Gametophytic pollen tube guidance

The concept of a pollen tube attractant was proposed in the late nineteenth century when pollen tubes were found to grow toward excised pistil tissues on medium. Since then, for about 140 years, plant biologists have tried to identify the pollen tube attractants. However, no molecule has been convincingly demonstrated to be the true attractant that actually controls the navigation of pollen tubes in the pistil. The past decade has seen substantial progress in this field in terms of our understanding of the various mechanisms of pollen tube guidance. It was suggested that diffusible pollen tube attractants might provide localized signals that affect the directional growth of the pollen tube, especially in the last phase of guidance by the target female gametophyte. Here, we review the mechanisms of pollen tube guidance, with special focus on the gametophytic guidance and the attractant. The necessary and appropriate conditions required by the true attractant will be discussed. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Phytosulfokine peptide signaling controls pollen tube growth and funicular pollen tube guidance in Arabidopsis thaliana

Phytosulfokine (PSK) is a peptide growth factor that requires tyrosine sulfation carried out by tyrosylprotein sulfotransferase (TPST) for its activity. PSK is processed from precursor proteins encoded by five genes in Arabidopsis thaliana and perceived by receptor kinases encoded by two genes in Arabidopsis. pskr1‐3 pskr2‐1 and tpst‐1 knockout mutants displayed reduced seed production, indicative of a requirement for PSK peptide signaling in sexual plant reproduction. Expression analysis revealed PSK precursor and PSK receptor gene activity in reproductive organs with strong expression of PSK2 in pollen. In support of a role for PSK signaling in pollen, in vitro pollen tube (PT) growth was enhanced by exogenously added PSK while PTs of pskr1‐3 pskr2‐1 and of tpst‐1 were shorter. In planta, growth of wild‐type pollen in pskr1‐3 pskr2‐1 and tpst‐1 flowers appeared slower than growth in wild‐type flowers. But PTs did eventually reach the base of the style, suggesting that PT elongation rate may not be responsible for the reduced fertility. Detailed analysis of anthers, style and ovules did not reveal obvious developmental defects. By contrast, a high percentage of unfertilized ovules in pskr1‐3 pskr2‐1 and in tpst‐1 siliques displayed loss of funicular PT guidance, suggesting that PSK signaling is required to guide the PT from the transmitting tract to the embryo sac. Cross‐pollination experiments with wild‐type, pskr1‐3 pskr2‐1 and tpst‐1 male and female parents revealed that both the PT and the female sporophytic tissue and/or female gametophyte contribute to successful PT guidance via PSK signaling and to fertilization success.     

Mechanistic insights from reversible splicing catalysis

Recent work demonstrating the ability of spliceosomes purified after the second catalytic step of splicing to efficiently reverse both steps of the reaction provides answers to several unresolved questions regarding the splicing reaction, and raises many more.     

Mechanistic insights from a refined three-dimensional model of integrin alphaIIbbeta3

The integrin alpha(IIb)beta(3) plays an important role in platelet function, and abnormalities of this protein result in a serious bleeding disorder, known as Glanzmann thrombasthenia. Although crystallographic data exist for the related integrin alpha(V)beta(3), to date, there are no high resolution structures of integrin alpha(IIb)beta(3) available in the literature. Therefore, it is still unclear how specific elements of the alpha(IIb) subunit contribute to integrin alpha(IIb)beta(3) function. Here we describe a refined model of the alpha(IIb) N-terminal portion of integrin alpha(IIb)beta(3) obtained by using the alpha(V)beta(3) template combined with a new method for predicting the conformations of the unique alpha(IIb) loop regions comprising residues 71-85, 114-125, and 148-164. The refined model was probed based on a structural prediction that differentiates it from standard homology models: specifically, that Lys-118 of alpha(IIb) contacts Glu-171 of beta(3). To test this hypothesis experimentally, the mutant integrin chains alpha(IIb) K118C and beta(3) E171C were cotransfected into HEK 293 cells. We show that the cells expressed the mutants alpha(IIb)beta(3) on their surface as a disulfide-linked dimer, supporting the close proximity between alpha(IIb) Lys-118 and beta(3) Glu-171 predicted from the refined model. This validated model provides a specific structural context for the analysis and interpretation of structure-function relations of integrin alpha(IIb)beta(3). In addition, it suggests mechanistic hypotheses pertaining to both naturally occurring mutations responsible for Glanzmann thrombasthenia and to point mutations that affect ligand binding.     

