Hormonal Regulation of Seed Development

Cytokinin Translocation to, and Biosynthesis and Metabolism within, Cereal and Legume Seeds: Looking Back to Inform the Future

Early in the history of cytokinins, it was clear that Zea mays seeds contained not just trans-zeatin, but its nucleosides and nucleotides. Subsequently, both pods and seeds of legumes and cereal grains have been shown to contain a complex of cytokinin forms. Relative to the very high quantities of cytokinin detected in developing seeds, only a limited amount appears to have been translocated from the parent plant. Translocation experiments, and the detection of high levels of endogenous cytokinin in the maternal seed coat tissues of legumes, indicates that cytokinin does not readily cross the maternal/filial boundary, indicating that the filial tissues are autonomous for cytokinin biosynthesis. Within the seed, trans-zeatin plays a key role in sink establishment and it may also contribute to sink strength. The roles, if any, of the other biologically active forms of cytokinin (cis-zeatin, dihydrozeatin and isopentenyladenine) remain to be elucidated. The recent identification of genes coding for the enzyme that leads to the biosynthesis of trans-zeatin in rice (OsCYP735A3 and 4), and the identification of a gene coding for an enzyme (CPN1) that converts trans-zeatin riboside to trans-zeatin in the apoplast, further cements the key role played by trans-zeatin in plants.

Overexpression of cytokinin dehydrogenase genes in barley (Hordeum vulgare cv. Golden Promise) fundamentally affects morphology and fertility

Barley is one of the most important cereal crops grown worldwide. It has numerous applications, but its utility could potentially be extended by genetically manipulating its hormonal balances. To explore some of this potential we identified gene families of cytokinin dehydrogenases (CKX) and isopentenyl transferases, enzymes that respectively irreversibly degrade and synthesize cytokinin (CK) plant hormones, in the raw sequenced barley genome. We then examined their spatial and temporal expression patterns by immunostaining and qPCR. Two CKX-specific antibodies, anti-HvCKX1 and anti-HvCKX9, predominantly detect proteins in the aleurone layer of maturing grains and leaf vasculature, respectively. In addition, two selected CKX genes were used for stable, Agrobacterium tumefaciens-mediated transformation of the barley cultivar Golden Promise. The results show that constitutive overexpression of CKX causes morphological changes in barley plants and prevents their transition to flowering. In all independent transgenic lines roots proliferated more rapidly and root-to-shoot ratios were higher than in wild-type plants. Only one transgenic line, overexpressing CKX under the control of a promoter from a phosphate transporter gene, which is expressed more strongly in root tissue than in aerial parts, yielded progeny. Analysis of several T1-generation plants indicates that plants tend to compensate for effects of the transgene and restore CK homeostasis later during development. Depleted CK levels during early phases of development are restored by down-regulation of endogenous CKX genes and reinforced de novo biosynthesis of CKs.

Genetic engineering of cytokinin metabolism: prospective way to improve agricultural traits of crop plants

Cytokinins (CKs) are ubiquitous phytohormones that participate in development, morphogenesis and many physiological processes throughout plant kingdom. In higher plants, mutants and transgenic cells and tissues with altered activity of CK metabolic enzymes or perception machinery, have highlighted their crucial involvement in different agriculturally important traits, such as productivity, increased tolerance to various stresses and overall plant morphology. Furthermore, recent precise metabolomic analyses have elucidated the specific occurrence and distinct functions of different CK types in various plant species. Thus, smooth manipulation of active CK levels in a spatial and temporal way could be a very potent tool for plant biotechnology in the future. This review summarises recent advances in cytokinin research ranging from transgenic alteration of CK biosynthetic, degradation and glucosylation activities and CK perception to detailed elucidation of molecular processes, in which CKs work as a trigger in model plants. The first attempts to improve the quality of crop plants, focused on cereals are discussed, together with proposed mechanism of action of the responses involved.

Spatial and temporal profiles of cytokinin biosynthesis and accumulation in developing caryopses of maize

BACKGROUND AND AIMS

Cytokinins are a major group of plant hormones and are associated with various developmental processes. Developing caryopses of maize have high levels of cytokinins, but little is known about their spatial and temporal distribution. The localization and quantification of cytokinins was investigated in maize (Zea mays) caryopsis from 0 to 28 d after pollination together with the expression and localization of isopentenyltransferase ZmIPT1 involved in cytokinin biosynthesis and ZmCNGT, the gene putatively involved in N9-glucosylation.

METHODS

Biochemical, cellular and molecular approaches resolved the overall cytokinin profiles, and several gene expression assays were used for two critical genes to assess cytokinin cell-specific biosynthesis and conversion to the biologically inactive form. Cytokinins were immunolocalized for the first time in maize caryopses.

KEY RESULTS

During the period 0-28 d after pollination (DAP): (1) large quantities of cytokinins were detected in the maternal pedicel region relative to the filial tissues during the early stages after fertilization; (2) unpollinated ovules did not accumulate cytokinins; (3) the maternal nucellar region showed little or no cytokinin signal; (4) the highest cytokinin concentrations in filial endosperm and embryo were detected at 12 DAP, predominantly zeatin riboside and zeatin-9-glucoside, respectively; and (5) a strong cytokinin immuno-signal was detected in specific cell types in the pedicel, endosperm and embryo.

