Seed development and vivipary in Zea mays L

Non-viviparous pre-dispersal seed germination in Amaranthaceae in the cold deserts of Central Asia

In the broad context of understanding the relationship between timing of seed germination and adaptation of a plant species to its habitat, the purpose of this study was to purse an observation of pre-dispersal seed germination of Salsola brachiata (Amaranthaceae) in late winter 2021 in the Amaranthaceae species-rich cold deserts in northwest China (Central Asia). We searched for pre-dispersal germination in species of Amaranthaceae growing in sand dunes (S), salt deserts (SD) and gravel deserts (GD). We examined 69 species in 155 populations in autumn 2021 and 52 species in 12 populations in early spring 2022. No seeds of any of the 69 species germinated on the mother plants in autumn 2021, while 30 of 52 species (57.7%) did so during snowmelt in early spring 2022. The rank order of species with few to many seeds germinated on the mother plants was annuals (66.7%) > small shrubs (23.3%) > small trees (6.7%) > shrubs (3.3%). The number of species in S, SD, and GD with pre-dispersal germinated seeds was 16 of 27 (59.3%), 15 of 31 (48.4%), and 15 of 30 (50.0%), respectively. The high species occurrence of pre-dispersal germination in early spring suggested that it might be adaptive in the unpredictable-rainfall growing-season environment of the cold deserts of Central Asia, a center of diversity of Amaranthaceae. However, preliminary studies on seedling/juvenile survival of S. brachiata showed that those from post-dispersal soil-germinated seeds had the best survival, suggesting that pre-dispersal seed germination may be maladaptive.

Changes in abscisic acid metabolism in relation to the maturation of grapevine (Vitis vinifera L., cv. Mencía) somatic embryos

BACKGROUND

Somatic embryogenesis in grapevines is a complex process that depends on many physiological and genetic factors. One of its main limitations is the process of precocious germination of the somatic embryos in differentiation medium. This process lowers plant conversion rates from the somatic embryos, and it is probably caused by a low endogenous abscisic acid (ABA) content.

RESULTS

Precocious germination of the somatic embryos was successfully avoided by culturing grapevine cv. Mencía embryogenic aggregates over a semipermeable membrane extended on top of the differentiation medium. The weekly analysis of the endogenous ABA and ABA-glucosyl ester (ABA-GE) contents in the aggregates showed their rapid accumulation. The expression profiles of 9-cis-epoxycarotenoid dioxygenase (VvNCED1), 8'-hydroxylase (VvHyd2), UDP-glucosyltransferase (VvUGT) and β-glucosidase (VvBG2) genes in grapevine revealed that the occurrence of a first accumulation peak of endogenous ABA in the second week of culture over the semipermeable membrane was mainly dependent on the expression of the VvNCED1 gene. A second increase in the endogenous ABA content was observed in the fourth week of culture. At this point in the culture, our results suggest that of those genes involved in ABA accumulation, one (VvNCED1) was repressed, while another (VvBG2) was activated. Similarly, of those genes related to a reduction in ABA levels, one (VvUGT) was repressed while another (VvHyd2) was activated. The relative expression level of the VvNCED1 gene in embryogenic aggregates cultured under the same conditions and treated with exogenous ABA revealed the significant downregulation of this gene.

CONCLUSIONS

Our results demonstrated the involvement of ABA metabolism in the control of the maturation of grapevine somatic embryos cultured over a semipermeable membrane and two important control points for their endogenous ABA levels. Thus, subtle differences in the expression of the antagonistic genes that control ABA synthesis and degradation could be responsible for the final level of ABA during the maturation of grapevine somatic embryos in vitro. In addition, the treatment of somatic embryos with exogenous ABA suggested the feedback-based control of the expression of the VvNCED1 gene by ABA during the maturation of grapevine somatic embryos.

Seed development and viviparous germination in one accession of a tomato rin mutant

In an experimental field, seed vivipary occurred in one accession of tomato rin mutant fruit at approximately 45-50 days after pollination (DAP). In this study, the possible contributory factors to this viviparous germination were investigated. Firstly, developing seeds were freshly excised from the fruit tissue every 5 days from 25-60 DAP. Germination occurred when isolated seeds were incubated on water, but was inhibited when they remained ex situ in fruit mucilage gel. The effect of abscisic acid (ABA) and osmoticum, separate and together, on germination of developing seeds was investigated. Additionally, ABA content in the seed and mucilage gel, as well as fruit osmolality were measured. The results showed that ABA concentrations in seeds were low during early development and increased later, peaking at about 50 DAP. ABA concentrations in rin accession were similar to those of the control cultivar and thus are not directly associated with the occurrence of vivipary. Developing seeds of rin accession are more sensitive than control seeds to all inhibitory compounds. However, osmolality in rin fruit at later developmental stages becomes less negative that is required to permit germination of developing seeds. Hence, hypo-osmolality in rin fruit may be an important factor in permitting limited viviparous germination.

Mapping the anther culture response genes in maize (Zea mays L.)

In order to map the genes conditioning the induction of embryos during our anther culture process, we evaluated F2 plants from three different crosses for their anther culture ability and also performed RFLP analysis on these plants. The results showed that six chromosomal regions appear to be associated with the ability to induce embryo-like structures from maize microspores. These regions are located on chromosomes 1 (two regions), 3, 5, 7, and 8. Some of these chromosomes are identical to those found in previous studies and we have localized the regions more precisely. Notably, all chromosome regions identified, except one, are near viviparous mutant loci. Since the viviparous mutations are known to involve the plant hormone abscisic acid (ABA), these results suggest that ABA or its antagonist, gibberellic acid (GA3), might somehow be related to anther culture ability. We also propose some combinations of probes to screen for anther culture ability in the three genotypes studied.

