Seed maturation is mainly governed by a few genes best studied in maize and Arabidopsis. The isolation of the LEC1 and FUS3 genes, besides the previously known VP1/ABI3 genes, and their identification as transcriptional regulators provides the first direct hints as to their molecular mode of action. With the identification of new effector genes, the investigation of the role of hormones with new methods such as immunomodulation and the increasingly recognised role of metabolites like sugars as important modulators of seed development, we increasingly understand the complexity and structure of the regulatory network underlying seed maturation.
Addresses
Institut für Pflanzengenetik und Kulturpflanzenforschung (IPK), D-06466 Gatersleben, Germany
*e-mail: wobusu@ipk-gatersleben.de
Current Opinion in Plant Biology1999, 2:33–38 http://biomednet.com/elecref/1369526600200033 © Elsevier Science Ltd ISSN 1369-5266
Abbreviation ABA abscisic acid
Introduction
Higher plants are characterised by the formation of seeds which contain the embryo, protected by the maternally-derived seed coat. The embryo first runs through a phase of cell division and morphogenesis, followed by a matu-ration phase which includes the accumulation of storage products, the suppression of precocious germination, the acquisition of desiccation tolerance, water loss and often the induction of dormancy (see [1•]). Maturation
inter-rupts seedling development. The process progresses in a wave-like manner forming temporally and spatially determined patterns or gradients between and within the different seed organs [2,3]. Several lines of evidence strongly suggest that maturation evolved late in evolu-tion and has been ‘inserted’ [1•] at different
developmental time points in different higher plant taxa [4•]. In depth molecular analyses, however, are mainly
carried out in Arabidopsis, grain legumes and cereals (especially maize), in each case with a specific research focus depending on the different advantages of the three experimental systems. In this review, the following exciting recent research achievements are briefly dis-cussed; the isolation and initial functional analysis of central transcriptional regulators of seed maturation and some of their target genes, a new methodology to control the process at the hormonal level and the role of metabo-lites as triggers of maturation processes. These data necessarily cover only certain aspects of the complex network of processes involved in seed maturation.
ABA3, FUS3 and LEC1: three major regulators
of seed maturation
Analyses of mutant phenotypes in Arabidopsishave provided strong evidence that three genes —ABA-INSENSITIVE3 (ABI3), FUSCA3 (FUS3) and LEAFY COTYLEDON1 (LEC1)— are of utmost importance for seed maturation as they affect a wide, broadly overlapping but not identical spectrum of seed-specific characters (see Figure 9 in [5••]
for a good graphic representation). In essence, mutant embryos enter the germination programme immediately by skipping maturation. The description of the abi3 mutant as abscisic acid insensitive already indicates a link to the most important hormonal regulator of seed matura-tion, abscisic acid (ABA). Until recently, only the molecular structure of the ABI3 gene [6] and its maize homologue, the transcriptional activator gene VP1 [7], were known. In 1998, however, the sequences of two other genes, FUS3 and LEC1, have been obtained [8••,9••] and led to the
important conclusions discussed below.
The FUS3 gene turned out to be related to the VP1/ABI3 gene family, but is of reduced length (ABI3 encodes 720 amino acid residues; FUS3 only 312 residues), and high homology in the protein is restricted to a stretch of at least 100 amino acid residues [9••] defined as the B3 region in
the VP1 protein [10]. The B3 domain can mediate sequence-specific DNA-binding in vitro[11•] and is critical
for gene activation at low or insignificant ABA concentra-tions [12•]. It was also used to define a B3-box family of
genes (or B3-domain family of proteins) [9••] of which
FUS3 clearly is a member of a — possibly more ancient — subgroup. An auxin-response factor ARF1 [13] represents another even more remote subfamily. In accordance with the ABA independence of fus3mutants, the FUS3 protein lacks an amino-terminal A-domain which, in VP1 (and cer-tainly in ABI3), mediates the ABA response [12•]. FUS3is
often classified as an LEC1-type gene on the basis of mutant phenotypes (see [1•,8••,14]), but the two genes are
structurally unrelated and should, therefore, stay separate. LEC1has recently been shown most probably to encode a protein related to a transcription factor subunit of the HAP3 type [8••] — HAP3 in mammals is part of the
heterotrimer-ic CCAAT box-binding factor CBF (see [15]).
