High temperature limits in vivo pollen tube growth rates by altering diurnal carbohydrate balance in field-grown Gossypium hirsutum pistils

It has recently been reported that high temperature slows in vivo pollen tube growth rates in Gossypium hirsutum pistils under field conditions. Although numerous physical and biochemical pollen-pistil interactions are necessary for in vivo pollen tube growth to occur, studies investigating the influence of heat-induced changes in pistil biochemistry on in vivo pollen tube growth rates are lacking. We hypothesized that high temperature would alter diurnal pistil biochemistry and that pollen tube growth rates would be dependent upon the soluble carbohydrate content of the pistil during pollen tube growth. G. hirsutum seeds were sown on different dates to obtain flowers exposed to contrasting ambient temperatures but at the same developmental stage. Diurnal pistil measurements included carbohydrate balance, glutathione reductase (GR; EC 1.8.1.7), soluble protein, superoxide dismutase (SOD; EC 1.15.1.1), NADPH oxidase (NOX; EC 1.6.3.1), adenosine triphosphate (ATP), and water-soluble calcium. Soluble carbohydrate levels in cotton pistils were as much as 67.5% lower under high temperature conditions (34.6 °C maximum air temperature; August 4, 2009) than under cooler conditions (29.9 °C maximum air temperature; August 14, 2009). Regression analysis revealed that pollen tube growth rates were highly correlated with the soluble carbohydrate content of the pistil during pollen tube growth (r² = 0.932). Higher ambient temperature conditions on August 4 increased GR activity in the pistil only during periods not associated with in vivo pollen tube growth; pistil protein content declined earlier in the day under high temperatures; SOD and NOX were unaffected by either sample date or time of day; pistil ATP and water soluble calcium were unaffected by the warmer temperatures. We conclude that moderate heat stress significantly alters diurnal carbohydrate balance in the pistil and suggest that pollen tube growth rate through the style may be limited by soluble carbohydrate supply in the pistil.     

Influence of drought stress on pistil physiology and reproductive success of two Gossypium hirsutum cultivars differing in drought tolerance

The causes of reproductive failure under drought stress (DS) are poorly understood. We hypothesized that reproductive failure was related to drought-induced changes in pistil biochemistry. To address this hypothesis, a water deficit-induced experiment was conducted with two cotton cultivars (Dexiamian 1, drought tolerant; Yuzaomian 9110, drought sensitive). Results showed that DS decreased the photosynthesis of subtending leaf and downregulated sucrose transporter gene (GhSUT-1) expression in pistil for both cultivars, resulting in lower pistil carbon accumulation which was reflected in the decreased starch accumulation. Lower starch, as potential energy, and adenosine triphosphate (ATP), as direct energy, in droughted pistils suggested less energy for pollen tube entrance into ovules, reducing the fertilized ovule number and fertilization efficiency. Further, although pistil peroxidase activity increased under DS, a higher hydrogen peroxide (H₂O₂) level still was measured in droughted pistils than well-watered pistils, damaging reproductive activities. Moreover, larger decreases in photosynthesis, pistil GhSUT-1 expression, carbon accumulation, starch and ATP contents caused by DS for Yuzaomian 9110 than Dexiamian 1, and different responses of superoxide dismutase and catalase activities, and ascorbic acid and H₂O₂ contents to DS between the two cultivars might be the reasons causing a greater decrease in fertilization efficiency for Yuzaomian 9110 than Dexiamian 1 under DS. Thus, we suggest that decreased ovule fertilization under DS was related to the disorganized carbohydrate metabolism and inefficient antioxidant defense in droughted pistils, and the effects of DS on pistil carbohydrate metabolism and antioxidant defense were more significant for drought-sensitive cultivars than drought-tolerant cultivars.     

