Genetic analysis as a tool to investigate the molecular mechanisms underlying seed development in maize

BACKGROUND: In angiosperms the seed is the outcome of double fertilization, a process leading to the formation of the embryo and the endosperm. The development of the two seed compartments goes through three main phases: polarization, differentiation of the main tissues and organs and maturation. SCOPE: This review focuses on the maize kernel as a model system for developmental and genetic studies of seed development in angiosperms. An overview of what is known about the genetic and molecular aspects underlying embryo and endosperm formation and maturation is presented. The role played by embryonic meristems in laying down the plant architecture is discussed. The acquisition of the different endosperm domains are presented together with the use of molecular markers available for the detection of these domains. Finally the role of programmed cell death in embryo and endosperm development is considered. CONCLUSIONS: The sequence of events occurring in the developing maize seed appears to be strictly regulated. Proper seed development requires the co-ordinated expression of embryo and endosperm genes and relies on the interaction between the two seed components and between the seed and the maternal tissues. Mutant analysis is instrumental in unravelling the genetic control underlying the formation of each compartment as well as the molecular signals interplaying between the two compartments.     

Class II tassel seed mutations provide evidence for multiple types of inflorescence meristems in maize (Poaceae)

The tassel seed mutations ts4 and Ts6 of maize cause irregular branching in its inflorescences, tassels, and ears, in addition to feminization of the tassel due to the failure to abort pistils. A comparison of the development of mutant and wild-type tassels and ears using scanning electron microscopy reveals that at least four reproductive meristem types can be identified in maize: the inflorescence meristem, the spikelet pair meristem, the spikelet meristem, and the floret meristem. ts4 and Ts6 mutations affect the fate of specific reproductive meristems in both tassels and ears. ts4 mutants fail to form spikelet meristems from spikelet pair meristems. Ts6 mutants are delayed in the conversion of certain spikelet meristems into floret meristems. Once floret meristems are established in both of these mutants, they form florets that appear normal but fail to undergo pistil abortion in the tassel. The abnormal branching associated with each mutant is suppressed at the base of ears, permitting the formation of normal, fertile spikelets. The classification of the different types of reproductive meristems will be useful in interpretation of gene expression patterns in maize. It also provides a framework for understanding meristem functions that can be varied to diversify inflorescence architectures in the Gramineae.     

Induction of callose formation is a sensitive marker for genotypic aluminium sensitivity in maize

The screening of 37 Zea mays L. cultivars in nutrient solution using root elongation (24 h) as a parameter showed large genotypic differences in Al resistance among the genetic material evaluated.Callose concentrations in root tips were closely and positively related to Al-induced inhibition of root elongation. Therefore, Al-induced callose formation in root tips appears to be an excellent indicator of Al injury and can be used as a selection criteria for Al sensitivity. In contrast, aluminium concentrations in root tips were not related to Al-induced inhibition of root elongation, nor to Al-induced callose formation. Callose formation was also induced by short-term A1 treatment in root tip protoplasts, and the response of protoplasts clearly reflected the cultivar-specific response to Al of intact roots. This indicates that in maize, Al sensitivity is expressed on the protoplast level.     

Relationship between development of endosperm transfer cells and grain mass in maize

The most basal endosperm cells of maize (Zea mays L.) began differentiating into transfer cells in 10 days after pollination (DAP). The thickening and ingrowths forming in the transfer cell wall were slow during 10 and 15 DAP. There were many vesicles, silky and string ball objects in cytoplasm, and the number of mitochondria and rough endoplasm reticulum increased. After 15 DAP, the wall thickening and ingrowths forming in the transfer cells sped up. By 20 DAP, the transfer cell zone had developed, there appeared 65 - 70 rows of cells in width and 3 - 4 layers of cell in depth, the obvious cell wall ingrowths presented strong positive reaction with periodic acid Schiff's reagent. After 20 DAP, no significant change appeared in the shape and structure of the transfer cells, and the transfer cells entered function stage. In the mature kernels (53 DAP), the most basal transfer cells were filled with ingrowths, however, dense cytoplasm was also found in these cells. The nuclei had quite irregular shapes in these cells. Some transfer cells contained black grains and crystals. A black layer formed in the pericarp tissue adjacent to the transfer cell zone. Full development of endosperm transfer cells was important for reduction of kernel abortion and increase of kernel mass. This revised version was published online in July 2006 with corrections to the Cover Date.     