Zymolyase removes callose from germinating pollen and pollen tube walls

The impermeable callosic wall that surrounds germinating pollen grains and pollen tubes is removed by brief treatment with Zymolyase, an enzyme preparation that hydrolyzes linear glucose polymers with beta-1,3 linkages. Zymolyase treatment does not interfere with pollen tube growth or cylosis in the vegetative cell.     

AtLURE1/PRK6-mediated signaling promotes conspecific micropylar pollen tube guidance

Reproductive isolation is a prerequisite to form and maintain a new species. Multiple prezygotic and postzygotic reproductive isolation barriers have been reported in plants. In the model plant, Arabidopsis thaliana conspecific pollen tube precedence controlled by AtLURE1/PRK6-mediated signaling has been recently reported as a major prezygotic reproductive isolation barrier. By accelerating emergence of own pollen tubes from the transmitting tract, A. thaliana ovules promote self-fertilization and thus prevent fertilization by a different species. Taking advantage of a septuple atlure1null mutant, we now report on the role of AtLURE1/PRK6-mediated signaling for micropylar pollen tube guidance. Compared with wild-type (WT) ovules, atlure1null ovules displayed remarkably reduced micropylar pollen tube attraction efficiencies in modified semi-in vivo A. thaliana ovule targeting assays. However, when prk6 mutant pollen tubes were applied, atlure1null ovules showed micropylar attraction efficiencies comparable to that of WT ovules. These findings indicate that AtLURE1/PRK6-mediated signaling regulates micropylar pollen tube attraction in addition to promoting emergence of own pollen tubes from the transmitting tract. Moreover, semi-in vivo ovule targeting competition assays with the same amount of pollen grains from both A. thaliana and Arabidopsis lyrata showed that A. thaliana WT and xiuqiu mutant ovules are mainly targeted by own pollen tubes and that atlure1null mutant ovules are also entered to a large extent by A. lyrata pollen tubes. Taken together, we report that AtLURE1/PRK6-mediated signaling promotes conspecific micropylar pollen tube attraction representing an additional prezygotic isolation barrier.

A modified ovule targeting assay revealed that AtLURE1/PRK6-mediated signaling promotes micropylar guidance of Arabidopsis thaliana pollen tubes while discriminating tubes of related Arabidopsis lyrata.     

The phospholipid flippase ALA3 regulates pollen tube growth and guidance in Arabidopsis

Pollen tube guidance regulates the growth direction and ovule targeting of pollen tubes in pistils, which is crucial for the completion of sexual reproduction in flowering plants. The Arabidopsis (Arabidopsis thaliana) pollen-specific receptor kinase (PRK) family members PRK3 and PRK6 are specifically tip-localized and essential for pollen tube growth and guidance. However, the mechanisms controlling the polar localization of PRKs at the pollen tube tip are unclear. The Arabidopsis P4-ATPase ALA3 helps establish the polar localization of apical phosphatidylserine (PS) in pollen tubes. Here, we discovered that loss of ALA3 function caused pollen tube defects in growth and ovule targeting and significantly affected the polar localization pattern of PRK3 and PRK6. Both PRK3 and PRK6 contain two polybasic clusters in the intracellular juxtamembrane domain, and they bound to PS in vitro. PRK3 and PRK6 with polybasic cluster mutations showed reduced or abolished binding to PS and altered polar localization patterns, and they failed to effectively complement the pollen tube-related phenotypes of prk mutants. These results suggest that ALA3 influences the precise localization of PRK3, PRK6, and other PRKs by regulating the distribution of PS, which plays a key role in regulating pollen tube growth and guidance.

AMINOPHOSPHOLIPID ATPASE3 guides pollen tubes by regulating the distribution of anionic phospholipids to affect the precise localization of certain pollen-specific receptor kinases at pollen tubes.

IN A NUTSHELL Background: In flowering plants, pollen tube guidance regulates the rapid growth and timely targeting of the pollen tube to the ovule in the pistil during sexual reproduction, when signaling between the male and female gametophytes occur. The small peptide-RLK signaling module is essential for the interaction between the male and female gametophyte. Certain members of the pollen-specific receptor kinase (PRK) family have different subcellular localization patterns in Arabidopsis pollen tubes and play critical roles in pollen tube growth and guidance. However, the molecular mechanisms that regulate and maintain the polar localization of PRKs at the pollen tube tip are still unknown. Question: We were interested in exploring how Arabidopsis P4-ATPase (aminophospholipid ATPase, ALA) precisely regulates pollen tube guidance and maintains the polar localization patterns of PRK6 and PRK3. How plant ALA family members regulate pollen tube guidance has not yet been documented. Findings: The loss of ALA3 function not only caused sluggish pollen tube growth and aberrant ovule targeting but also affected the polar localization patterns of several PRKs at the pollen tube tip. Members of the PRKs family can directly interact with anionic phospholipids such as phosphatidylserine (PS), and the capacity of PRK3/6 to bind anionic phospholipids is crucial for both their polar localization and physiological functions. ALA3 establishes and maintains the polar distribution of PS, which influences secretory vesicles-mediated polar trafficking at the pollen tube tip to affect the distribution of PRK3 and PRK6. On the other hand, PS might also directly recruit PRK3 and PRK6 to the pollen tube tip and sustain their localization. Next steps: The localization of PRKs is a complex, finely regulated process. The C-termini of PRKs may also affect their polar distribution. More research is required to reveal how the C-terminus domain precisely controls the localization of PRKs.     