Abscisic acid and stress signals induce Viviparous1 expression in seed and vegetative tissues of maize

Viviparous1 (Vp1) encodes a B3 domain-containing transcription factor that is a key regulator of seed maturation in maize (Zea mays). However, the mechanisms of Vp1 regulation are not well understood. To examine physiological factors that may regulate Vp1 expression, transcript levels were monitored in maturing embryos placed in culture under different conditions. Expression of Vp1 decreased after culture in hormone-free medium, but was induced by salinity or osmotic stress. Application of exogenous abscisic acid (ABA) also induced transcript levels within 1 h in a dose-dependent manner. The Vp1 promoter fused to beta-glucuronidase or green fluorescent protein reproduced the endogenous Vp1 expression patterns in transgenic maize plants and also revealed previously unknown expression domains of Vp1. The Vp1 promoter is active in the embryo and aleurone cells of developing seeds and, upon drought stress, was also found in phloem cells of vegetative tissues, including cobs, leaves, and stems. Sequence analysis of the Vp1 promoter identified a potential ABA-responsive complex, consisting of an ACGT-containing ABA response element (ABRE) and a coupling element 1-like motif. Electrophoretic mobility shift assay confirmed that the ABRE and putative coupling element 1 components specifically bound proteins in embryo nuclear protein extracts. Treatment of embryos in hormone-free Murashige and Skoog medium blocked the ABRE-protein interaction, whereas exogenous ABA or mannitol treatment restored this interaction. Our data support a model for a VP1-dependent positive feedback mechanism regulating Vp1 expression during seed maturation.

The developmental expression of the maize regulatory gene Hopi determines germination-dependent anthocyanin accumulation

The Hopi gene is a member of the maize r1 gene family. By genetic and molecular analyses we report that Hopi consists of a single gene residing on chromosome 10 approximately 4.5 cM distal to r1. Hopi conditions anthocyanin deposition in aleurone, scutellum, pericarp, root, mesocotyl, leaves, and anthers, thus representing one of the broadest specifications of pigmentation pattern reported to date of all the r1 genes. A unique feature of the Hopi gene is that seeds are completely devoid of pigment at maturity but show a photoinducible germination-dependent anthocyanin accumulation in aleurone and scutellum. Our analysis has shown that the Hopi transcript is not present in scutellum of developing seeds but is induced only upon germination and that the simultaneous presence of both C1 and Hopi mRNAs is necessary to achieve A1 activation in scutella. We conclude that the expression pattern of the Hopi gene accounts for the germination-dependent anthocyanin synthesis in scutella, whereas the developmental competence of germinating seeds to induce anthocyanin production in scutella results from the combination of the light-inducible expression of C1 and the developmentally regulated expression of the Hopi gene.

Molecular and genetic analysis of signal transduction in higher plants

Plant scientists have long recognized the complexity of responses to environmental and hormonal signals that provide the basis for plant growth and development. The systematic isolation and analysis of mutations that disrupt signal transduction and prevent the appropriate physiological response provides an important resource for studying these processes and, ultimately, for describing the molecular events that control growth and developmental responses in plants.

Regulation of the abscisic acid-responsive gene rab28 in maize viviparous mutants

We have isolated a new maize gene, rab28, that responds to abscisic acid (ABA) treatment. This gene has been characterized by determining the sequence of the cDNA and corresponding genomic copy, and by mapping the start site of its transcript. The rab 28 gene encodes a protein of predicted molecular weight 27713 Da which shows strong homology with the Lea D-34 protein identified in cotton. The proximal promoter region contains the conserved ABA-response element, CACGTGG, reported in other plant genes to be responsible for ABA induction. rab 28 mRNA has been identified as ABA-inducible in embryos and young leaves. It is also induced by water-stress in leaves of wild-type plants. Regulation of the rab 28 gene was studied in maize viviparous mutants. The results obtained with the ABA-insensitive vp1 mutant show that rab 28 transcripts do not accumulate to a significant level during embryogenesis. Surprisingly, induction of rab 28 mRNA can be achieved in young embryos by exogenous ABA treatment. Moreover, water-stressed or ABA-treated seedlings of vp1 contain significant levels of rab 28 mRNA which is not detectable in well-watered seedlings. Regulation of the rab 28 gene in excised young embryos of ABA-deficient vp2 mutants, in which influences of the maternal environment are absent, closely resembles that found in non-mutant excised young embryos. The significance of these results is discussed.

The Viviparous-1 developmental gene of maize encodes a novel transcriptional activator

The Viviparous-1 (Vp1) gene of maize is specifically required for expression of the maturation program in seed development. We show that Vp1 encodes a 73,335 dalton protein with no detectable homology to known proteins. An acidic transcriptional activation sequence was identified by fusion to the GAL4 DNA-binding domain. Expression of VP1 in maize protoplasts resulted in strong activation (greater than 130-fold) of a reporter gene fused to the promoter of a presumptive target gene. The acidic domain in VP1 was essential for transactivation and could be functionally replaced by the activator sequence of the herpes simplex virus VP16 protein. Our results indicate that VP1 is a novel transcription factor possibly involved in potentiation of a seed-specific hormone response.