RESUMEN

Pollen tube (PT) serves as a vehicle that delivers male gametes (sperm cells) to a female gametophyte during double fertilization, which eventually leads to the seed formation. It is one of the fastest elongating structures in plants. Normally, PTs traverse through the extracellular matrix at the transmitting tract after penetrating the stigma. While the endeavor may appear simple, the molecular processes and mechanics of the PT elongation is yet to be fully resolved. Although it is the most studied "tip-growing" structure in plants, several features of the structure (e.g., Membrane dynamics, growth behavior, mechanosensing etc.) are only partially understood. In many aspects, PTs are still considered as a tissue rather than a "unique cell." In this review, we have attempted to discuss mainly on the mechanics behind PT-elongation and briefly on the molecular players involved in the process. Four aspects of PTs are particularly discussed: the PT as a cell, its membrane dynamics, mechanics of its elongation, and the potential mechanosensors involved in its elongation based on relevant findings in both plant and non-plant models.

RESUMEN

Carbohydrates (sugars) are an essential energy-source for all life forms. They take a significant share of our daily consumption and are used for biofuel production as well. However, sugarcane and sugar beet are the only two crop plants which are used to produce sugar in significant amounts. Here, we have discovered and fine-tuned a phenomenon in rice which leads them to produce sugary-grain. We knocked-out GCS1 genes in rice by using CRISPR technology, which led to fertilization failure and pollen tube-dependent ovule enlargement morphology (POEM) phenomenon. Apparently, the POEMed-like rice ovule ('endosperm-focused') can grow near-normal seed-size unlike earlier observations in Arabidopsis in which gcs1 ovules ('embryo-focused') were aborted quite early. The POEMed-like rice ovules contained 10-20% sugar, with extremely high sucrose content (98%). Trancriptomic analysis revealed that the osgcs1 ovules had downregulation of starch biosynthetic genes, which would otherwise have converted sucrose to starch. Overall, this study shows that pollen tube content release is sufficient to trigger sucrose unloading at rice ovules. However, successful fertilization is indispensable to trigger sucrose-starch conversion. These findings are expected to pave the way for developing novel sugar producing crops suited for diverse climatic regions.

Asunto(s)

Regulación de la Expresión Génica de las Plantas/fisiología , Oryza/genética , Oryza/metabolismo , Proteínas de Plantas/metabolismo , Sacarosa/metabolismo , Repeticiones Palindrómicas Cortas Agrupadas y Regularmente Espaciadas , Mutación , Proteínas de Plantas/genética , Tubo Polínico/fisiología , Transcriptoma

RESUMEN

Gametophytic mutants share very small proportion of the total mutants generated by any mutagenic approach; even rarer are the fertilization-defective gametophytic mutants. They require an efficient and targeted strategy instead of 'brute force' screening approach. The classical gametophyte mutant screening method, mainly based on the segregation distortion, can distinguish gametophytic mutants from the others. However, the mutants pooled after the screening constitute both fertilization-defective and developmental-defective gametophytic mutants. Until recently, there has not been any straightforward way to screen the former from the latter. Additionally, most of the mutations affecting both gametes are lost during the screening process. The novel gametophyte screening approach tends to circumvent those shortcomings. This review discusses on the classical approach of gametophytic mutant screening and focuses on the novel approach on distinguishing fertilization-/developmental-defective gametophytic mutants (both male and female). It offers an empirical basis of screening such mutants by taking in the consideration of earlier studies on fertilization failure, initiation of seed coat formation, and fertilization recovery system in plants.

RESUMEN

KEY MESSAGE: In light of the available discoveries in the field, this review manuscript discusses on plant reproduction mechanism and molecular players involved in the process. Sperm cells in angiosperms are immotile and are physically distant to the female gametophytes (FG). To secure the production of the next generation, plants have devised a clever approach by which the two sperm cells in each pollen are safely delivered to the female gametophyte where two fertilization events occur (by each sperm cell fertilizing an egg cell and central cell) to give rise to embryo and endosperm. Each of the successfully fertilized ovules later develops into a seed. Sets of macromolecules play roles in pollen tube (PT) guidance, from the stigma, through the transmitting tract and funiculus to the micropylar end of the ovule. Other sets of genetic players are involved in PT reception and in its rupture after it enters the ovule, and yet other sets of genes function in gametic fusion. Angiosperms have come long way from primitive reproductive structure development to today's sophisticated, diverse, and in most cases flamboyant organ. In this review, we will be discussing on the intricate yet complex molecular mechanism of double fertilization and how it might have been shaped by the evolutionary forces focusing particularly on the model plant Arabidopsis.