Cysteine-rich peptides: signals for pollen tube guidance,species isolation and beyond

<正>In flowering plants, the two sperm cells are passive cargo transported into the ovule by pollen tubes (Zhang et al.,2017). Therefore, the precise guidance of pollen tubes is critical for successful double fertilization (Dresselhaus and Franklin-Tong, 2013). A series of male-female interactions are required to guarantee the spatiotemporal regulation of pollen tube guidance because during growth, pollen tubes     

A quantitative model of human DNA base excision repair. I. Mechanistic insights

Base excision repair (BER) is a multistep process involving the sequential activity of several proteins that cope with spontaneous and environmentally induced mutagenic and cytotoxic DNA damage. Quantitative kinetic data on single proteins of BER have been used here to develop a mathematical model of the BER pathway. This model was then employed to evaluate mechanistic issues and to determine the sensitivity of pathway throughput to altered enzyme kinetics. Notably, the model predicts considerably less pathway throughput than observed in experimental in vitro assays. This finding, in combination with the effects of pathway cooperativity on model throughput, supports the hypothesis of cooperation during abasic site repair and between the apurinic/apyrimidinic (AP) endonuclease, Ape1, and the 8-oxoguanine DNA glycosylase, Ogg1. The quantitative model also predicts that for 8-oxoguanine and hydrolytic AP site damage, short-patch Polβ-mediated BER dominates, with minimal switching to the long-patch subpathway. Sensitivity analysis of the model indicates that the Polβ-catalyzed reactions have the most control over pathway throughput, although other BER reactions contribute to pathway efficiency as well. The studies within represent a first step in a developing effort to create a predictive model for BER cellular capacity.     

Rapid tip growth: insights from pollen tubes

Pollen tubes extend rapidly in an oscillatory manner by the extreme form of polarized growth, tip growth, and provide an exciting system for studying the spatiotemporal control of polarized cell growth. The Rho-family ROP GTPase is a key signaling molecule in this growth control and is periodically activated at the apical plasma membrane to spatially define the apical growth region and temporally precede the burst of growth. The spatiotemporal dynamics of ROP GTPase is interconnected with actin dynamics and polar exocytosis that is required for tip-targeted membrane and wall expansion. Recent advances in the study of the mechanistic interlinks between ROP-centered signaling and spatiotemporal dynamics of cell membrane and wall remodeling will be discussed.     

Attractive and repulsive interactions between female and male gametophytes in Arabidopsis pollen tube guidance

Sexual reproduction in plants, unlike that of animals, requires the action of multicellular haploid gametophytes. The male gametophyte (pollen tube) is guided to a female gametophyte through diploid sporophytic cells in the pistil. While interactions between the pollen tube and diploid cells have been described, little is known about the intercellular recognition systems between the pollen tube and the female gametophyte. In particular, the mechanisms that enable only one pollen tube to interact with each female gametophyte, thereby preventing polysperm, are not understood. We isolated female gametophyte mutants named magatama (maa) from Arabidopsis thaliana by screening for siliques containing half the normal number of mature seeds. In maa1 and maa3 mutants, in which the development of the female gametophyte was delayed, pollen tube guidance was affected. Pollen tubes were directed to mutant female gametophytes, but they lost their way just before entering the micropyle and elongated in random directions. Moreover, the mutant female gametophytes attracted two pollen tubes at a high frequency. To explain the interaction between gametophytes, we propose a monogamy model in which a female gametophyte emits two attractants and prevents polyspermy. This prevention process by the female gametophyte could increase a plant's inclusive fitness by facilitating the fertilization of sibling female gametophytes. In addition, repulsion between pollen tubes might help prevent polyspermy. The reproductive isolations observed in interspecific crosses in Brassicaceae are also consistent with the monogamy model.