Genetic and molecular approaches to the study of the anaerobic response and tissue specific gene expression in maize

Abstract. Plant gene expression is regulated during development and in response to environmental stimuli. For example, the pattern of gene expression in roots changes dramatically when seedlings are placed under low oxygen conditions. Roots respond to anaerobiosis by increasing cytosolic-glycolytic enzyme activity. Through the use of genetic and molecular techniques we have begun to characterize the differential expression of isozymes which show increased synthesis under anaerobic conditions and tissue specific expression during plant development.     

夏玉米植株及叶片生长发育热量需求的试验与模拟研究

通过比较国内多种积温计算方法造成的结果差异,推荐一种有较强生物学意义和普适性的职温计算方法,并以此计算了不同播期,不同品种,不同密度以及不同水肥管理条件下夏玉米各生育阶段,从出苗到各叶片完全展开以及各叶片生期的积温,讨论了影响植株及叶片生长发育期积温稳定性的主要因子,并模拟了单株出速度与出苗后温度累积及叶片生活期积温需求与叶龄的定量关系。     

Dosage regulation of Zea mays homeobox (ZmHox) genes and their relationship with the dosage-sensitive regulatory factors of Shrunken 1 (Sh1) in maize

In gene dosage studies on the expression of the maize Shrunken I gene several dosage-sensitive regulatory factors were identified that modulate Sh 1 mRNA levels in various aneuploids. ZmHox 1a and 1b have been shown to interact with the Sh1 promoter sequences in vitro. The present study was conducted to test whether these molecularly defined regulatory genes are subject to dosage modulation and to determine whether ZmHox 1a and 1b are involved with the dosage sensitive modification of Sh1 expression. The result was that ZmHox 1a and 1b were affected by several transacting dosage modifiers and that both genes exhibited a structural gene dosage effect but did not modulate Sh1 mRNA levels. For some chromosomal regions, a correlation was found between the dosage regulation of Sh1 and that of ZmHox 1a, while other effects were not correlated. The results suggest that the dosage effects on Sh1 are not mediated by ZmHox 1a or 1b, but interestingly Sh1 and ZmHox 1a share some dosage-regulatory effects. Dev Genet 20:67–73, 1997. © 1997 Wiley-Liss, Inc.     

Phytochrome is required for the occurrence of time-dependent phototropism in maize coleoptiles

Time-dependent phototropism (TDP), sometimes called second positive curvature, occurs when the duration of phototropic stimulation with blue light (B) exceeds a few minutes. TDP was characterized in maize (Zea mays L.) coleoptiles raised under continuous red light (R). Subsequently, coleoptiles adapted to darkness were used to investigate the effect of R on TDP. It was found that TDP, which is induced in R-grown coleoptiles, does not occur in dark-adapted coleoptiles and that dark-adapted coleoptiles begin to show TDP after treatment with R. The TDP responsiveness became maximal 1-2 h after treatment with a R pulse and decreased during the next few hours. At least 10 min was required after a short pulse of R before the coleoptile began to respond to B for the induction of TDP. The effect of R in establishing the TDP responsiveness was totally suppressed by a pulse of far-red light given immediately after an inductive pulse of R. It is concluded that the mechanism of TDP requires for its establishment a R signal perceived by phytochrome. The TDP of R-grown and R-pretreated coleoptiles showed relationships to stimulation times and fluence rates that are similar to those reported for oat coleoptiles, except that TDP of maize showed a sharp increase in its magnitude within a narrow range of stimulation times as short as 5-10 min.     

