Progamic Phase Research Articles

The PTI1-like kinase ZmPti1a from maize (Zea maysL.) co-localizes with callose at the plasma membrane of pollen and facilitates a competitive advantage to the male gametophyte

BackgroundThe tomato kinase Pto confers resistance to bacterial speck disease caused by Pseudomonas syringae pv. tomato in a gene for gene manner. Upon recognition of specific avirulence factors the Pto kinase activates multiple signal transduction pathways culminating in induction of pathogen defense. The soluble cytoplasmic serine/threonine kinase Pti1 is one target of Pto phosphorylation and is involved in the hypersensitive response (HR) reaction. However, a clear role of Pti1 in plant pathogen resistance is uncertain. So far, no Pti1 homologues from monocotyledonous species have been studied.ResultsHere we report the identification and molecular analysis of four Pti1-like kinases from maize (ZmPti1a, -b, -c, -d). These kinase genes showed tissue-specific expression and their corresponding proteins were targeted to different cellular compartments. Sequence similarity, expression pattern and cellular localization of ZmPti1b suggested that this gene is a putative orthologue of Pti1 from tomato. In contrast, ZmPti1a was specifically expressed in pollen and sequestered to the plasma membrane, evidently owing to N-terminal modification by myristoylation and/or S-acylation. The ZmPti1a:GFP fusion protein was not evenly distributed at the pollen plasma membrane but accumulated as an annulus-like structure which co-localized with callose (1,3-β-glucan) deposition. In addition, co-localization of ZmPti1a and callose was observed during stages of pollen mitosis I and pollen tube germination. Maize plants in which ZmPti1a expression was silenced by RNA interference (RNAi) produced pollen with decreased competitive ability. Hence, our data provide evidence that ZmPti1a plays an important part in a signalling pathway that accelerates pollen performance and male fitness.ConclusionZmPti1a from maize is involved in pollen-specific processes during the progamic phase of reproduction, probably in crucial signalling processes associated with regions of callose deposition. Pollen-sporophyte interactions and pathogen induced HR show certain similarities. For example, HR has been shown to be associated with cell wall reinforcement through callose deposition. Hence, it is hypothesized that Pti1 kinases from maize act as general components in evolutionary conserved signalling processes associated with callose, however during different developmental programs and in different tissue types.

The male gametophytic mutant tepitzin1 indicates a requirement of the homeobox gene WOX5 for pollen tube growth in Arabidopsis

The WUSCHEL HOMEOBOX (WOX) gene family of Arabidopsis thaliana transcription factors is composed of at least 15 members encoding homeodomain proteins. We have characterized tepitzin1 (tpz1), a mutant harboring a transposon insertion in the 5′ regulatory region of WOX5. Heterozygous tpz1 plants do not have defects in vegetative growth or female reproductive development but exhibit aberrant pollen transmission, indicating that this mutation acts at the gametophytic level. WOX5 is abundantly expressed in mature pollen grains and elongating pollen tubes. Expression of WOX5 in heterozygous tpz1/+ anthers corresponds to 66% of WOX5 expression in wild-type anthers, confirming that WOX5 activity is affected in the mutant. WOX5-deficient pollen grains did not show developmental abnormalities and germinated at a frequency similar to wild-type pollen; however, in vitro and in vivo germination of pollen from tpz1 heterozygous individuals produced pollen tubes showing delayed germination and reduced length, confirming that the male gametophytic expression of WOX5 is necessary for normal pollen tube elongation. The reduction in average pollen tube length produced by the absence of WOX5 activity is less severe when heterozygous tpz1 individuals are in a homozygous quartet1 background, indicating that the absence of QRT1 activity partially compensates WOX5-deficient pollen tube elongation. Taken together, these results indicate that WOX5 activity is required for normal pollen tube growth in Arabidopsis. Our findings suggest that WOX5 identifies a new genetic pathway integrating homeobox transcription factors and auxin-mediated signals with the progamic phase of plant reproduction.