On the basis of an extensive descriptive study of double and the parental single mutants, Parcy et al. [5••] had
already concluded that the three genes act synergistical-ly. The authors proposed a model for the concerted action of the gene trio in controlling several major aspects of seed maturation. They further speculated that the three proteins could act as transcription factors and form hetero-oligomeric combinations, which regulate different developmental processes. These protein com-plexes could also feed-back regulate their own gene
Seed maturation: genetic programmes and control signals
promoters as shown, for instance, for the DEF A/GLO heterodimer in Antirrhinum [16]. DEF A and GLO are two MADS-box transcription factors involved in flower development. Preliminary experiments at the molecular level involving ABI3 and FUS3 [17•] provide first hints
that these suggestions may be valid. In a transient expression assay both FUS3 and ABI3 strongly induce the activity of maturation-specific promoters in a co-operative and synergistic way. This induction was strictly dependent on an intact RY (CATGCAT) promoter-cis -element and suggests a rather direct mode of action, perhaps direct binding. Also LEC1, due to its proposed HAP3-like function [8••], could directly bind to the RY
repeat. In maize and other grasses, the same element (here called the Sph motif) is functionally dependent on the VP1 gene product, specifically on the B3 domain, in promoters of genes like C1, a transcriptional regulator of anthocyanin biosynthesis in seeds [11•]. VP1 not only
recognises the Sph cis-element [11•], but may also act by
its B2 domain as a protein association enhancer [10]. In an Em-promoter G-box binding complex, VP1 is associ-ated with a 14-3-3 protein (GF14), and VP1, the G-box transcription factor EmBP1 and GF14 may all interact to stabilise and/or activate the transcription complex [18•,19•]. Em is an ABA-responsive late embryogenesis
abundant (LEA) gene expressed exclusively in maturing embryos. Additional b-ZIP factors, modifying ABI3 homologue-dependent gene expression, have been char-acterised from pea [20,21] and underscore the complex nature of the molecular machinery directing gene regu-lation during seed maturation.
Studies on genes affected by FUS3 [22•,23•] revealed
another interesting new aspect; among the genes con-trolled by FUS3 during maturation are upregulated [22•]
and downregulated [23•] MYB transcription factors.
AtMYB13, for instance, is involved in some aspects of meristematic vegetative growth. It is likely to be a rep-resentative of several regulators of vegetative growth repressed during maturation by the concerted action of at least FUS3and LEC1[23•]. The data also indicate that
FUS3 is, on the one hand, a central regulator controlling other regulators such as MYB transcription factors but, on the other hand, the protein probably directly regu-lates effector genes like maturation-specific storage protein genes via the RY promoter cis-element.
The ABI3/VP1 transcriptional activator family is well known from mutant phenotypes to be involved in estab-lishing embryo dormancy. Recent work with Avena fatua inbred lines suggests that VP1 plays a central role since AfVP1 mRNA levels established during the final stages of embryo maturation were positively correlated with the dor-mant phenotype in a way that qualifies the gene as a molecular marker for dormancy potential/after-ripening time [24•]. The most recent discovery of a role of an ABI3
homologue in quiescent apices of Arabidopsis[25•] signals
that the function of genes of the ABI3-type, and perhaps
also of the FUS3-type (low expression levels in non-seed tissues [9••]), is not restricted to seeds. Interestingly,
another newly isolated transcriptional regulator, ABI4, most likely to be involved in regulating seed responses to ABA, is not expressed seed-specifically in spite of the fact that a clear mutant phenotype is only seen in seeds [26•].
According to published data [8••], only LEC1 expression
seems to be strictly embryo-specific but studies with more sensitive assays like RT-PCR have not yet been reported.