Respiration rate in maize roots is related to concentration of reduced nitrogen and proliferation of lateral roots

The relationship between specific rate of respiration (respiration rate per unit root dry weight) and concentration of reduced nitrogen was examined for maize ( Zea mays L.) roots. Plants with 2 primary nodal root axes were grown for 8 days in a split-root hydroponic system in which NO-₃ was supplied to both axes at 1.0 mol m⁻³, to one axis at 1.0 mol m⁻³ and the other axis at 0.0 mol m⁻³ or to both axes at 0.0 mol m⁻³ Respiration rates and root characteristics were measured at 2-day intervals. Specific rate of respiration was positively correlated in a nonlinear relationship with concentration of reduced nitrogen. The lowest specific rates of respiration occurred when neither axis received exogenous NO⁻³ and the concentration of reduced nitrogen in the axes was less than 9 mg g⁻¹. The greatest rates occurred in axes that were actively absorbing NO⁻³ and contained more than 35 mg g⁻¹ of reduced nitrogen. At 23 mg g⁻¹ of reduced nitrogen, below which initiation of lateral branches was decreased by 30–50%. specific rate of respiration was 17% greater for roots actively absorbing NO⁻³ than for roots not absorbing NO⁻³ Increases in specific rate of respiration associated with concentrations of reduced nitrogen greater than 23 mg g⁻¹ were concluded to be attributable primarily to proliferation of lateral branches.     

Sloughing of cap cells and carbon exudation from maize seedling roots in compacted sand

Sloughing of root cap cells and exudation of mucilage plays an important role in the penetration of compacted soils by roots. For the first time we have quantified the rate of sloughing of root cap cells in an abrasive growth medium that was compacted to create mechanical impedance to root growth. The number of maize ( Zea mays ) root cap cells sloughed into sand increased as a result of compaction, from 1930 to 3220 d⁻¹ per primary root. This represented a 12-fold increase in the number of cells sloughed per mm root extension (from 60 to >700). We estimated that the whole of the cap surface area was covered with detached cells in compacted sand, compared with c . 7% of the surface area in loose sand. This lubricating layer of sloughed cells and mucilage probably decreases frictional resistance to soil penetration. The total carbon deposited by the root was estimated at c . 110 μg g⁻¹ sand d⁻¹. Sloughed cells accounted for <10% of the total carbon, the vast majority of carbon being contained in mucilage exudates.     

Identification and characterisation of an RPD3 homologue from maize ( Zea mays L.) that is able to complement an rpd3 null mutant of Saccharomycescerevisiae

In mammals, yeast and Drosophila, the histone deacetylase RPD3 proteins can alter the expression of genes involved in fundamental biological processes by affecting the degree of acetylation of histones and changing chromatin structure. Here we report the isolation of a cDNA sequence encoding an RPD3 homologue from maize, which is able to complement the phenotype of an rpd3 null mutant of the yeast Saccharomyces cerevisiae. The expression of the corresponding gene(s) was assessed in different maize tissues. The number of homologous loci was estimated by Southern hybridisation to be in the range of two to three, and the chromosomal location of one of these loci was determined. Phylogenetic analysis and tests for relative divergence rates, using related RPD3 sequences from different species, were performed, and suggest that different polymorphic forms of RPD3-like proteins that evolve at distinct rates are present in the species considered. Received: 5 December 1997 / Accepted: 16 January 1998     

Immunological evidence for the presence of plant homologues of the actin- related protein Arp3 in tobacco and maize: subcellular localization to actin-enriched pit fields and emerging root hairs

Summary. The actin-nucleating and -organizing Arp2/3 protein complex is well known to be conserved throughout the eukaryotic kingdom. For higher plants, however, only limited evidence is available for the presence of the Arp2/3 complex so far. Using heterologous antibodies against the Dictyostelium discoideum and Schizosaccharomyces pombe proteins and a bovine peptide, we found immunological evidence for the presence of Arp3 homologues in plants. First, proteins with a molecular mass of about 47–50 kDa were clearly recognized in extracts of both a dicotyledonous plant (tobacco) and a monocotyledonous plant (maize) in immunoblots with the anti-Arp3 antibodies. Second, immunolocalization with these Arp3 antibodies was performed on different plant cells, selected for their diverse actin organizations and functions. On isolated plasma membrane ghosts derived from tobacco leaf protoplasts, a putative Arp3 was localized along cortical actin filaments. In the inner cortex of maize roots, Arp3 was localized to actin-rich plasmodesmata and pit fields and to multivesicular bodies in the cytoplasm. During root hair formation, distinct site-specific localization was found at the protruding apical plasma membrane portions of these tip-growing cells.Correspondence and reprints: Department of Biology, Universitaire Instelling Antwerpen, Universiteitsplein 1, 2610 Wilrijk, Belgium.Received January 3, 2003; accepted February 7, 2003; published online August 26, 2003     

