect to the previously characterized cDNA clones of the same gene type. Therefore, differences in nucleotide sequences can account for some of the isozyme variations seen between the sub-families of α-amylases and among members of the same subfamily. Although the nucleotide sequences of the promoter regions of α-Amy 1 and α-Amy 2 genes show little homology, both contain pairs of inverted repeat elements which could constitute regulatory sites.

The effect of abscisic acid on the differential expression of α-amylase isozymes in barley aleurone layers

The treatment of barley aleurone layers with gibberellic acid (GA3) results in the synthesis of two groups of α-amylase isozymes. Addition of abscisic acid (ABA) at the same time as GA3 inhibited the synthesis of both groups of isozymes. However, midcourse ABA addition (12 h or later after GA3) had a more inhibitory effect on the high pI α-amylase group than on the low pI α-amylase group. This midcourse inhibition was detectable within 2 h of ABA addition. Northern analysis results using cDNA probes for the high pI and low pI α-amylase groups paralleled the protein synthesis results for both isozyme groups. High pI α-amylase mRNA levels began to decrease within 2 h of midcourse ABA treatment and were less than 10% of the original level by 4 h. The levels of low pI α-amylase mRNA were decreased less by midcourse ABA addition than were high pI mRNA levels. Cordycepin and cycloheximide blocked the effects of midcourse ABA addition on α-amylase mRNA. These observations indicate that ABA inhibits α-amylase expression at the pretranslational level and that protein and RNA synthesis are required for midcourse ABA action to occur. Our results also show that α-amylase mRNA, which has been thought to be very stable, is degraded after midcourse ABA treatment.

Involvement of the Golgi apparatus in the secretion of α-amylase from gibberellin-treated barley aleurone cells

Localisation of α-amylase (EC 3.2.1.1) in barley aleurone cells treated with gibberellic acid has been achieved using protein A-gold-labelled polyclonal antibodies. Gold particles were located almost exclusively over the lumen of the rough endoplasmic reticulum and cisternae of the Golgi apparatus. The label was most concentrated over the Golgi apparatus. This indicates that the Golgi is involved in the secretion of α-amylase protein from aleurone cells.

Selective expression of a probable amylase/protease inhibitor in barley aleurone cells: Comparison to the barley amylase/subtilisin inhibitor

We have cloned and sequenced a 650-nucleotide cDNA from barley (Hordeum vulgare L.) aleurone layers encoding a protein that is closely related to a known α-amylase inhibitor from Indian finger millet (Eleusine coracana Gaertn.), and that has homologies to certain plant trypsin inhibitors. mRNA for this probable amylase/protease inhibitor (PAPI) is expressed primarily in aleurone tissue during late development of the grain, as compared to that for the amylase/subtilisin inhibitor, which is expressed in endosperm during the peak of storage-protein synthesis. PAPI mRNA is present at high levels in aleurone tissue of desiccated, mature grain, and in incubated aleurone layers prepared from rehydrated mature seeds. Its expression in those layers is not affected by either abscisic acid or gibberellic acid, hormones that, respectively, increase and decrease the abundance of mRNA for the amylase/subtilisin inhibitor. PAPI mRNA is almost as abundant in gibberellic acid-treated aleurone layers as that for α-amylase, and PAPI protein is synthesized in that tissue at levels that are comparable to α-amylase. PAPI protein is secreted from aleurone layers into the incubation medium.

The action of exogenous abscisic and gibberellic acids on gene expression in germinating castor beans

Exogenously applied abscisic acid inhibits isocitrate-lyase activity of the endosperm during germination of castor-bean seeds. Amounts of isocitrate-lyase mRNA have been estimated by immunoprecipitation of in-vitro-translated polypeptide products. Exogenous abscisic acid leads to an inhibition of isocitrate lyase-mRNA accumulation. A large proportion of this effect of the growth factor may be accounted for by its action in inhibiting the overall accumulation of ribosomal RNA and total mRNA. However, the effect of abscisic acid on protein synthesis is not general, as the production of some mRNAs was stimulated. The major mRNA stored in the dry seed, coding for a 25600-Mr polypeptide that normally disappears within the first 12 h of germination, exhibited high levels in abscisic-acid-treated endosperms throughout the germination period. Three complementary DNA clones, of which two clones are complementary to isocitrate lyase, have been used to measure levels of transcripts during seed germination. The accumulation of both transcripts was inhibited by exogenous abscisic acid. The data strongly indicate that the action of abscisic acid on isocitrate lyase synthesis is either to inhibit the transcription, or to increase the transcript turnover. Exogenous gibberellic acid is able to counteract the inhibitory effects of abscisic acid.