Asunto(s)

Flores/fisiología , Magnoliopsida/fisiología , Polinización/fisiología , Evolución Biológica , Gametogénesis en la Planta , Óvulo Vegetal/fisiología , Polen/fisiología , Semillas

RESUMEN

Plant reproduction is an extremely important phenomenon, as it is strongly associated with plant genetics and early development. Additionally, foundations of the reproductive system have direct implications on plant breeding and agriculture. Investigation of the functions of male and female gametophytes is critical since their fusion is required for seed formation. Although a large number of mutants have been generated to understand the functions of male and female gametophytes, only a small number of genes required for plant fertilization have been identified to date. This is because the screening method used previously required the dissection of siliques, and fertilization-specific mutants exhibiting semi-fertility (or ∼50% fertility) were difficult to identify. Here, we report a new efficient screening method for the identification of fertilization defective mutants in Arabidopsis thaliana using vanillin staining. This method is based on the pollen tube-dependent ovule enlargement morphology (POEM) phenomenon, which generates a partial seed coat within the ovule without fertilization. Using this method, we successfully identified 23 putative fertilization defective mutants in Arabidopsis.

Asunto(s)

Arabidopsis/fisiología , Mutación , Arabidopsis/genética , Óvulo Vegetal/genética , Óvulo Vegetal/fisiología , Plantas Modificadas Genéticamente , Tubo Polínico/genética , Tubo Polínico/fisiología , Reproducción , Semillas/genética

RESUMEN

Plant seeds are essential for human beings, constituting 70% of carbohydrate resources worldwide; examples include rice, wheat, and corn. In angiosperms, fertilization of the egg by a sperm cell is required for seed formation; therefore, fertilization failure results in no seed formation, except in the special case of apomixis. Initially, plants produce many pollen grains inside the anthers; once the pollen grain is deposited onto the top of the pistil, the pollen tube elongates until it reaches the ovule. Generally, only one pollen tube is inserted into the ovule; however, we previously found that if fertilization by the first pollen tube fails, a second pollen tube could rescue fertilization via the so-called fertilization recovery system (FRS). Our previous reports also demonstrated that failed fertilization results in pollen tube-dependent ovule enlargement morphology (POEM), enlarged seeds, and partial seed coat formation if the pollen tube releases the pollen tube contents into the ovule. However, we have not determined whether all the ovules enlarge or produce seed coats if an ovule accepts the pollen tube contents. Therefore, we conducted a partial seed coat formation experiment taking into account both the FRS and POEM phenomena. Notably, the ratios of failed fertilization and the ovules with partial seed coats matched, indicating that all ovules initiate seed coat formation if the fertilization fails but the pollen tube contents enter the ovule. In addition, we confirmed that the agl62 mutant , defective in early endosperm formation, showed seed coat initiation with and without fertilization, indicating that for a normal seed coat initiation, fertilization is not required; however, for the completion of normal seed coat formation, both normal fertilization and endosperm formation are required. Further molecular evidence is required to understand these phenomena because very few factors related to FRS and POEM have been identified.

Asunto(s)

Arabidopsis , Tubo Polínico , Semillas , Arabidopsis/genética , Arabidopsis/crecimiento & desarrollo , Mutación , Óvulo Vegetal

RESUMEN

In angiosperms, pollen tubes carry two sperm cells toward the egg and central cells to complete double fertilization. In animals, not only sperm but also seminal plasma is required for proper fertilization. However, little is known regarding the function of pollen tube content (PTC), which is analogous to seminal plasma. We report that the PTC plays a vital role in the prefertilization state and causes an enlargement of ovules without fertilization. We termed this phenomenon as pollen tube-dependent ovule enlargement morphology and placed it between pollen tube guidance and double fertilization. Additionally, PTC increases endosperm nuclei without fertilization when combined with autonomous endosperm mutants. This finding could be applied in agriculture, particularly in enhancing seed formation without fertilization in important crops.