In summary, three major genes involved in seed matura-tion initiamatura-tion and maintenance are structurally either not (LEC1) or only distantly (ABI3/FUS3) related and their expression patterns are temporally and spatially different, but broadly overlapping especially during seed maturation. At least ABI3and FUS3are expressed in tissues other than seed, that is to say they are not strictly seed-specific, con-trary to accepted beliefs. The door is now open to study the molecular interactions of the gene products with each other and other factors and their role in determining cell fate and function in each specific process.
A fresh look at the role of abscisic acid:
probing abscisic acid function by expressing
ABA-antibodies
The important role of ABA in seed maturation is well doc-umented but since the hormone triggers diverse processes not only in the seed, analysis by established procedures faces limitations. A method which blocks the ABA mole-cule itself very specifically in a tissue-, cell- and compartment-specific manner [27•], therefore, promises
new insights. Phillips et al.[28••] directed an ABA-specific
single chain Fv (scFv) antibody into the lumen of the endo-plasmic reticulum (ER) of developing tobacco seeds by using the ER-target signal KDEL and a seed-specific pro-moter, active from the late phase of early development to the first part of late embryogenesis. The anti-ABA scFv caused a developmental switch to the germination pro-gramme characterised by the formation of chloroplasts containing photosynthetic pigments and a dramatic reduc-tion in the embryo of the most abundant storage proteins as well as protein and oil bodies. The authors calculated that the recombinant antibody bound nearly all available ABA until day 20 of tobacco development. The overall concen-tration of antibody-bound ABA plus unbound ABA increased considerably above wild-type level, possibly due to a feed-back mechanism or protection from ABA metabo-lism. The seed phenotype was most similar to the Arabidopsis aba/abi3 double mutant seeds [29]. The well established fact that ABA biosynthetic mutants do not markedly reduce storage protein mRNAs [29] led to the question — how important is ABA in controlling storage protein accumulation [30•]? The anti-ABA seeds now
clear-ly underscore the dominating role of ABA in the process at least in tobacco seeds [28••]. By using a broader range of
Development and metabolism: renewed
interest in a long known connection
Metabolites as developmental signals in seeds
Metabolites as signal molecules in plant developmental processes have been mainly neglected until recently when sugars especially were shown to regulate diverse gene activ-ities [31] and to act as important components in signal transduction networks [32•]. In grain legume seeds,
exten-sive, mainly correlative analyses (see [33•,34•] for reviews)
including transgenic plants which express a yeast-derived invertase in developing seeds [35], suggested that soluble sugars provide signals for cotyledon development but that glucose and sucrose can play different roles; whereas a high glucose/sucrose ratio was correlated with cell division, a high sucrose/hexose ratio seems to be a major trigger of the stor-age product pathways. Of special interest is the discovery of steep glucose gradients across the cotyledons of the grain legume Vicia faba, which change during development and correlate positively with mitotic activity but negatively with storage starch accumulation [36•]. These gradients are likely
to be primarily established by the specific expression pattern of a cell wall-bound invertase in the developing maternal seed coat [37] and further modulated (enhanced?) by a hex-ose- and a sucrose transporter (mainly excluding each other in the timing of expression) in the epidermal cells of the embryo [38•]. Gradients were also visualised in developing
maize kernels at the level of the activity of a key enzyme — sucrose synthase — of starch biosynthesis in sink organs [39•]. The combination of methods which allow the (semi-)
quantitative determination not only of RNA and protein but also of enzyme activity and metabolites at (or nearly at) cel-lular resolution will certainly reveal a refined net of relationships. Whether sugar levels in such gradients func-tion as morphogenic substances [36•], however, has not been
shown unequivocally. Whatever the exact role of sugars is, the elicited signals have to interact with other signals to evoke the complex processes of seed maturation.
One site of integration for different signals are promoter sequences. Interestingly, it has recently been demonstrated by analysing the phaseolin seed storage protein promoter in transgenic tobacco seeds that the essential signal for the tran-scriptional activity of the phaseolin promoter provided by ABA is modulated beside unknown developmental factors by sucrose and calcium ions [40•]. In the barley embryo,
glu-cose acting as a signal molecule interferes with the gibberellic acid response of α-amylase gene expression, but in a tissue-specific manner [41•]; the induction of α-amylase
by GA in the aleurone layer is unaffected by sugars, whereas in the embryo, sugars repress the response. The site of action of the sugar molecule has not been determined, but could be at the transcriptional and also at the post-transcriptional level, as rice α-amylase mRNA stability and abundance have been shown to be controlled by sucrose [42•,43•].
