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Hexose sugars, such as glucose and fructose produced in plants, are ubiquitous in most organisms and are the origin of most of the organic matter found in nature. To be utilized, hexose sugars must first be phosphorylated. The central role of hexose-phosphorylating enzymes has attracted the attention of many researchers, leading to novel discoveries. Only two families of enzymes capable of phosphorylating glucose and fructose have been identified in plants; hexokinases (HXKs), and fructokinases (FRKs). Intensive investigations of these two families in numerous plant species have yielded a wealth of knowledge regarding the genes number, enzymatic characterization, intracellular localization, and developmental and physiological roles of several HXKs and FRKs. The emerging picture indicates that HXK and FRK enzymes found at specific intracellular locations play distinct roles in plant metabolism and development. Individual HXKs were shown for the first time to be dual-function enzymes -- sensing sugar levels independent of their catalytic activity and controlling gene expression and major developmental pathways, as well as hormonal interactions. FRK, on the other hand, seems to play a central metabolic role in vascular tissues, controlling the amounts of sugars allocated for vascular development. While a clearer picture of the roles of these two types of enzymes is emerging, many questions remain unsolved, such as the specific tissues and types of cells in which these enzymes function, the roles of individual HXK and FRK genes, and how these enzymes interact with hormones in the regulation of developmental processes. It is anticipated that ongoing efforts will broaden our knowledge of these important plant enzymes and their potential uses in themodification of plant traits. [ABSTRACT FROM AUTHOR]

The miniature1 (mn1) seed phenotype in maize (Zea mays) is due to a loss-of-function mutation at the Mn1 locus that encodes a cell wall invertase (INCW2) that localizes exclusively to the basal endosperm transfer cells (BETCs) of developing seeds. A common feature of all transfer cells is the labyrinth-like wall-in-growth (WIG) that increases the plasma membrane area, thereby enhancing transport capacity in these cells. To better understand WIG formation and roles of INCW2 in the BETC development, we examined wild-type and mn1 mutant developing kernels by cryofixation and electron microscbpy. In Mni seeds, WIGs developed uniformly in the BETC layer during 7 to 17 d after pollination, and the secretory/endocytic organelles proliferated in the BETCs. Mitochondria accumulated in the vicinity of WIGs, suggesting a functional link between them. In the mn1 BETCs, WIGs were stunted and their endoplasmic reticulum was swollen; Golgi density in the mutant BETCs was 51% of the Mn1 Golgi density. However, the polarized distribution of mitochondria was not affected. INCW2-specific immunogold particles were detected in WIGs, the endoplasmic reticulum, Golgi stacks, and the trans-Golgi network in the Mn1 BETCs, while immunogold particles were extremely rare in the mutant BETCs. Levels of WIG development in the empty pericarp4 mutant was heterogeneous among BETCs, and INCW2 immunogold particles were approximately four times more abundant in the larger WIGs than in the stunted WIGs. These results indicate that polarized secretion is activated during WIG formation and that INCW2 is required for normal development of WIGs to which INCW2 is localized. [ABSTRACT FROM AUTHOR]

We report here on a comparative developmental profile of plant hormone cytokinins in relation to cell size, cell number and endoreduplication in developing maize caryopsis of a cell wall invertase-deficient miniature1 ( mn1) seed mutant and its wild type, Mn1, genotype. Both genotypes showed extremely high levels of total cytokinins during the very early stages of development, followed by a marked and genotype specific reduction. While the decrease of cytokinins in Mn1 was associated with their deactivation by 9-glucosylation, the absolute and the relative part of active cytokinin forms was higher in the mutant. During the exponential growth phase of endosperm between 6 d after pollination and 9 d after pollination, the mean cell doubling time, the absolute growth rate and the level of endoreduplication were similar in the two genotypes. However, the entire duration of growth was longer in Mn1 compared with mn1, resulting in a significantly higher cell number in the Mn1 endosperm. These data correlate with the previously reported peak levels of the Mn1-encoded cell wall invertase-2 (INCW2) at 12 d after pollination in the Mn1 endosperm. A model showing possible crosstalk among cytokinins, cell cycle and cell wall invertase as causal to increased cell number and sink strength of the Mn1 developing endosperm is discussed. [ABSTRACT FROM AUTHOR]