The Effect of Temperature on Pollen Germination, Pollen Tube Growth, and Stigmatic Receptivity in Peach

Temperature is a major climatic factor that limits geographical distribution of plant species, and the reproductive phase has proven to be one of the most temperature-vulnerable stages. Here, we have used peach to evaluate the effect of temperature on some processes of the progamic phase, from pollination to the arrival of pollen tubes in the ovary. Within the range of temperatures studied, 20 degrees C in the laboratory and, on average, 5.7 degrees C in the field, the results show an accelerating effect of increasing temperature on pollen germination and pollen tube growth kinetics, as well as an increase in the number of pollen tubes that reach the style base. For the last two parameters, although the range of temperature registered in the field was much lower, the results obtained in the laboratory paralleled those obtained in the field. Increasing temperatures drastically reduced stigmatic receptivity. Reduction was sequential, with stigmas first losing the capacity to sustain pollen tube penetration to the transmitting tissue, then their capacity to offer support for pollen germination and, finally, their capacity to support pollen grain adhesion. Within a species-specific range of temperature, this apparent opposite effect of temperature on the male and female side could provide plants with the plasticity to withstand changing environmental effects, ensuring a good level of fertilization.

Hormonal status of the pollen-pistil system at the progamic phase of fertilization after compatible and incompatible pollination in Petunia hybrida L.

The hormonal status of the pollen-pistil system in Petunia hybrida L. during the progamic phase of fertilization was investigated. The contents of indolyl-3-acetic acid (IAA), abscisic acid (ABA), and cytokinins, as well as the rate of ethylene production in the pistils and their parts (stigma, style, and ovary) were measured over an 8-h period following compatible and self-incompatible pollination. In both pollinations, the phytohormones were present in various proportions in the stigma, style and ovary: the stigma was the main site of ethylene synthesis and contained 90% of the ABA, while the style contained 80% of the total cytokinin content in the pollinated pistil. Relatively low levels of hormones in the ovary did not influence the hormonal status of the pollen-pistil system. The interaction of the male gametophyte with the stigmatic tissues was accompanied by a 7- to 10-fold increase in ethylene production and a 1.5- to 2.0-fold increase in IAA content in the pollen-pistil system over 0–4 h. Pollen tube growth after self-incompatible pollination, in contrast to compatible pollination, was accompanied by a 3-fold increase in the ABA content in the stigma and style and by a 5-fold higher cytokinin content in the stylar tissues. Thus, the ethylene/ABA status of the stigma may play a role in controlling the processes of adhesion, hydration, and germination of pollen grains during pollination while the auxin/cytokinin status of the style may be involved in controlling pollen tube growth.

Inheritance and Expression of Gametophyte Mutant Gene Adh-PAffecting the Modification of Alcohol Dehydrogenase in Pollen Grains of the Beet Beta vulgaris L.

An additional alcohol dehydrogenase (ADH) activity zone denoted ADH-P (“pollen”) has a slightly lower mobility than the major protein ADH1 (the product of structural locus Adh1). This zone is detected in maturing and mature pollen grains and has not been found in any other tissue. ADH-P is detected by electrophoresis in a neutral medium (at pH 7.0–7.2). In an alkaline medium (pH > 8), protein ADH-P is completely inactivated, whereas protein ADH1 retains its activity. ADH-P is a modified variant of the major protein ADH1. Both alleles of the main structural gene (Adh1-F and Adh1-S) undergo modification. The pollen of an FS heterozygote has two variants of the modified enzyme: ADH-PS and ADH-PF. Analysis of segregation in F2 offsprings and test crosses has confirmed that this character is controlled by the only gene Adh-P with allelic variantsAdh-P+ (the presence of the modified ADH protein in the pollen) and Adh-P– (the normal protein). Allele Adh-P+ is transmitted through female gametes at a normal frequency (about 1) and through male gametes at a decreased frequency (0.2–0.6), the mean frequency being about 0.4. The frequency of the transmission of allele Adh-P+ through male gametes depends on the genotype of the female parent and the conditions of pollination. Cytoembryological study of microsporogenesis in the Adh-P+/Adh-P– heterozygotes demonstrated an absence of any disturbances in the formation of microspores and pollen grains. Some differences in the formation of pollen tubes on an artificial medium have been observed. It is assumed that the differences between theAdh-P+ and Adh-P– microgametophytes manifest themselves at the progamic phase of fertilization. The possible mechanisms of the formation of the modified ADH-P protein are discussed in connection with the differential activity of genes in the microgametophytes of angiosperms.