Assimilate import and seed filling
If metabolites are able to act as signals in developmental processes, the regulated import of photoassimilates into the
growing seed will influence seed development beside by providing nutrients. Developing seeds import sucrose and amino acids from the phloem. These assimilates are unloaded from the seed coat and must be taken up by the apoplastically isolated embryo or endosperm. Seed-located apoplastic transport processes, therefore, can influence assimilate partitioning and storage product synthesis. One obvious regulatory control point could be provided by vari-ous transporters situated in the membranes of seed coat and/or embryo epidermal cells. In Vicia faba a sucrose trans-porter gene is highly expressed in the epidermal transfer cell layer covering the outer surface of developing cotyledons [38•,44]. These transfer cells develop at the contact area of
the cotyledonary epidermis to the seed coat, starting first at early cotyledon stage and subsequently spreading to the abaxial region. The tissue underlying the epidermal transfer cell layer differentiates into storage parenchyma cells, indi-cating an important role of the sucrose transporter in providing sugars for storage product synthesis. Feeding high concentrations of sugars in vitro suppressed both the sucrose transporter gene expression and transfer cell differentiation, suggesting a control mechanism by carbohydrate availability [38•]. A more detailed study [45•] also demonstrated the
induction of transfer cell differentiation by sugars and the modulation by the sugar state of sugar transport activity and consequently storage product synthesis. Less detailed data are available for amino acid transporters which have a broad substrate specificity and are probably regulated by tissue-specific expression. Two members were found to be expressed in developing seeds of Arabidopsis. Using promot-er-GUS fusions it was shown that AAP1 is expressed in the endosperm and the cotyledons whereas AAP2 is expressed in the vascular strands of siliques and in funiculi [46•].
In addition to these seed-specific processes, there are genes acting on the whole plant level in controlling assimilate transport into the seed and seed growth. One example is the SN gene in pea which regulates assimilate partitioning between vegetative and reproductive growth [47•]. One important factor could be gibberellic
acid (GA) since it appears that in the pea lh2 mutation seeds are unable to use available assimilates. In develop-ing lh2-seeds, GA levels are reduced and lead to reduced growth and to a high frequency of seed abortion. GAs may either promote assimilate uptake into developing seeds or affect seed growth, that is sink strength [48•].
The general significance of seed-specific transport processes for development, however, is still unknown.
Conclusions
hitherto neglected factors modulating or even triggering developmental processes. The still scarce data suggest the existence of an extended regulatory network rather than a clear hierarchy of actions and allows first glimpses on its structure. Large scale expression analysis by DNA chip technology will uncover many more possible network com-ponents especially at the gene level. The real challenge, however, will be to unravel their role and interconnection. Beside the genetic approach, with its focus on gene struc-ture and mRNA expression level, more extensive studies at the protein and higher order structural level (subcellular, cellular and also whole-plant level), together with more integrated experimental approaches are necessary to fur-ther deepen and extend our knowledge of seed maturation as part of seed development and plant life in general.
Acknowledgements
The authors would like to thank Helmut Bäumlein and Udo Conrad for help with the manuscript and Simon Miséra and Antje Rhode for providing manuscripts in press. Work in our lab is supported by the Deutsche Forschungsgemeinschaft and the European Union.
References and recommended reading
Papers of particular interest, published within the annual period of review, have been highlighted as:
• of special interest ••of outstanding interest
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In this important paper, the authors, by quantifying several developmental processes in a range of mutants, clearly show that the three genes work syn-ergistically and not in different pathways as earlier models suggested. They also provide evidence that the abundance of ABI3 is regulated by FUS3and
LEC1. The genetic model provided can now be tested at the molecular level due to the availability of FUS3and LEC1.