Asunto(s)

Arabidopsis/metabolismo , Óvulo Vegetal/metabolismo , Tubo Polínico/metabolismo , Semillas/metabolismo , Arabidopsis/genética , Mutación , Óvulo Vegetal/genética , Tubo Polínico/genética , Semillas/genética

RESUMEN

In flowering plants, double fertilization is normally accomplished by the first pollen tube, with the fertilized ovule subsequently inhibiting the attraction of a second pollen tube. However, the mechanism of second-pollen-tube avoidance remains unknown. We discovered that failure to fertilize either the egg cell or the central cell compromised second-pollen-tube avoidance in Arabidopsis thaliana. A similar disturbance was caused by disrupting the fertilization-independent seed (FIS) class polycomb-repressive complex 2 (FIS-PRC2), a central cell- and endosperm-specific chromatin-modifying complex for gene silencing. Therefore, the two female gametes have evolved their own signaling pathways. Intriguingly, second-pollen-tube attraction induced by half-successful fertilization allowed the ovules to complete double fertilization, producing a genetically distinct embryo and endosperm. We thus propose that each female gamete independently determines second-pollen-tube avoidance to maximize reproductive fitness in flowering plants.

Asunto(s)

Proteínas de Arabidopsis/metabolismo , Fertilización , Óvulo Vegetal/metabolismo , Tubo Polínico/metabolismo , Proteínas Represoras/metabolismo , Compuestos de Anilina/metabolismo , Arabidopsis/citología , Arabidopsis/metabolismo , Proteínas de Arabidopsis/genética , Evolución Biológica , Muerte Celular , Ensamble y Desensamble de Cromatina , Fluorescencia , Silenciador del Gen , Aptitud Genética , Óvulo Vegetal/citología , Tubo Polínico/citología , Polinización , Complejo Represivo Polycomb 2 , Proteínas Represoras/genética , Transducción de Señal , Factores de Transcripción/genética , Factores de Transcripción/metabolismo

RESUMEN

For over a century, plant fertilization has been thought to depend on the fertility of a single pollen tube. However, we reported recently a "fertilization recovery system" in flowering plants that actively rescues failed fertilization of a defective mutant pollen tube by attracting a second, functional pollen tube. In typical flowering plants, two synergid cells beside the egg cell attract pollen tubes, one of which degenerates upon pollen tube discharge. We observed that fertilization was rescued when the second synergid cell accepted a wild-type pollen tube. Our results suggest that flowering plants precisely control the number of pollen tubes that arrive at each ovule and use a fertilization recovery mechanism to maximize the likelihood of successful seed set. Restricted pollination experiments showed that if sufficient pollen grains are provided, ovules attract a second pollen tube for recovery. These results support our previous finding that a long period of time is required for ovules to complete the system.

Asunto(s)

Arabidopsis/fisiología , Fertilización , Óvulo Vegetal/fisiología , Tubo Polínico/crecimiento & desarrollo , Polen , Polinización , Semillas/fisiología , Arabidopsis/genética , Proteínas de Arabidopsis/genética , Proteínas Portadoras/genética , Mutación

RESUMEN

In animal fertilization, multiple sperms typically arrive at an egg cell to "win the race" for fertilization. However, in flowering plants, only one of many pollen tubes, conveying plant sperm cells, usually arrives at each ovule that harbors an egg cell. Plant fertilization has thus been thought to depend on the fertility of a single pollen tube. Here we report a fertilization recovery phenomenon in flowering plants that actively rescues the failure of fertilization of the first mutant pollen tube by attracting a second, functional pollen tube. Wild-type (WT) ovules of Arabidopsis thaliana frequently (∼80%) accepted two pollen tubes when entered by mutant pollen defective in gamete fertility. In typical flowering plants, two synergid cells on the side of the egg cell attract pollen tubes, one of which degenerates upon pollen tube discharge. By semi-in vitro live-cell imaging we observed that fertilization was rescued when the second synergid cell accepted a WT pollen tube. Our results suggest that flowering plants precisely control the number of pollen tubes that arrive at each ovule and employ a fertilization recovery mechanism to maximize the likelihood of successful seed set.