Diurnal pollen tube growth rate is slowed by high temperature in field-grown Gossypium hirsutum pistils

For Gossypium hirsutum pollination, germination, and pollen tube growth must occur in a highly concerted fashion on the day of flowering for fertilization to occur. Because reproductive success could be influenced by the photosynthetic activity of major source leaves, we hypothesized that increased temperatures under field conditions would limit fertilization by inhibiting diurnal pollen tube growth through the style and decreasing subtending leaf photosynthesis. To address this hypothesis, G. hirsutum seeds were sown on different dates to obtain flowers exposed to contrasting ambient temperatures while at the same developmental stage (node 8 above the cotyledons). Collection and measurement were conducted at 06:00, 09:00, 12:00, 15:00, and 18:00 h on August 4 (34.6 °C maximum air temperature) and 14, 2009 (29.9 °C maximum air temperature). Microclimate measurements included photosynthetically active radiation, relative humidity, and air temperature. Pistil measurements included pistil surface temperature, pollen germination, pollen tube growth through the style, fertilization efficiency, fertilized ovule number, and total number of ovules per ovary. Subtending leaf measurements included leaf temperature, photosynthesis, and stomatal conductance. Under high temperatures the first measurable pollen tube growth through the style was observed earlier in the day (12:00 h) than under cooler conditions (15:00 h). Also, high temperature resulted in slower pollen tube growth through the style (2.05 mm h⁻¹) relative to cooler conditions (3.35 mm h⁻¹), but there were no differences in fertilization efficiency, number of fertilized ovules, or ovule number. There was no effect of sampling date on diurnal photosynthetic patterns, where the maximum photosynthetic rate was observed at 12:00 h on both dates. It is concluded that, of the measured physiological and reproductive processes, pollen tube growth rate showed the greatest sensitivity to high temperature under field conditions.     

Mode of pollen tube growth in pistils of Ticodendron incognitum (Ticodendraceae,Fagales) and the evolution of chalazogamy

Ticodendron incognitum is the sole species of the Ticodendraceae, which was established as a new family in the Fagales less than 20 years ago. Considering the diverse modes of pollen tube growth observed in other Fagales, we investigated the growth of pollen tubes in the pistil of Ticodendron. At the time of pollination, T. incognitum had four immature ovules in a bilocular ovary, thus exhibiting delayed fertilization, as in other Fagales. During the period when fertilization was delayed, pollen tube growth in the pistil was intermittent, consisting of five steps associated with development of the ovules and embryo sacs. Four cessation sites occurred: in the style, in the tissue of the upper part of the ovary, inside and outside of the funicle and at the chalaza. A single pollen tube eventually reaches a mature embryo sac through the chalaza in one of the four ovules. While both delayed fertilization and intermittent pollen tube growth play a role in male and female gametophyte selection, as in other Fagales, the five‐step process of pollen tube growth through the chalaza (i.e. chalazogamy) is characteristic of lineages of the Casuarinaceae, Ticodendraceae and Betulaceae (the latter with the loss of one step). © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society, 2008, 157 , 621–631.     

Cell–cell communications and molecular mechanisms in plant sexual reproduction

Sexual reproduction is achieved by precise interactions between male and female reproductive organs. In plant fertilization, sperm cells are carried to ovules by pollen tubes. Signals from the pistil are involved in elongation and control of the direction of the pollen tube. Genetic, reverse genetic, and cell biological analyses using model plants have identified various factors related to the regulation of pollen tube growth and guidance. In this review, I summarize the mechanisms and molecules controlling pollen tube growth to the ovule, micropylar guidance, reception of the guidance signal in the pollen tube, rupture of the pollen tube to release sperm cells, and cessation of the tube guidance signal. I also briefly introduce various techniques used to analyze pollen tube guidance in vitro.     

Reactive oxygen species produced by NADPH oxidase are involved in pollen tube growth

Tip-localized reactive oxygen species (ROS) were detected in growing pollen tubes by chloromethyl dichlorodihydrofluorescein diacetate oxidation, while tip-localized extracellular superoxide production was detected by nitroblue tetrazolium (NBT) reduction. To investigate the origin of the ROS we cloned a fragment of pollen specific tobacco NADPH oxidase (NOX) closely related to a pollen specific NOX from Arabidopsis. Transfection of tobacco pollen tubes with NOX-specific antisense oligodeoxynucleotides (ODNs) resulted in decreased amount of NtNOX mRNA, lower NOX activity and pollen tube growth inhibition. The ROS scavengers and the NOX inhibitor diphenylene iodonium chloride (DPI) inhibited growth and ROS formation in tobacco pollen tube cultures. Exogenous hydrogen peroxide (H2O2) rescued the growth inhibition caused by NOX antisense ODNs. Exogenous CaCl2 increased NBT reduction at the pollen tube tip, suggesting that Ca2+ increases the activity of pollen NOX in vivo. The results show that tip-localized ROS produced by a NOX enzyme is needed to sustain the normal rate of pollen tube growth and that this is likely to be a general mechanism in the control of tip growth of polarized plant cells.     