6. Giraudat J, Hauge BM, Valon C, Smalle J, Parcy F, Goodman HM:
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The authors isolated the LEC1gene and demonstrated its major role, not only during late, but also in early embryogenesis including suspensor and endosperm development. Ectopic LEC1expression, in contrast to ABI3, induced embryonic programmes in vegetative cells in rare cases.
Homology of LEC1 to HAP3 subunit sequences of the CCAAT-box bind-ing factor CBF is restricted to the HAP3 B-domain.
9. Luerßen H, Kirik V, Herrmann P, Miséra S: FUSCA3encodes a
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Arabidopsis thaliana.Plant J1998, 15:755-764.
A third key regulatory gene of seed maturation beside ABI3and LEC1,
FUS3, has been isolated by postional cloning and its product shown to share the B3 domain with ABI3-like proteins, whereas significant features of A- and B1-domains are missing, obviously due to the short amino-terminal part of this protein. Whether some similarities to B2-domain sequences are of functional importance has still to be shown. The analysed allelic variants (most of them causing abnormal splicing) provide hints for the essential role of the B3 domain in seed maturation (but see [12•]).
10. Hill A, Nantel A, Rock CD, Quatrano RS: A conserved domain of the VIVIPAROUS-1 gene product enhances the DNA binding activity of the bZIP protein EmBP-1 and other transcription factors.J Biol Chem1996, 271:3366-3374.
11. Suzuki M, Kao CY, McCarty DR: The conserved B3 domain of
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Since its isolation, VP1 has been implicated in gene regulation, but the ques-tion remained open as to whether or not the protein binds directly to DNA. Now the McCarty lab provides convincing evidence that the B3 domain of VP1 has a highly co-operative, sequence-specific DNA-binding activity in an
in vitroassay. Specific binding to the Sph-motif TCCATGCAT provides a good link to the expected functions and leads to the suggestion that the B3 domain in general binds to this element (also known as the RY-motif) present in many seed protein gene promoters.
12. Carson CB, Hattori T, Rosenkrans L, Vasil V, Vasil IK, Peterson P,
• McCarty DE: The quiescent/colourless alleles of viviparous-1 show that the conserved B3 domain of VP1 is not essential for
ABA-regulated gene expression in the seed.Plant J1997,
12:1231-1240.
By analysing several vp1 alleles in a further attempt to map functional domains within the VP1 protein in more detail, the authors conclude that B3 is not absolutely essential for completion of seed maturation. Contrary to predictions in [9••], a two amino acid deletion in the B3 domain of FUS3 still led to the Fus3–phenotype in Arabidopsis.Whether this apparent discrep-ancy is of general importance has still to be worked out.
13. Ulmasov T, Hagen G, Guilfoyle TJ: ARF1, a transcription factor that binds to auxin response elements.Science 1997, 276:1865-1868. 14. Vernon DM, Meinke DW: Late embryo-defective mutants of
Arabidopsis.Dev Genet1995, 16:311-320.
15. Maity SN, de Crombrugghe B: Role of the CCAAT-binding protein CBF/NF-Y in transcription.Trends Biochem Sci1998, 23:174-178. 16. Schwarz-Sommer Z, Hue I, Huijser P, Flor PJ, Hansen R, Tetens F,
Lönnig W-E, Saedler H, Sommer H: Characterization of the
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17. Wohlfarth T, Braun H, Kirik V, Kölle K, Czihal A, Tewes A, Luerssen H,
• Miséra S, Shutov A, Bäumlein H:Regulation and evolution of seed globulin genes. J Plant Physiol1998, 152:600-606.
This symposium paper contains interesting data which have still to be pub-lished in detail. They suggest a co-operative interaction of FUS3 and ABI3 in the regulation of seed-specific promoters with the RY cis-motif as target.
18. Schultz TF, Medina J, Hill A, Quatrano RS: 14-3-3 proteins are part
• of an abscisic acid-VIVIPAROUS1 (VP1) response complex in the Em promoter and interact with VP1 and EmbP1. Plant Cell1998,
10:837-847.
The paper provides a first and important example of a transcription complex involving VP1 and identifies members of the complex including 14-3-3 pro-teins which are supposed to act as mediators in protein–protein interactions involved in kinase-related steps of signal-transduction pathways.
