Today no one questions the assertion that Ca2+ is a crucial regulator of growth and development in plants. The myriad processes in which this ion participates is large and growing and involves nearly all aspects of plant development (recent reviews in [Harper et al., 2004][1]; [Hetherington and

/pdf/calcium-a-central-regulator-of-plant-growth-and-development-z4sfsxskgo.pdf

Calcium: A Central Regulator of Plant Growth and Development

Budding in the yeast Saccharomyces cerevisiae involves a polarized deposition of new cell surface material that is associated with a highly asymmetric disposition of the actin cytoskeleton. Mutants defective in gene CDC24, which are unable to bud or establish cell polarity, have been of great interest with regard to both the mechanisms of cellular morphogenesis and the mechanisms that coordinate cell-cycle events. To gain further insights into these problems, we sought additional mutants with defects in budding. We report here that temperature-sensitive mutants defective in genes CDC42 and CDC43, like cdc24 mutants, fail to bud but continue growth at restrictive temperature, and thus arrest as large unbudded cells. Nearly all of the arrested cells appear to begin nuclear cycles (as judged by the occurrence of DNA replication and the formation and elongation of mitotic spindles), and many go on to complete nuclear division, supporting the hypothesis that the events associated with budding and those of the nuclear cycle represent two independent pathways within the cell cycle. The arrested mutant cells display delocalized cell-surface deposition associated with a loss of asymmetry of the actin cytoskeleton. CDC42 maps distal to the rDNA on chromosome XII and CDC43 maps near lys5 on chromosome VII.

/pdf/cdc42-and-cdc43-two-additional-genes-involved-in-budding-and-30kaf5xic1.pdf

CDC42 and CDC43, two additional genes involved in budding and the establishment of cell polarity in the yeast Saccharomyces cerevisiae.

Publisher Summary Pollen germination and pollen-tube growth events when viewed either in culture or in the style are considered as biochemically, physiologically, and structurally complex. The various processes involved are each independent in some measure, but still all are interactive—namely, the successful attainment of the delivery of two male gametes into the embryo sac, requires a high degree of coordination within the male gametophyte and at the same time a remarkable sensitivity to a considerable range of influences both in the physical environment and in the pistil. Much of the activity of the vegetative cell has the air of being preprogrammed; and this must be so in those species where there is little or no gene transcription in the vegetative nucleus. The capacity to function successfully in a complex and varying environment betokens a high degree of self-regulation. Some of the feedback mechanisms likely to contribute to stability are reviewed. They are analogous with the homeorhetic systems postulated in other instances of biological development. However, two features make the angiosperm male gametophyte almost unique among higher organisms, namely the versatility expressed by a single haploid cell, and the degree of coadaptation it shows with its diploid host.

Pollen germination and pollen-tube growth

This review considers pollen tube growth with regard to current information on pollen tube cytoplasm, wall structure and calcium ion interactions with pollen tubes. Pollen tubes have a marked cytoplasmic Polarity with a number of distinct zones along the tube, each with a characteristic complement of cytoplasmic and nuclear structures. The cytoplasmic structures are characteristic of secretory cells with extensive endoplasmic reticulum cisternae and numerous dictyosomes. The dictyosomes produce secretory vesicles that are mainly directed to the extending tip of the tube, where they provide new plasma membrane and wall components. The rates of secretory vesicle production and delivery have been estimated, allowing quantitative assessments of the rate of delivery of materials to the tip. Pollen tubes contain cytoskeletal components, with microtubules and microfilament strands lying axially in the main tube and diffuse microfilament strands at the tip. The tube wall consists of an outer fibrous layer containing pectins and an inner, more homogeneous layer containing callose and cellulose-like microfibrils, possessing both β-1,4 and β-1,3 linkages. Protein is also present in the wall. The tube tip lacks the inner callosic wall. This type of structure is considered to be different from that of elongating sporophyte tissue cells which are enclosed by a wall containing layers of cellulose microfibrils. Calcium ions are required for pollen tube growth and, in at least some species, act as a chemotropic agent. High concentrations of calcium ions in the external medium inhibit growth. Pollen tubes contain some calcium ions bound to the cell wall and larger amounts located intracellularly, which enter the tube at the tip. This intracellular calcium is present as ions that exist freely within the cytoplasmic Matrix and as ions bound to membrane systems. The highest concentrations in both of these pools are found at the tip and in both they decline towards the base. The structure of the tip and the activity involved in providing components for plasma membrane and Wall assembly provide a basis for considering possible mechanisms of tip growth. Two hypotheses to account for the regulation of tip extension are considered, cell wall control and cytoskeletal control. In the cell wall hypothesis, control depends on an interaction between internal turgor pressure and a plastic cell wall. The mechanical properties of the wall are assumed to be partly dependent on the availability of external calcium ions to crosslink acidic pectin chains. According to this hypothesis, high external calcium ion concentrations cause cessation of tip growth due to increased mechanical resistance of the tip wall. Various observations on plant cell-wall interactions with calcium ions and on experimentally-treated pollen tubes provide evidence that does not support this hypothesis. The cytoskeletal control hypothesis of tip growth depends on the internal tip cytoskeleton to contain the tube tip cytoplasm against the internal turgor pressure during cell wall assembly. The activities and mechanical properties of the cytoskeleton are assumed to depend on the availability of external calcium ions. High external concentrations are believed to cause a state of rigor in the cytoskeleton and hence a cessation of tip growth. Some experimental evidence is presented which suggests that the effects of excess calcium ions are on intracellular processes, and not extracellular ones. The mitochondrial zone behind the tip is believed to maintain the tip calcium ion concentration at an optimal level for growth. Some comparisons are made between tip growth in pollen tubes and that in other tip growing cells. CONTENTS Summary 323 I. Introduction 324 II. Cytoplasm 326 III. Wall structure 332 IV. Calcium 335 V. Tip growth 339 VI. Conclusions 350 References 351.