Absence of turnover and futile cycling of sucrose in leaves of<Emphasis Type="Italic"> Lolium temulentum</Emphasis> L.: implications for metabolic compartmentation

To study the interdependence of sucrose accumulation and its hydrolyzing enzyme, soluble acid invertase (AI; EC 3.2.1.26), in fructan-accumulating temperate grasses and cereals, experiments were performed in which sucrose synthesis was abolished in leaves of Lolium temulentum by four independent inhibitory factors, each having a distinct mechanism of action. Trials in the light with mannose or vanadate and in the dark with anoxia or cyanide showed that previously accumulated sucrose was stable in the tissue over a 5- to 6-h period. Conversely, putatively vacuolar AI activity in tissue homogenates was sufficient to completely convert endogenous sucrose to monosaccharide within the same period. Continuous invertase-mediated breakdown of sucrose was thus not a feature of this tissue. It is concluded that AI and sucrose were not in metabolic contact in vivo, implying differential compartmentation. In darkness, in uninhibited leaves, sucrose concentrations fell linearly with respect to time at a rate of –0.6 mg g^–1 FW h^–1, over a 5- to 6-h period. This value is equivalent to rates of dark respiration measured by gas exchange. Dark-utilisation of sucrose was not accompanied by monosaccharide accumulation in the tissue. The rate of sucrose loss was 3-fold lower than rates of extractable AI activity. Hence, if AI was involved in dark-utilisation, then this implies at least a partial differential localisation of enzyme and substrate. However, the dark-consumption of sucrose was completely abolished by anoxia and by cyanide. It follows that dark-mobilisation (unlike invertase hydrolysis per se) was respiration-dependent and did not result from a simple co-localisation of sucrose and invertase. Taken together, the results show that sucrose and invertase do not share the same metabolic compartment in grass leaves. It is possible that invertase has no role in the mobilisation of stored sucrose in leaves of the fructan-accumulating grasses.Abbreviations AI Acid invertase - PAR Photosynthetically active radiation - TLC Thin-layer chromatography     

An excision and squash technique for analysis of the cell cycle in the root quiescent centre of maize

The quiescent centre of primary roots of Zea mays L. (cvs LG 11 and Golden Bantam) consists of a population of slowly cycling diploid cells. These metabolically inactive cells may be triggered to synthesise DNA under specific conditions and constitute a good model for studying the regulation of the cell cycle. An excision and squash technique is described for the quiescent centre which, when coupled with Feulgen and fluorochrome staining, allows nuclear DNA contents to be determined by microdensitometry in less than a day. This technique was coupled with experiments in which excised quiescent centres were placed on solid culture medium into which hormones and radioactive DNA precursors were incorporated. In complementary and control experiments [methyl-³H]thymidine was supplied to intact roots (with or without root caps) by means of fibre-glass cubes as donors.
Progression of the cell cycle was followed by microdensitometry and autoradiography. Distribution of DNA content was similar in excised and squashed quiescent centres and in histological sections. Labelling experiments showed that the quiescent centre is made up of cells that differ in their cycle time. While some labelled cells had reached mitosis after 8 h, others were still in G₂ after 16 h of continuous labelling. Excision and culture of the quiescent centre resulted in a dramatic activation of the cell cycle as shown by the labelling index that increased from 15% in intact roots fed during 16 h with [methyl-³H]thymidine to 31% in excised quiescent centres to which radioactive precursor was supplied during the same time. Supplying hormones (50 μ M abscisic acid [ABA], 0. 1 μ M zeatin, 1 μ M zeatin riboside) to quiescent centres via the culture medium restored their inactivity (labelling indices dropped to 1% after ABA. and to 11% after zeatin and zeatin riboside treatments. respectively).     