19. Quatrano RS, Bartels D, Ho T-HD, Pagés M: New insights into
• ABA-mediated processes.Plant Cell1997, 9:470-475. A meeting-based review including the role of ABA in seed maturation.
20. Chern MS, Bob AJ. Bustos MM: The regulator of MAT2 (ROM2) protein binds to early maturation promoters and represses PvALF-activated transcription.Plant Cell1996, 8:305-321. 21. Chern MS, Eiben HG, Bustos MM: The developmentally regulated
bZIP factor ROM1 modulates transcription from lectin and storage protein genes in bean embryos.Plant J1996,
22. Kirik V, Kölle K, Miséra S, Bäumlein H: Two novel MYB homologues
• with changed expression in late embryogenesis-defective
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Arabidopsisinflorescence.Plant J 1998,13:729-742.
Papers [22•] and [23•] contribute to the elucidation of the key role of the
ABI3/LEC1/FUS3 gene trio in late embryogenesis by the identification, via differential display, of effector genes — in this case members of the MYB group of transcriptional regulators. The genes described are either positively [22•] or negatively [23•] regulated.AtMYB13is activated in late embryogenesis defective fus3and lec1mutants but not abi3mutants, that is to say, in wild-type embryos this MYB protein should be at least in part responsible for repressing genes involved in vegetative meristem activation. The report [23•] provides the first evidence at the molecular level for different action spectra of ABI3on the one hand and FUS3plus
LEC1on the other.
24. Jones HD, Peters NCB, Holdsworth MJ: Genotype and environment
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This is yet another good example that the well-chosen use of plants other than Arabidopsiscan considerably contribute to the better understanding of specific developmental processes.
25. Rhode A, Van Montagu M, Boerjan W: The abscisic acid-insensitive
• gene ABI3is expressed during vegetative quiescence processes in Arabidopsis.Plant Cell Environ 1998, in press.
Here, Rhode et al.show, in contrast to present conceptions, that ABI3most probably has additional functions to those in seed development. During growth in the dark ABI3is expressed in the seedling apex after the arrest of cell division. Concomitantly, the 2S napin seed storage protein is induced.
ABI3-promoter/GUS fusion constructs are expressed in still other vegeta-tive organs. These results — together with the observed expression of an
ABI3 homologue in dormant poplar buds (A Rhode, Molecular strategies to study bud dormancy in Populus [PhD thesis]. Universiteit Gent 1998) — led the authors to conclude that ABI3is one of several components of a molecular network underlying all types of quiescence.
26. Finkelstein RR, Wang ML, Lynch TJ, Rao S, Goodman H: The
• Arabidopsisabscisic acid response locus ABI4 encodes an APETALA2 domain protein.Plant Cell1998, 10:1043-1054. ABA-sensitive abi4mutants are defective in seed development but exhibit a less severe phenotype than known abi3mutants. The gene, isolated by posi-tional cloning, might represent a new element in the signal transduction pathway involving ABI3.
27. Conrad U, Fiedler U: Compartment-specific accumulation of
• recombinant immunoglobulins in plant cells: an essential tool for antibody production and immunomodulation of physiological functions and pathogen activity.Plant Mol Biol1998, 38:101-109. A review focusing on the importance of cell compartment-specific expression of antibodies with respect to both functional studies or biotechnological goals.
28. Phillips J, Artsaenko O, Fiedler U, Horstmann C, Mock H-P, Müntz K,
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The paper demonstrates for the first time the successful use of transgene encoded antibodies directed against low molecular weight regulatory com-pounds for the study of plant developmental processes. This new experi-mental tool will be certainly used in the future in a variety of situations including other hormones and non-protein regulators.
29. Koornneef M, Hanhart CJ, Hilhorst HWM, Karssen CM: In vivo
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30. Merlot S, Giraudat J: Genetic analysis of abscisic acid signal
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One of several recommendable recent reviews (see also [19•]) on ABA sig-nal transduction.
31. Koch KE: Carbohydrate-modulated gene expression in plants.
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Recommended review on a rapidly expanding research field.