Interspecific crosses in the genus Tulipa L.: identification of pre-fertilization barriers

 Pollen tube growth in the pistil and pollen tube penetration of ovules have both been studied in crosses between cultivars from Tulipa gesneriana L. and 12 tulip species from all eight sections of the genus Tulipa to identify pre-fertilization barriers. Depending on the cross, pollen tubes grew as far as the stigma or the style or continued growing down into the ovary. Pollen tubes penetrated none or only a few percent of the ovules of some crosses, despite the presence of many pollen tubes in the ovary. In other crosses, from which no or only a few hybrids have been obtained after seed maturation on the plant, pollen tube penetration was found in up to 79% of the ovules. Apparently, various kinds of barriers preventing fertilization or normal embryogenesis occur in interspecific tulip crosses. Received: 26 July 1996 / Revision accepted: 31 January 1997     

From pollination to fertilization in fruit trees

The phase that elapses from pollination to fertilization is re-examined giving special attention to pollen pistil interaction in compatible matings. Pollination induces an activation of the pistil. A number of changes take place in the different tissues of this organ that appear to support male gametophyte development and to assist fertilization. Thus pollination induces stigma secretion, the release of starch from the transmitting tissue and prolongs embryo sac viability. It appears that even those pollen grains that do not achieve fertilization have a synergistic role supporting others to do so.The pistil also has an effect on pollen tube growth. Pollen tube growth along the pistil is not continuous, accelerations and decelerations take place depending on the different tissues they traverse. The fact that pollen tube growth is heterotrophic, at the expenses of the pistil reserves, and that these reserves are not continuously produced confers the pistil with a role controlling pollen tube growth kinetics.     

钙在被子植物受精过程中的作用

近年来,花粉管中的钙信号和生理功能的研究取得了明显的进展,同时在雌蕊系统中有关钙分布的研究也初步显示了其时、空特征与被子植物的受精作用密切相关。该文总结了花粉萌发和花粉管生长过程中外源钙和内源钙的作用机制,结合雌蕊组织中钙分布的特征,进一步探讨了钙在被子植物受精过程中的功能。     

Pollen-pistil interaction in kiwifruit (Actinidia deliciosa; Actinidiaceae)

The pollen-pistil interaction has been examined in kiwifruit (Actinidia deliciosa). In this species a large number of seeds are produced in each fruit and a great many pollen grains germinate and grow to reach the ovules. This growth is assisted by an abundant secretion that is present all along the pistilar tract. At anthesis, the stigma is covered by a secretion where the pollen grains germinate and grow. The stylar transmitting tissue is initially rich in starch reserves, but the starch gradually disappears and, concomitantly, an abundant secretion that stains for carbohydrates appears in all of the intercellular spaces. Pollen tube growth relies on this secretion since it is depleted after pollen tube passage, while in unpollinated flowers it remains unaltered throughout the flower life-span. In the ovary a similar situation occurs. The placental surface, where the pollen tubes grow before reaching the ovules, is covered by a number of obturators. At anthesis, these obturators are rich in starch reserves and have an abundant secretion on their outer surface. As time passes, starch disappears while the secretion increases. It is in this secretion that the pollen tubes grow on their way toward the ovules. These observations are discussed in terms of the support given by the pistil to pollen tube growth to achieve the highly successful reproductive performance of this species.     

The pollen receptor kinase LePRK2 mediates growth-promoting signals and positively regulates pollen germination and tube growth

In flowering plants, the process of pollen germination and tube growth is required for successful fertilization. A pollen receptor kinase from tomato (Solanum lycopersicum), LePRK2, has been implicated in signaling during pollen germination and tube growth as well as in mediating pollen (tube)-pistil communication. Here we show that reduced expression of LePRK2 affects four aspects of pollen germination and tube growth. First, the percentage of pollen that germinates is reduced, and the time window for competence to germinate is also shorter. Second, the pollen tube growth rate is reduced both in vitro and in the pistil. Third, tip-localized superoxide production by pollen tubes cannot be increased by exogenous calcium ions. Fourth, pollen tubes have defects in responses to style extract component (STIL), an extracellular growth-promoting signal from the pistil. Pollen tubes transiently overexpressing LePRK2-fluorescent protein fusions had slightly wider tips, whereas pollen tubes coexpressing LePRK2 and its cytoplasmic partner protein KPP (a Rop-GEF) had much wider tips. Together these results show that LePRK2 positively regulates pollen germination and tube growth and is involved in transducing responses to extracellular growth-promoting signals.     