Gametophyte–Sporophyte Interactions in the Pollen–Pistil System: 3. Hormonal Status at the Progamic Phase of Fertilization

The content of hormones, IAA, ABA, and cytokinins, as well as the rate of ethylene production in petunia (Petunia hybrida L.) pistils and their parts (stigma, style, and ovary) were determined over 8 h after compatible pollination. At the progamic phase of fertilization in the pollen–pistil system, the phytohormones were virtually absent from the ovary but were present in various proportions in stigma and style. The stigma was the main site of ethylene synthesis and contained 90% of ABA while the style contained 80% of cytokinins of their contents in the whole pollinated pistil. Stigma and style did not differ in their IAA levels. The interaction of the male gametophyte with the stigmatic tissues was accompanied by a threefold increase in the ethylene production and a 1.5-fold increase in the IAA content in the pollen–pistil system within 0–4 h. Growth of pollen tubes in the stylar tissues (4–8 h) was accompanied by a further increase in IAA content and a decrease in the ethylene production by stigmatic tissues, as well as by a decrease in the cytokinin content in the stylar tissues. The ethylene/auxin status of the stigma may be suggested to control the processes of adhesion, hydration, and germination of pollen grains during pollination, while the auxin/cytokinin status of the style controls the pollen tube growth.

Pollen and pistil in the progamic phase

The progamic phase, the period of pollen tube growth through the pistil, is a period of specific interactions between the male gametophyte and the pistil. Understanding of pollen germination and pollen tube growth are relevant for the study of pollen-pistil interactions and for understanding the function of components specifically accumulated in the transmitting tissue cell walls and intercellular matrix that may interact with pollen tubes.

Successful Fertilization and Seed Set from Pollination on Immature Non-dehisced Flowers of Eucalyptus globulus

To minimize the number of visits required to achieve seed-set from hand pollination whilst maintaining full control of the reproductive process in Eucalyptus globulus, we investigated the importance of style maturation on the progamic phase of reproduction. Controlled pollinations were performed in: (1) fully receptive styles; (2) immature cut styles at or after flower dehiscence; and (3) immature cut styles before flower dehiscence. We show here that pollen germination and tube growth to the ovule occurred in all three treatments, and all produced viable seeds. Remarkably, tubes growing on immature sliced styles of non-dehisced flowers reached the ovules by day 7, 1–2d faster than tubes growing on normal receptive stigmas. Moreover, more tubes entered the ovary in this kind of pollination and, consequently, more viable seeds were produced. We established histochemically that tube growth in immature sliced styles is concomitant with the presence of lipid secretion. These results fully support the efficiency of the so-called ‘one-step pollination’ method in Eucalyptus globulus, which we now prove can be applied more successfully to immature non-dehisced flowers. General application of this procedure is likely to have beneficial impacts on the planning and efficiency of large-scale genetic improvement programs.

Postulated interaction between branching pollen tubes and ovules inOenothera hookeri de Vries (onagraceae)

The pollen grain germinationin vitro and progamic phase till fertilization inOenothera hookeri de Vries was observed after open and controlled pollination. The same pattern of pollen grain germination was foundin vitro and on the stigma. The pollen tubes can germinate from 1,2 or 3 poruses of the pollen grain, divide and branch during their growth in the ovary. The branches are of different length and give secondary splits. Special short branches are formed near the micropyle of the ovule. They grow into top part of integments. The pollen tubes start to branch profusely near the placental surface. In that place they are likely to react to the stimulus from mature ovules which seems to be dispersed in the exudate covering placenta.

Progamic phase and fertilization in Gasteria verrucosa (Mill.) H.Duval: pollination signals

A summary is given of the pollen tube pathway, the interactions during the progamic phase and the incompatibility system ofGasteria. Pollen coating substances stimulate pollen tube ovular penetration and fruit set in cases of cross-pollination. Pollen coating substances also stimulate the water supply of the pistil which completes the pollen tube pathway. These two types of signals are related to recognition reactions, as well as to the activation of the pistil. Because the pollen coating substance is of sporophytic origin, the incompatibility system type is discussed.