Regulation of the expression of α-amylase gene by sodium butyrate

Sodium butyrate exerts a pronounced inhibition on the gibberellic acid-induced synthesis and secretion of α-amylase by aleurone cells of barley seeds. This inhibition, which is reversible and non-competitive with cespect to gibberellic acid, is concentration dependent, with virtually total inhibition being accomplished between 4 and 5 mM sodium butyrate. The pattern of inhibition of α-amylase formation correlates well with a decrease in the accumulation of its messenger RNA. The addition of butyrate 12 h after the addition of gibberellic acid to half-seeds, has no effect on the formation and secretion of α-amylase. It has been shown in earlier studies that the synthesis of α-amylase mRNAs takes about 12 h for completion. The conclusion that butyrate interferes with some step in the transcriptional process is supported by a decrease observed in the RNAs that hybridize with a cloned α-amylase cDNA. The results of in vitro translation confirm the inhibition of the formation of several translatable mRNAs. Further, immunological probing detected only trace amounts of α-amylase proteins in the secretion of butyrate-treated seeds. Translation of functional mRNAs, post-translational modifications and the secretion α-amylase are not affected by sodium butyrate. It is concluded that butyrate selectively inhibits the transcription of several genes that are under the influence of gibberellic acid. This report is the first one documenting the inhibitory effect of sodium butyrate on a hormone-induced synthesis and accumulation of mRNAs in a plant system.

Gibberellic-acid-responsive protoplasts from mature aleurone of Himalaya barley

Gibberellic acid (GA3)-responsive protoplasts were prepared from mature aleurone layers of Himalaya barley. Protoplasts prepared in air (air-protoplasts) synthesized α-amylase (EC 3.2.1.1) in the presence of GA3 at a rate which was 4-5 times greater that in its absence. Protoplasts prepared in nitrogen (N2-protoplasts) took longer than air-protoplasts to respond to GA3 but α-amylase synthesis ultimately attained a rate which was similar to that for air-protoplasts and which was many times that occurring in the absence of the hormone. Many characteristics of the protoplast response were similar to those of intact aleurone layers. α-Amylase arose by new synthesis, its synthesis was inhibited by abscisic acid, it was isozymically similar to aleurone layer enzyme, most of it was secreted into the incubation medium and its synthesis was accompanied by accumulation of α-amylase mRNA. GA3-induced changes in protein synthesis and cell structure also resembled those of intact aleurone cells. We conclude that the response of the protoplasts to GA3 is normal and that they present a useful system for the study of GA3 action in barley aleurone.

The effects of gibberellic acid and abscisic acid on α-amylase mRNA levels in barley aleurone layers studies using an α-amylase cDNA clone

Two cDNA clones were characterized which correspond to different RNA species whose level is increased by gibberellic acid (GA3) in barley (Hordeum vulgare L.) aleurone layers. On the criteria of amino terminal sequencing, amino acid composition and DNA sequencing it is likely that one of these clones (pHV19) corresponds to the mRNA for α-amylase (1,4-α-D-glucan glucanohydrolase, EC 3.2.1.1.), in particular for the B family of α-amylase isozymes (Jacobsen JV, Higgins TJV: Plant Physiol 70:1647-1653, 1982). Sequence analysis of PHV19 revealed a probable 23 amino acid signal peptide. Southern hybridization of this clone to barley DNA digested with restriction endonucleases indicated approximately eight gene-equivalents per haploid genome.The identity of the other clone (pHV14) is unknown, but from hybridization studies and sequence analysis it is apparently unrelated to the α-amylase clone.Both clones hybridize to RNAs that are similar in size (∼1500b), but which accumulate to different extents following GA3 treatment: α-amylase mRNA increases approximately 50-fold in abundance over control levels, whereas the RNA hybridizing to pHV14 increases approximately 10-fold. In the presence of abscisic acid (ABA) the response to GA3 is largely, but not entirely, abolished. These results suggest that GA3 and ABA regulate synthesis of α-amylase in barley aleurone layers primarily through the accumulation of α-amylase mRNA.

Ethylene-regulated gene transcription in carrot roots

The plant hormone ethylene elicits many biochemical changes in target tissues. To investigate ethylene effects on expression of genetic information, cDNA clones corresponding to ethylene-induced carrot root mRNAs were constructed and isolated. RNA dot blot analysis showed that for the three clones studied peak cytosolic mRNA prevalence occurred at 21 h of treatment followed thereafter by rapid messenger decay. DNA filter excess hybridization to in vitro synthesized nuclear RNA showed that the ethylene-induced mRNA increase is engendered by transcription of previously quiescent genes. The kinetics and magnitude of changes in mRNA prevalence parallel changes in transcriptional activity; therefore the ethylene effect is primarily at the level of transcription. In vivo pulse labelling with [(35)S]-methionine showed that between 18 and 27 h of ethylene treatment a 2.5 fold increase in translational efficiency occurred for one message studied. The resulting protein is the predominant protein synthesized in carrots treated with ethylene for 27 h. Thus ethylene seemingly exerts multiple regulatory controls on the expression of genetic information.