Asunto(s)

Arabidopsis/fisiología , Fertilización/fisiología , Óvulo Vegetal/fisiología , Tubo Polínico/fisiología , Polen/genética , Arabidopsis/citología , Indoles , Modelos Biológicos

RESUMEN

For more than 140 years, pollen tube guidance in flowering plants has been thought to be mediated by chemoattractants derived from target ovules. However, there has been no convincing evidence of any particular molecule being the true attractant that actually controls the navigation of pollen tubes towards ovules. Emerging data indicate that two synergid cells on the side of the egg cell emit a diffusible, species-specific signal to attract the pollen tube at the last step of pollen tube guidance. Here we report that secreted, cysteine-rich polypeptides (CRPs) in a subgroup of defensin-like proteins are attractants derived from the synergid cells. We isolated synergid cells of Torenia fournieri, a unique plant with a protruding embryo sac, to identify transcripts encoding secreted proteins as candidate molecules for the chemoattractant(s). We found two CRPs, abundantly and predominantly expressed in the synergid cell, which are secreted to the surface of the egg apparatus. Moreover, they showed activity in vitro to attract competent pollen tubes of their own species and were named as LUREs. Injection of morpholino antisense oligomers against the LUREs impaired pollen tube attraction, supporting the finding that LUREs are the attractants derived from the synergid cells of T. fournieri.

Asunto(s)

Factores Quimiotácticos/metabolismo , Defensinas/metabolismo , Magnoliopsida/citología , Magnoliopsida/crecimiento & desarrollo , Tubo Polínico/crecimiento & desarrollo , Secuencia de Aminoácidos , Factores Quimiotácticos/química , Factores Quimiotácticos/farmacología , Defensinas/química , Defensinas/farmacología , Etiquetas de Secuencia Expresada , Magnoliopsida/efectos de los fármacos , Magnoliopsida/genética , Datos de Secuencia Molecular , Oligonucleótidos Antisentido/genética , Tubo Polínico/efectos de los fármacos , Tubo Polínico/genética , ARN de Planta/antagonistas & inhibidores , ARN de Planta/genética , ARN de Planta/metabolismo , Transcripción Genética

RESUMEN

The synergid cells of the female gametophyte play a role in many steps of the angiosperm fertilization process, including guidance of pollen tube growth to the female gametophyte. However, the mechanisms by which the synergid cells become specified and develop their unique features during female gametophyte development are not understood. We identified MYB98 in a screen for Arabidopsis thaliana genes expressed in the female gametophyte. MYB98 is a member of the R2R3-MYB gene family, the members of which likely encode transcription factors. In the context of the ovule, MYB98 is expressed exclusively in the synergid cells, and mutations in this gene affect the female gametophyte specifically. myb98 female gametophytes are affected in two unique features of the synergid cell, pollen tube guidance and the filiform apparatus, but are otherwise normal. MYB98 also is expressed in trichomes and endosperm. Homozygous myb98 mutants exhibit no sporophytic defects, including trichome and endosperm defects. Together, these data suggest that MYB98 controls the development of specific features within the synergid cell during female gametophyte development.

Asunto(s)

Proteínas de Arabidopsis/metabolismo , Arabidopsis/crecimiento & desarrollo , Diferenciación Celular/fisiología , Flores/crecimiento & desarrollo , Factores de Transcripción/metabolismo , Secuencia de Aminoácidos , Arabidopsis/genética , Arabidopsis/metabolismo , Proteínas de Arabidopsis/genética , Proteínas de Arabidopsis/aislamiento & purificación , Secuencia de Bases , ADN Complementario/análisis , ADN Complementario/genética , Fertilización/fisiología , Flores/genética , Flores/metabolismo , Flores/ultraestructura , Regulación de la Expresión Génica de las Plantas/fisiología , Células Germinativas/crecimiento & desarrollo , Células Germinativas/metabolismo , Células Germinativas/ultraestructura , Microscopía Electrónica de Transmisión , Datos de Secuencia Molecular , Mutación/fisiología , Semillas/genética , Semillas/metabolismo , Semillas/ultraestructura , Factores de Transcripción/genética , Factores de Transcripción/aislamiento & purificación