CONCLUSIONS

The cytokinins of developing maize caryopsis may originate from both local syntheses as well as by transport. High levels of fertilization-dependent cytokinins in the pedicel suggest filial control on metabolism in the maternal tissue; they may also trigger developmental programmed cell death in the pedicel.

Aleurone cell identity is suppressed following connation in maize kernels

Expression of the cytokinin-synthesizing isopentenyl transferase enzyme under the control of the Arabidopsis (Arabidopsis thaliana) SAG12 senescence-inducible promoter reverses the normal abortion of the lower floret from a maize (Zea mays) spikelet. Following pollination, the upper and lower floret pistils fuse, producing a connated kernel with two genetically distinct embryos and the endosperms fused along their abgerminal face. Therefore, ectopic synthesis of cytokinin was used to position two independent endosperms within a connated kernel to determine how the fused endosperm would affect the development of the two aleurone layers along the fusion plane. Examination of the connated kernel revealed that aleurone cells were present for only a short distance along the fusion plane whereas starchy endosperm cells were present along most of the remainder of the fusion plane, suggesting that aleurone development is suppressed when positioned between independent starchy endosperms. Sporadic aleurone cells along the fusion plane were observed and may have arisen from late or imperfect fusion of the endosperms of the connated kernel, supporting the observation that a peripheral position at the surface of the endosperm and not proximity to maternal tissues such as the testa and pericarp are important for aleurone development. Aleurone mosaicism was observed in the crown region of nonconnated SAG12-isopentenyl transferase kernels, suggesting that cytokinin can also affect aleurone development.

Comparison of cytokinin-binding proteins from wheat and oat grains

Cytokinin-binding proteins (CBPs) isolated from mature grains of oat (Avena sativa L.) and wheat (Triticum aestivum L.) by acid precipitation, ion-exchange and affinity chromatography had similar characteristics, although they differed somewhat in apparent molecular weight of the native protein as determined by gel filtration (109 and 133 kDa, respectively) and subunit size as estimated by SDS-polyacrylamide gel electrophoresis (47 and 55 kDa, respectively). Highly purified oat CBP showed very weak but distinct immunochemical cross-reactivity with anti-wheat CBP IgG, indicating different immunogenic properties of the two CBPs. Nevertheless, both CBPs exhibited very similar binding of different cytokinins and were characterized by high affinity for N6-benzyladenine (BA)-type and by low affinity for zeatin-type cytokinins to both wheat and oat CBPs and by somewhat higher binding activities of oat CBP compared to wheat CBP (Kds for BA: 4.6 x 10-7 M and 6.8 x 10-7 M, respectively). The potential role of CBPs in regulating free BA-type cytokinin levels during cereal grain development and germination is discussed.

ENDOSPERM DEVELOPMENT: Cellularization and Cell Fate Specification

The endosperm develops from the central cell of the megagametophyte after introduction of the second male gamete into the diploid central cell. Of the three forms of endosperm in angiosperms, the nuclear type is prevalent in economically important species, including the cereals. Landmarks in nuclear endosperm development are the coenocytic, cellularization, differentiation, and maturation stages. The differentiated endosperm contains four major cell types: starchy endosperm, aleurone, transfer cells, and the cells of the embryo surrounding region. Recent research has demonstrated that the first two phases of endosperm occur via mechanisms that are conserved among all groups of angiosperms, involving directed nuclear migration during the coenocytic stage and anticlinal cell wall deposition by cytoplasmic phragmoplasts formed in interzones between radial microtubular systems emanating from nuclear membranes. Complete cellularization of the endosperm coenocyte is achieved through centripetal growth of cell files, extending to the center of the endosperm cavity. Key points in cell cycle control and control of the MT (microtubular) cytoskeletal apparatus central to endosperm development are discussed. Specification of cell fates in the cereal endosperm appears to occur via positional signaling; cells in peripheral positions, except over the main vascular tissues, assume aleurone cell fate. Cells over the main vascular tissue become transfer cells and all interior cells become starchy endosperm cells. Studies in maize have implicated Crinkly4, a protein receptor kinase-like molecule, in aleurone cell fate specification.

The forms and sources of cytokinins in developing white lupine seeds and fruits

A comprehensive range of cytokinins (CK) was identified and quantified by gas chromatography-mass spectrometry in tissues of and in xylem and phloem serving developing white lupine (Lupinus albus) fruits. Analyses were initiated at anthesis and included stages of podset, embryogenesis, and seed filling up to physiological maturation 77 d post anthesis (DPA). In the first 10 DPA, fertilized ovaries destined to set pods accumulated CK. The proportion of cis-CK:trans-CK isomers was initially 10:1 but declined to less than 1:1. In ovaries destined to abort, the ratio of cis-isomers to trans-isomers remained high. During early podset, accumulation of CK (30-40 pmol ovary(-1)) was accounted for by xylem and phloem translocation, both containing more than 90% cis-isomers. During embryogenesis and early seed filling (40-46 DPA), translocation accounted for 1% to 14% of the increases of CK in endosperm (20 nmol fruit(-1)) and seed coat (15 nmol fruit(-1)), indicating synthesis in situ. High CK concentrations in seeds (0.6 micromol g(-1) fresh weight) were transient, declining rapidly to less than 1% of maximum levels by physiological maturity. These data pose new questions about the localization and timing of CK synthesis, the significance of translocation, and the role(s) of CK forms in reproductive development.