Changes in levels of transcripts in endosperms of castor beans treated with exogenous gibberellic acid

A complementary-DNA library to mRNA from castor-bean endosperm has been prepared. Three clones have been examined in detail. One of these is complementary to isocitrate-lyase mRNA. The other two clones code for proteins with M r , 42000 and 38000. All three clones have been used to measure levels of transcripts during seed germination. The three transcripts all increased during germination and the rate of their appearance is stimulated by exogenous GA3. The data strongly support the view that the action of GA3 in these seeds is to stimulate non-specifically the rate of transcription and, in turn, protein synthesis. Possible mechanisms for the action of the growth regulator are discussed.

Abscisic acid promotes lectin biosynthesis in developing and germinating rice embryos

Immature rice (Oryza sativa, L) embryos isolated about 12 days post anthesis are fully able to develop into young seedlings when cultured in vitro. Concomitantly, they rapidly loose their lectin synthesis activity. Abscisic acid added to the nutrient medium prevents precocious germination of the immature embryos and simultaneously strongly promotes lectin biosynthesis activity. Similarly, abscisic acid keeps mature embryos grown in a nutrient medium in a dormant state and maintains their lectin synthesis activity, whereas control embryos rapidly germinate but also quickly loose their lectin synthesis activity. It appears, therefore, that rice lectin is typically synthesized in embryos which are kept in a dornant state.

Modulation by abscisic acid and S-2-aminoethyl-L-cysteine of α-amylase mRNA in barley aleurone cells

The changes in the levels of α-amylase mRNA is barley aleurone layers in response to addition of plant growth regulators have been studied using a cloned α-amylase cDNA as the hybridization probe. An increase in gibberellic acid (GA) concentration in the incubation medium from 10(-9) M to 10(-6) M results in a progressive increase in α-amylase mRNA concentration in the aleurone cells. Detectable levels of α-amylase mRNA appear in the aleurone cells as early as 1 h after addition of GA. The concentration of this mRNA increases for several hours and then declines rapidly. Abscisic acid (ABA) and the amino acid analog S-2-aminoethyl-L-cysteine (AEC) suppress the GA-mediated induction of α-amylase. These compounds appear to affect the level of α-amylase mRNA in aleurone cells as measured byin vitro translation assays and by analysis of RNA blots with α-amylase cDNA probes. It is concluded that the regulation of α-amylase gene expression by ABA is at the level of transcription. Further, a protein factor appears to be required in addition to GA for transcription of α-amylase genes.

The role of the endoplasmic reticulum in the synthesis and transport of α-amylase in barley aleurone layers

The subcellular site of α-amylase (EC 1.6.2.1) synthesis and transport was studied in barley aleurone layers incubated in the presence or absence of gibberellic acid (GA3). Using [(35)S]methionine as a marker, the site of amino-acid incorporation into organelles isolated from aleurone layers incubated with and without GA3 was determined following purification by isopycnic sucrose-density-gradient centrifugation. Incorporation of radioactivity into trichloroacetic-acid-insoluble proteins was greatest in those fractions exhibiting activity of an endoplasmic reticulum (ER) marker enzyme. Further fractionation of densitygradient fractions by sodium-dodecyl-sulfate polyacrylamide-gel electrophoresis showed that a major portion of the radioactivity in the ER fractions was present in a protein co-migrating with marker α-amylase. This protein was identified as authentic α-amylase by immunoadsorbent chromatography and affinity chromatography. The newly synthesized α-amylase associated with the ER was shown to be sequenstered within the lumen of the ER by experiments which showed that the enzyme was resistant to proteolytic degradation. The labelled α-amylase sequestered in the ER can be chased from this organelle when tissue is incubated in unlabelled methionine following a 1-h pulse of labelled methionine. The isoenzymic forms of α-amylase found in tissue homogenates and incubation media of aleurone layers incubated with and without GA3 were characterized after chromatography on diethylaminoethyl cellulose. In homogenates of GA3-treated aleurone layers, five peaks of α-amylase activity were detected, while in homogenates of aleurone layers incubated with-out GA3 only three peaks of activity were found. In incubation media, four isoenzymes were found after GA3 treatment and two were found after incubation without GA3. We conclude that at least five α-amylase isoenzymes are synthesized by the ER of barley aleurone layers and that this membrane system is involved in the sequestration and transport of four of these isoenzymes.