Pistil strategies controlling pollen tube growth

The progamic phase appears especially well suited for pollen-pistil interaction. During this phase the pistil supports pollen germination and tube growth, and provides an adequate environment, nutrition and directional cues. However, this support does not occur indiscriminantly and some mechanisms operating in the pistil constrain pollen tube growth. An active, regulated constraint is the self-incompatibility reaction, but moderate restrictions of pollen tube growth also occur in compatible matings. These moderate restrictions involve reduced support by the pistil and they operate through two main strategies; one is by decreasing the amount of support and the other is by varying the time at which this support is provided. In this minireview, we examine the evidence that is accumulating for both support and constraint of pollen tube growth by the pistil and discuss the benefits of this dual system.

Sporogenesis, gametogenesis, and progamic phase in Alstroemeria

The sexual reproduction biology of Alstroemeria was studied histologically. The processes of sporogenesis and gametogenesis were described in relation to the development of the male and female organs. Comparative developmental stages in Alstroemeria take much longer than they do in tobacco. Alstroemeria has the monosporic Polygonum type of embryo sac development. Bicellular pollen is formed, which after germination on the receptive stigma immediately undergoes the mitotic division of the generative cell, thus finalizing gametogenesis. Part of the progamic phase has been studied as well. As early as 12 h after pollination, some pollen tubes enter the micropyle of the ovules. This knowledge is important for the application of post-fertilization ovule culture to rescue abortive embryos during interspecific hybridization. Keywords: Alstroemeria, Alstroemeria, Inca Lily, sporogenesis, gametogenesis, progamic phase.

Initial fruit set at high temperature in olive,Olea europaeaL.

SUMMARYThe effects of temperature on the progamic phase, fertilization and initial fruit set in cross pollinated olives were studied to determine the reasons for poor fruit set when high temperatures occur during the bloom period. Fruit set in ‘Manzanillo’ olive was completely inhibited at 30°C constant temperature. This temperature significantly reduced pollen germination but did not prevent pollen tube growth. Ovule penetration by the pollen tube was observed in 47% of the flowers at 30°C; however, the lack of growth of both functional ovules and ovaries suggests problems in zygote formation or endosperm development. The most favourable temperature was 25°C, in which faster pollen tube growth, more and earlier fertilization, and more fruit set occurred. At 20°C pollen tube growth was slower, resulting in delayed and reduced fertilization.

The progamic phase in Cichorium intybus L. Pollen tube growth in the style, incompatibility reaction and gametophytic competition

Pollen germination and tube growth were studied following compatible, incompatible and pseudo-compatible pollinations in chicory. Pollen germination begins 3 minutes after compatible pollinations. The earliest pollen tubes reach the ovary 17 minutes later. Many of the later germinating pollen tubes are arrested and burst at the stigma papillae. In the transmitting tissue inhibitional effects due to negative interactions between pollen tubes are frequently observed. Complete self-incompatibility results in total inhibition of germination. In case of pseudo-self-compatibility, some pollen germinate but germination and stigma penetration are delayed and often result in pollen bursting. There is no self-incompatibility reaction in the transmitting tract but if the pollen tubes reaching this tissue are relatively numerous, negative interactions between them occur as after compatible pollinations. An hypothesis is presented which attributes the negative interactions between pollen tubes to the diffusion of a substance from the growing pollen tubes. This substance would also provoke pollen bursting on the stigma.

The mentor pollen phenomenon in poplars: a new concept

The mentor effect has been investigated in poplars. Attempts to overcome interspecific incompatibility are analysed by pollen germination and pollen tube behaviour in situ, both for compatible and incompatible crosses. We have demonstrated that following the mixed pollination, the two pollen sets interact at different levels of the progamic phase. A hypothetical model is proposed which describes mentor effect as the result of interactions of antagonist and cynergic forces applying on compatible pollen and tubes. These forces promote pollen tube growth both on the female partner surface and within the tissues.

Sexual reproduction in Populus I. Some physiological and biochemical events of the progamic phase

Sexual reproduction in Populus I. Some physiological and biochemical events of the progamic phase

Cytochalasin B Affects Protein Secretion in Germinating Pollen of Malus domestica Borkh.