Effects of cytokinin on photosynthetic gas exchange, chlorophyll fluorescence parameters, antioxidative system and carbohydrate accumulation in cucumber (Cucumis sativus L.) under low light

The effects of 6-benzylaminopurine (6-BA) on plant growth, net photosynthetic rate, relative chlorophyll content, soluble protein, carbohydrates contents and antioxidant systems of cucumber (Cucumis sativus L.) under low-light environment were investigated using two different cucumber cultivars. The results showed that the weak light resulted in the remarkable decrease in plant net photosynthetic rate, relative chlorophyll content, soluble protein and carbohydrates contents, but promoted the superoxide dismutase and guaiacol peroxidase activities. However, application of 6-BA alleviated the reduction of the correlative parameters and mediated the changes of antioxidant systems. The potential mechanisms may involve the following aspects: 6-BA clearly enhanced the plants’ tolerance to low light by increasing chlorophyll content, reducing the production of superoxide radical (O ₂ ^·? ), and enhancing the quenching of hydrogen peroxide (H₂O₂), consequently alleviating the injury of photosynthetic system, and further increasing the efficiency of CO₂ assimilation, producing more carbohydrates which can meet the growth need of cucumber. Meanwhile, the present study indicated that cucumber of Europe mini type (Chunqiuwang) was more tolerant to low light than HuaNan type (Huza No.3).     

Delayed fertilization and pollen-tube growth in pistils of Fagus japonica (Fagaceae)

In contrast to most angiosperms, in which fertilization occurs 1 or 2 days after pollination, in some plant orders, including the Fagales, fertilization is delayed from 4 days to more than 1 year, raising questions regarding why fertilization is delayed and where and how pollen tubes remain in the pistil during the delay. To answer these questions, we investigated pollen-tube growth in pistils of Fagus japonica (Fagaceae), which are tricarpellate and have six ovules, using light, fluorescence, and scanning electron microscopy. The ovules were immature at the time of pollination and required 5 weeks to become fully developed. During this 5 weeks, pollen tubes grew from the stigma to the embryo sac in association with the development of ovules and intermittently in three steps with two growth-cessation sites, i.e., on the funicle and near the micropyle. The number of pollen tubes was gradually reduced from many to one at the two growth-cessation sites, and fertilization occurred in one ovule that apparently developed earlier than the others in the pistil. Thus, delayed fertilization plays an important role in gametophyte competition and selection leading to nonrandom fertilization. Intermittent pollen-tube growth is also likely widespread in angiosperms because it is known in other Fagales and an unrelated order Garryales.     

Arabidopsis HAP2 (GCS1) is a sperm-specific gene required for pollen tube guidance and fertilization

In flowering plants, sperm cells develop in the pollen cytoplasm and are transported through floral tissues to an ovule by a pollen tube, a highly polarized cellular extension. After targeting an ovule, the pollen tube bursts, releasing two sperm that fertilize an egg and a central cell. Here, we identified the gene encoding Arabidopsis HAP2, demonstrating that it is allelic to GCS1. HAP2 is expressed only in the haploid sperm and is required for efficient pollen tube guidance to ovules. We identified an insertion (hap2-1) that disrupts the C-terminal portion of the protein and tags mutant pollen grains with the beta-glucuronidase reporter. By monitoring reporter expression, we showed that hap2-1 does not diminish pollen tube length in vitro or in the pistil, but it reduces ovule targeting by twofold. In addition, we show that the hap2 sperm that are delivered to ovules fail to initiate fertilization. HAP2 is predicted to encode a protein with an N-terminal secretion signal, a single transmembrane domain and a C-terminal histidine-rich domain. These results point to a dual role for HAP2, functioning in both pollen tube guidance and in fertilization. Moreover, our findings suggest that sperm, long considered to be passive cargo, are involved in directing the pollen tube to its target.     