A full-length cDNA clone, SbSnRK1b (1530 bp, GenBank accession no. EF544393), encoding a putative serine/threonine protein kinase homologue of yeast (Saccharomyces cerevisiae) SNF1, was isolated from developing endosperm of sorghum [Sorghum bicolor (L.) Moench]. Multiple sequence alignment data showed a phylogenetic affiliation of the sorghum clone with the SnRK1b group of protein kinases that are highly expressed in cereal seed endosperm. The DNA gel blot analyses indicated that SbSnRK1b gene is present as a single- or low copy number gene in sorghum. The RNA and protein gel blot analyses confirmed the expression of SbSnRK1b in developing sorghum caryopses, overlapping with the starch biosynthesis phase, 12–24 days after fertilization. In situ hybridization and immunolocalization data resolved the spatial specificity of SbSnRK1b expression in the basal endosperm transfer cell layer, the unique port of assimilate unloading in the growing sorghum seed. Expression of SbSnRK1b was also evident in the developing sorghum microspores, coincident with the onset of starch deposition phase. As in sorghum, similar spatiotemporal specificity of SnRK1b expression was observed during maize (Zea mays L.) seed development. However, discordant in situ hybridization and immunolocalization data indicated that the expression of SbSnRK1b homologue in maize is under posttranscriptional control during endosperm development. [ABSTRACT FROM AUTHOR]

Cotton (Gossypium hirsutum L.) seed develops single-celled long fibres (lint) from the seed-coat epidermis at anthesis. Previous studies have shown that the initiation and rapid elongation of these fibres requires the expression of sucrose synthase (Sus) and, potentially, a transient closure of plasmodesmata. This study extends the previous work to examine the patterns of Sus expression and plasmodesmata gating in fuzz-like short fibres of a mutant that shows delayed initiation and much slower and reduced elongation of the fibre cells. Immunolocalization studies revealed delayed expression of Sus in the mutant seed-coat epidermis that correlates temporally and spatially with the initiation of the fibre cells. Anatomically, these short fibres differed from the normal lint in that their basal ends enlarged immediately after initiation, while the majority of the normal lint on wild-type seed did not show this enlargement until the end of elongation. Suppression of Sus expression in the seed-coat epidermis of the transgenic plants reduced the length of both lint and short fuzz fibres at maturity, suggesting that the growth of short fibres also requires high levels of Sus expression. Confocal imaging of the membrane-impermeant fluorescent solute carboxyfluorescein (CF) revealed no closure of plasmodesmata during the entire elongation period of short fibres from the mutant seed. These results show (i) the delayed initiation of fuzz-like short fibres from the mutant seed correlates with delayed or insufficient expression of Sus in a subset of seed-coat epidermal cells destined to become fibres and (ii) the much shortened elongation of the fibres from the mutant may be related to their inability to close plasmodesmata. [ABSTRACT FROM PUBLISHER]

The structure of the maize (Zea mays L.) root cap was studied to quantitatively evaluate the relationship among the size of the cells, their endopolyploidy level, and the abundance of the sucrose synthase isozyme SUS1. Median longitudinal root cap sections were analysed using immunolocalization, quantitative DNA staining, and image cytometry. Both the immunolocalization signal for the SUS1 protein and the endopolyploidy level increased from calyptrogen towards the root cap periphery and were thus the highest in the outer cells. These cells had a nuclear DNA content of mostly 8C or higher and the largest volumes of all root cap cells. The high amount of SUS1 protein in the outer, endopolyploid cells suggests an association between endoreduplication and the abundance of this enzyme. The outer cells are involved in mucilage production; hence, there is a possibility that sucrose synthase provides monosaccharide precursors for mucilage synthesis. [ABSTRACT FROM AUTHOR]

The pedicel region of Zea mays kernels contains a unique form of maize glutamine synthetase (GS), GSpl. RNA blot analysis using GS gene-specific probes revealed that the expression of the GS,.2 gene was specific to the pedicel and that it increased in the kernels during development. This pattern of the maize GS,.2 gene expression is consistent with the tissue specificity of the GSpl protein and suggests that it encodes the GSpl isoform of maize GS. [ABSTRACT FROM AUTHOR]