Summary Apple pollen sensitivity to cytochalasin B was studied during germination in vitro , with consideration of the effect on tube emergence and growth, on protein release in the culture medium, and on secretion of tube wall proteins. Cytochalasin B from 0.05 to 1.0 μg ml −1 induced strong inhibition of tube elongation, whereas the pure emergence was less affected by the drug. At 0.1μg ml −1 the cytochalasin blocked protein export from 90 min germinated pollen tubes and markedly reduced 3 H-proteins of the ionically-bound fraction of tube walls. The inhibition of protein secretion agrees with numerous ultrastructural observations on the cytochalasin B effect in pollen tubes or other plant systems. In the case of pollen, the ionic protein fraction of the tube wall is important for communication with the stigma or style during the progamic phase of fertilization.

Effect of carbon dioxide and relative humidity on self incompatibility in cauliflower, Brassica oleracea

The events of the progamic phase of fertilization have been monitored by in vitro experiments in self compatible (SC), partial self-incompatible (PSI) and self incompatible (SI) lines. The duration of the progamic phase is about 30 h. Treatment with low concentrations of CO2 (3 to 5%) at high relative humidity (rH, 100%) had the following effects: pollen quality, which declines normally during flower ageing, was prematurely reduced; pollen adhesion and germination, both low in SI matings, were increased; the stigma callose response in SI matings was reduced to the low level of SC matings; and the number of pollen tubes in the style after SI matings significantly increased. CO2 concentrations of 4 to 6% applied for 8, 16 or 24 h at 100% rH proved to be the most effective treatment for blocking the SI response in cauliflower.

RNA, Proteins and Polyamines During Tube Growth in Germinating Apple Pollen

Variations of RNA, protein, and free- and trichloroacetic acid-soluble bound polyamine levels were determined during tube growth in germinating Malus domestica Borkh. cv. Starkrimson pollen.During rehydration of pollen no marked differences were observed, whereas, during germination, RNA, proteins, and polyamines showed parallel decreases. At the same time, there was synthesis of RNA and polyamines as indicated by use of labeled precursors. The data indicate that during germination: (a) the genes for rRNA, tRNA, and probably mRNA are active; (b) the enzymes involved in polyamine biosynthesis are very active. High levels of free arginine during the first 15 minutes were observed, probably in response to a demand for this precursor in polyamine biosynthesis. Moreover, profiles of the variations in the specific activities of RNA and polyamines showed similar patterns. The results indicate that biosynthesis of RNA and polyamines precedes tube emergence. The possible role of these compounds, which are known to be released into the medium in the progamic phase of the fertilization processes, is considered.

EFFECT OF SEASON, PLANT AGE AND TEMPERATURE DURING PLANT GROWTH ON COMPATIBLE AND INCOMPATIBLE POLLEN TUBE GROWTH IN PETUNIA HYBRIDA

SUMMARY In Petunia hybrida the length of pollen tubes was measured after growth in the style for 24 h. The length attained depends upon the temperature regime under which the pollen as well as the styles developed prior to pollination. The temperature regimes used were 25.5/18°C and 19.5/18°c respectively, both in a 16/8 h day/night regime. A high temperature regime during the progamic phase causes a greater length of the tubes than the low temperature regime; this difference is larger after cross- than after self-pollination. The length after 24 hours' growth of incompatible pollen tubes is larger when the pollen developed, prior to pollination, under the high temperature regime than under the low one, but their length is not affected by the temperature at which the styles developed. With compatible pollen tubes, on the other hand, that length is larger when the styles developed under the high temperature regime than under the low one and it is also enhanced by a pretreatment of styles, developed under a low temperatue regime, with the higher temperature for 24 h. The enhancement is largest when the treatment precedes pollination at the low temperature regime immediately and is absent when the time between treatment and pollination exceeds four weeks. The length of incompatible pollen tubes after 24 h is in general lower in plants cultivated from summer- than from winter-cuttings; the length of compatible pollen tubes in plants cultivated from winter-cuttings decreases with the age of the plant, if style development and tube growth take place under the low temperature regime. Compatible pollen germination is affected by a low temperature regime during style development and progamic phase. The incompatible pollen germination rate is lower in pollen from summer- than from winter-cuttings, when pollen development and pollination occur at the low temperature regime.