Pollen tube tip growth.

Lily pollen tubes possess a steep, tip-focused intracellular Ca2+ gradient and a tip-directed extracellular Ca2+ influx. Ratiometric ion imaging revealed that the gradient extends from above 3.0 microM at the apex to approximately 0.2 microM within 20 microns from the tip, while application of the Ca(2+)-specific vibrating electrode indicated that the extracellular influx measured between 1.4 and 14 pmol cm-2 sec-1. We examined the relationship between these phenomena and their role in tube growth by using different 1,2-bis(o-aminophenoxy)ethane N,N,N',N'-tetraacetic acid (BAPTA)-type buffers and hypertonic media. Injection of active BAPTA-type buffers or application of elevated levels of sucrose reversibly inhibited growth, destroyed tip zonation of organelles, and modified normal patterns of cytoplasmic streaming. Simultaneously, these treatments dissipated both the intracellular tip-focused gradient and the extracellular Ca2+ flux. Of the BAPTA-type buffers, 5,5'-dibromo-BAPTA (dissociation constant [Kd] is 1.5 microM) and 4,4'-difluoro-BAPTA (Kd of 1.7 microM) exhibited greater activity than those buffers with either a higher affinity (5,5'-dimethyl-BAPTA, Kd of 0.15 microM; BAPTA, Kd of 0.21 microM; 5,5'-difluoro-BAPTA, Kd of 0.25 microM) or lower affinity (5-methyl, 5'-nitro-BAPTA, Kd of 22 microM) for Ca2+. Our findings provide evidence that growing pollen tubes have open Ca2+ channels in their tip and that these channels become inactivated in nongrowing tubes. The studies with elevated sucrose support the view that stretching of the apical plasma membrane contributes to the maintenance of the Ca2+ signal.

Pollen tube growth is coupled to the extracellular calcium ion flux and the intracellular calcium gradient: effect of BAPTA-type buffers and hypertonic media.

Pollen of L. longiflorum was grown in 45Ca-labeled medium and washed with nonradioactive medium. Whole, labeled pollen was then frozen and autoradiographed at -78 degrees C. The autoradiographs show striking accumulations of 45Ca in the growing tips of the pollen tubes. This result is obtained when the pollen is labeled for times as short as 1 min, or as long as 5 h. In most cases, the tip concentration is about two to four times greater than that in the bulk of the pollen tube, and extends for a length of about 20 mum. In autoradiographs of tubes longer than 1 mm, a small fraction of cells show a distinctly larger 45Ca accumulation, the tip containing more than 100 times that in the rest of the cell. The 1- to 5-h labeling experiments show that calcium is relatively concentrated within the cytoplasm of the growing tip. The 1- to 3-min labeling experiments suggest that calcium may enter the tip faster than it enters other regions. These patterns of calcium accumulation and flux may be related to the localized secretion of vesicles at the grow;ng tip.

/pdf/calcium-accumulations-within-the-growing-tips-of-pollen-rwshyohi3w.pdf

Calcium accumulations within the growing tips of pollen tubes.

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Calcium accumulations within the growing tips of pollen tubes.