钙在有花植物受精过程中的作用

钙作为第二信使在植物信号转导中的作用一直是植物生理学、细胞生物学和发育生物学研究的热点。近年已有不少综述和专著从不同角度对此作了详细评论［1～5］。虽然这些文章中只有部分内容涉及本文的主题———钙在植物受精中的作用,但是它们所论述的关于钙信使系统在植物...     

INFLUENCE OF THE PISTIL ON POLLEN TUBE KINETICS IN PEACH (PRUNUS PERSICA)

Pollen tube growth has been studied in peach and has been related to changes in the pistil structures which the pollen tube has to traverse in its way from the stigma down to the ovule. Growth of the pollen tubes along the pistil is not continuous. While pollen tubes reach the base of the style 7 days after pollination, fertilization does not take place until 12 days later. Pollen tubes stop for 5 days at the top of the obturator and they further stop for 3 days before entering the ovule. The pollen tube growth is heterotrophic; starch, present all along the pistilar tract at anthesis, vanishes as the pollen tubes pass by. Discontinuous pollen tube growth appears to be controlled by the pistil. At anthesis the pistil is not fully matured. Maturation of the pistil implies a number of secretory processes that occur in a basipetal way starting from the stigma down to the style and ending in the ovule. Some of these secretions at the stigma and the style are triggered by pollination; others appear to be a maturative stage of the pistil and are produced in a discrete way. The fact that the pollen tube depends on these secretions together with the fact that these secretions are not continuously produced confer upon the pistil a role of controlling pollen tube kinetics and point out that, for a successful fertilization, male gametophyte development and pistil maturation need to by synchronized.     

The effect of lichen‐dominated biological soil crusts on growth and physiological characteristics of three plant species in a temperate desert of northwest China

Biocrusts (biological soil crusts) cover open spaces between vascular plants in most arid and semi‐arid areas. Information on effects of biocrusts on seedling growth is controversial, and there is little information on their effects on plant growth and physiology. We examined impacts of biocrusts on growth and physiological characteristics of three habitat‐typical plants, Erodium oxyrhynchum, Alyssum linifolium and Hyalea pulchella, growing in the Gurbantunggut Desert, northwest China. The influence of biocrusts on plant biomass, leaf area, leaf relative water content, photosynthesis, maximum quantum efficiency of PSII (F_v/F_m), chlorophyll, osmotic solutes (soluble sugars, protein, proline) and antioxidant enzymes (superoxide dismutase, catalase, peroxidase) was investigated on sites with or without biocrust cover. Biomass, leaf area, leaf water content, photosynthesis, F_v/F_m and chlorophyll content in crusted soils were higher than in uncrusted soils during early growth and lower later in the growth period. Soluble sugars, proline and antioxidant enzyme activity were always higher in crusted than in uncrusted soils, while soluble protein content was always lower. These findings indicate that biocrusts have different effects on these three ephemeral species during growth in this desert, primarily via effects on soil moisture, and possibly on soil nutrients. The influence of biocrusts changes during plant development: in early plant growth, biocrusts had either positive or no effect on growth and physiological parameters. However, biocrusts tended to negatively influence plants during later growth. Our results provide insights to explain why previous studies have found different effects of biocrusts on vascular plant growth.     

The Role of Calcium in the Fertilization Process in Flowering Plants

The fertilization process in flowering plants in a broad sense includes a progamic phase preceding the phase of double fertilization. To our present knowledge, calcium as a second messenger in the signal transduction plays important roles in all the links of this process. The present review attempts to highlight the recent advances in this research field, including: calcium in relation to in vitro pollen tube growth (distribution of calcium in pollen tube tip; regulation of pollen tube growth by calcium; calcium oscillation in pollen tube); distribution of calcium in pistil and its relation to in vivo pollen tube growth (calcium in relation to pollen-pistil recognition; pollen tube growth in pistil; pollen tube entry into embryo sac and the discharge and transportation of sperms); and calcium in relation to sperm-egg fusion and egg cell activation. In conclusion the author summarizes into several main view points, and gives recommendation for further researches on this topic.