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Reviews (condensed [33•]and more extended [34•]) focusing on the pos-sible role of sugars in regulating cotyledon development.
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A luciferase-coupled enzyme reaction together with quantitative biolumines-cence and single photon imaging is used to visualise glucose concentration gradients which are related to specific cell types. In addition, they correlate positively with mitotic activity and negatively with starch accumulation. The authors speculate about glucose concentration ‘landscapes’ as possible morphogenetic gradients.
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39. Wittich PE, Vreugdenhil D: Localization of sucrose synthase activity
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A much welcomed piece of work bringing enzyme activity studies in seeds down to the cellular level. Such studies, together with high-resolution metabolite analysis (see [36•]) and gene expression studies, will increasing-ly contribute to a better understanding of developmental processes.
40. Bustos MM, Iyer M, Gagliardi SJ: Induction of a ß-phaseolin
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Phaseolin (seed storage globulin) promoter-GUS mRNA and GUS activity in transgenic tobacco seeds can be induced by ABA, a process superimposed on development-dependent regulation. The ABA response itself is repressed by exogenously added sucrose alone, but stimulated by the presence of both sucrose and CaCl2. These data add to earlier published work [49] on the involvement of Ca2+in the sugar-inducible expression of plant genes.
41. Perata P, Matsukura C, Vernieri P, Yamaguchi J: Sugar repression of
• a gibberellin-dependent signaling pathway in barley embryos.
Plant Cell1997, 9:2197-2708
This study deals mainly with α-amylase gene expression in germinating bar-ley grain, but is also certainly of relevance for processes in seed maturation, as it clearly demonstrates the complex and tissue-dependent interaction of sugar signals with hormone regulation.
42. Chan M-T, Yu S-M: The 3¢untranslated region of a rice a-amylase
• gene functions as a sugar-dependent mRNA stability determinant. Proc Natl Acad Sci USA1998, 95:6543-6547. See [43•] for an annotation to this reference.
43. Chan M-T, Yu S-M: The 3¢untranslated region of a rice a-amylase
• gene mediates sugar-dependent abundance of mRNA.Plant J
1998, 15:685-695.
Evidence is provided [42•,43•] that the studied α-Amy3 3′UTR is probably the major determinant for controlling transcript abundance in response to glucose deprivation. This is the first example of a structural element in a developmentally regulated plant mRNA gene involved in sugar regulation.
44. Harrington GN, Franceschi VR, Offler CE, Patrick JW, Tegeder M, Frommer WB, Harper JF, Hitz WD: Cell specific expression of three genes involved in plasma membrane sucrose transport in developing Vicia faba seed. Protoplasma1997,197:160-173. 45. Offler CE, Liet E, Sutton EG: Transfer cell induction in cotyledons
• of V. faba. Protoplasma 1997, 200:51-64.
46. Hirner B, Fischer WN, Rentsch D, Kwart M, Frommer WB:
• Developmental control of H+/amino acid permease gene expression during seed development of Arabidopsis.Plant J
1998, 14:535-544.
The two characterised genes, AAP1 and AAP2, show non-overlapping expres-sion patterns and might play a role in supplying the seeds with organic nitrogen.
47. Kelly MO, Spanswick RM: Maternal, single-gene regulation of
• assimilate partitioning in pea.Plant Physiol1997, 114:1055-1059. The paper stresses the point that important seed characteristics like rate and duration of seed growth are strongly influenced by physiological processes determined maternally.
48. Swain SM, Reid JB, Kamiya Y: Gibberellins are required for embryo
• growth and seed development in pea. Plant J 1997,12:1329-1338. The pea mutants lh-1and lh-2provide the most unequivocal evidence for an important role of gibberellins (GAs) in early seed development. They exclude assimilate availability and lacking maternal GAs as causes for reduced final seed weight or seed abortion, but suggest that GAs either promote assimi-late uptake by the embryo or exert a direct influence on seed growth.
49. Ohto M, Hayashi K, Isobe M, Nakumura K: Involvement of Ca2+ signalling in the sugar-inducible expression of genes coding for sporamin and b-amylase of sweet potato. Plant J1995,