Changes in leaf protein and pigment contents and photosynthetic activities during senescence of detached maize leaves: influence of different ultraviolet radiations

Senescence induced loss in pigments and proteins of detached maize (Zea mays L. cv. Col) leaves was significantly enhanced on the exposure of leaves to different ranges of ultraviolet (UV) radiation. Compared to UV-A (320-400 nm) and UV-B (280-320 nm), the UV-C (200-320 nm) was the most damaging for the pigments and macromolecules. A severe decline in photosystem (PS) 2 mediated photoreduction during senescence of detached leaves exposed to UV irradiation suggested a damage of the system. The PS1 mediated photoreduction of methylviologen with 2,6-dichlorophenol indophenol as electron donor was stimulated by UV-A and UV-B radiations, suggesting a reorganisation of the PS1 complex. These results were fortified by the values of fast and slow kinetics of chlorophyll (Chl) a fluorescence transients. This revised version was published online in June 2006 with corrections to the Cover Date.     

Production of normal,germinable and viable pollen from in vitro-cultured maize tassels

Summary Immature tassel meristems (1.0–1.5 cm long) of Zea mays L. inbred, Oh43, and single cross hybrid, Se60, cultured on a nutrient liquid medium underwent extensive development through to maturity and produced normal, mature, trinucleate pollen grains. The grains germinated on nutrient agar and on receptive silks and also produced viable kernels. No differences were observed between in vitro-produced pollen and in vivo pollen (pollen from greenhouse-grown plants) in characteristics such as pollen size, in vitro and in situ germination, and pollen tube growth in vitro. The kernels produced with in vitro pollen grew into mature plants (in vitro plants) which were similar to in vivo plants (plants produced with in vivo pollen), with no significant differences for all the morphological characteristics measured, and no phenotypic and cytological abnormalities. Gel electrophoresis of polypeptides revealed no major differences between in vitro and in vivo seedlings. This demonstration of fertilization and production of normal, uniform plants with pollen from cultured tassels has significant potential in basic and applied research studies.     

Impact of iron deficiency and iron re-supply during the early stages of vegetative development in maize (Zea mays L.)

The consequences of iron deficiency and iron re-supply were evaluated during the early stages of growth and development of young maize plantlets grown hydroponically in the absence of iron. Various parameters, such as fresh and dry weights, and the concentration of chlorophylls, iron, copper, manganese, calcium, magnesium and potassium in leaves, were measured at various times during the first 15 d of culture. Ten-day-old maize plantlets grown without iron displayed severe alterations, with a 50% decrease in iron and chlorophyll concentrations in leaves, and serious impairments in mitochondria and chloroplast ultrastructure. In contrast, neither leaf nor root growth, nor other mineral concentrations other than iron were significantly affected at this stage of development. In an attempt to characterize proteins potentially involved in iron nutrition or the adaptative response to iron starvation, comparative 2D-gel electrophoretic analysis of polypeptides was carried out on soluble and membrane fractions prepared from leaves and roots of iron-deficient and iron-sufficient 10-d-old maize plantlets. Two polypeptides (11 and 17 kDa, pI of about 6.8) from the microsomal fraction of leaves were found to be repressed under iron-deficient conditions. Some other polypeptides were found to he induced in microsomal fractions either from roots or leaves. Significant variations in the concentration of most of these polypeptides were observed from one experiment to another. It can be concluded from this study that, at this early stage of maize vegetative growth and development, molecular variations induced by iron deficiency do not affect major house-keeping proteins, but probably affect very specific events depending on low abundance proteins.     

Genome size and numerical polymorphism for the B chromosome in races of maize (Zea mays ssp. mays, Poaceae)

Twenty-one native populations (1120 individuals) of maize from Northern Argentina were studied. These populations, which belong to 13 native races, were cultivated at different altitudes (80-3620 m). Nineteen of the populations analyzed showed B chromosome (Bs) numerical polymorphism. The frequency of individuals with Bs varied from 0 to 94%. The number of Bs per plant varied from 0 to 8 Bs, with the predominant doses being 0, 1, 2, and 3. Those populations with varying number of Bs showed a positive and statistically significant correlation of mean number of Bs with altitude. The DNA content, in plants without Bs (A-DNA)(2n = 20), of 17 populations of the 21 studied was determined. A 36% variation (5.0-6.8 pg) in A-DNA content was found. A significant negative correlation between A-DNA content and altitude of cultivation and between A-DNA content and mean number of Bs was found. This indicates that there is a close interrelationship between the DNA content of A chromosomes and doses of Bs. These results suggest that there is a maximum limit to the mass of nuclear DNA so that Bs are tolerated as long as this maximum limit is not exceeded.