Angiosperm Fertilization and Pollen Tube Biology

One or more thin areas present in the exine of pollen grains are known as germ pores. The germ pores are apertures in the exine layer of the pollen grain where the sporopollenin is absent. The germ pore helps in the formation of the pollen tube and the release of the male gametes during fertilisation. There are usually three germ pores in dicots(tricolpate) and one in monocots(monocolpate).

In an ovule, an opening is formed at the tip of ovule by the integuments called as micropyle. The micropyle acts as a pore for the entry of pollen tube into the ovule for fertilization. In bitegmic ovules, the opening is formed by the inner integument and is called endostome. If the opening is formed by the outer integument it is called exostome. Sometimes it is formed by both the integuments, then it is called as amphistome. 

• Casuarina equisetifolia is a monoecious, wind-pollinated species that undergoes chalazogamy fertilization.
  
• In this type of fertilization, the pollen tube penetrates the ovule through the tissue of the chalaza.
  
• It was first observed by Treub in 1981 in Casuarina species.
  

The pollen tube consists of two male gametes. One male gamete fertilizes the egg cell placed in between the egg apparatus and forms the diploid oospore/zygote which forms embryo after maturation. While the other male gamete fuses with the two polar nuclei of the central cell and forms the triploid PEN (Primary Endosperm Nucleus) which results in the formation of endosperm after maturation.

Chemotaxis is the movement of an organism or plant in response to a chemical stimulus. Somatic cells, bacteria and other single cell or multicellular organisms direct their movements according to certain chemicals in their environment. It is clearly seen in the case of growth of pollen tube towards the embryo sac. The pollen tube does not directly reach the ovary in a straight line. It travels near the skin of the style and curls to the bottom of the ovary, then near the receptacle, it breaks through the ovule through the micropyle and the pollen tube bursts into the embryo sac.

In mesogamy, pollen tube enters the ovule through the integuments or funiculus. After the pollen tube enters the embryo sac, its apex dissolves and the male gametes are liberated. One of the male gametes are liberated. One of the male gametes fuses with the egg and produces a diploid zygote which by further division develops into an embryo. The other male gamete fuses with the diploid secondary nucleus and produces a triploid primary endosperm nucleus. This is called double fertilization.

Synergid cells are two specialized cells that lie adjacent to the egg cell in the female gametophyte of angiosperms and play an essential role in  pollen tube guidance and function. The term synergid comes from the Greek word ' Synergos' which means 'working together' and was reportedly coined by Eduard Strasburger. The pollen tube grows towards and into one of the synergid cells, which begins to degenerate as the pollen tube enters it. The pollen tube then ceases growth, ruptures and releases its two sperm cells which migrate to the egg and central cell, fertilization of which gives rise to the embryo and endosperm, respectively. The synergid cells are necessary for pollen tube guidance in the ovule.

Paratonic growth movements are the induced movements of growth, caused by external stimuli, such as gravity, light, water or contact. The movements show a directional relation to the stimulus. The stimuli are effective in causing growth movements only when they are unidirectional. Such movements are generally known as tropic movements and the phenomenon is referred to as tropism. 

Movements of locomotion are generally very fast movements, which may be
(A) Autonomic or spontaneous and
(B) induced or paratonic.
The induced locomotory movements are also called as tactic movements induced by external stimuli. Based on the nature of stimuli, these may be of three types:
(i) Phototactic movement: Seen in zoospores and gametes, which are provided with a light sensitive eye spot, which is attracted by low light intensities.
(ii) Thermotactic movement: Movement in response to heat stimulus is seen in certain motile algae, like Chlamydomonas, which moves from a colder to a warmer place.
(iii) Chemotactic movement: In bryophytes and pteridophytes, the swimming antherozoids are attracted towards the archegonium by chemical stimuli, such as organic substances, like sugar, malic acid and so on.

The pollen tube grows towards embryo sac because of influence of certain chemicals. Pollen tube most commonly enters embryo sac through micropyle and bursts in one of the synergid. It is believed that filiform apparatus of synergids secrete pollen tube attracting chemicals. Mineral; boron is required for germination of pollen grains and elongation of pollen tube. The growth of pollen tube is an example of chemotropism.

Geotropism is movement or growth with respect to gravitation force; it may be positive geotropism (in direction of gravity) or negative geotropism (away from gravity). This makes option A incorrect as growth of pollen tube does not occur in response to gravity. Phototaxis refers to movement/growth of plant or plant part in response to sunlight. This makes option C incorrect as growth of pollen tube occurs towards the embryo sac, not towards or away the sunlight. Thigmotaxis refers to unequal growth caused by contact of plant part with a solid surface which makes option D incorrect. A tropic response of plant or plant part towards a chemical stimulus is referred to as chemotaxis. Pollen tube growth towards the embryo sac is driven by ion flux (reactive oxygen species) and calcium gradient which makes option B correct. 

The eight nucleate female gametophytes of flowering plants have an egg apparatus at micropylar end which in turn has one egg cell and two flanking cells. The flanking cells are known as synergids and have finger like projections of wall in their cytoplasm, the filiform apparatus. They serve as site for entry of pollen tube into one of the synergid followed by disintegration of later. The pollen tube pierces through nucellus and traverses the wall of embryo sac between egg and synergid. Filiform apparatus guide the pollen tube to egg nucleus and thereby facilitate syngamy. This makes option A correct. Option B is incorrect as egg nucleus does not have filiform apparatus to guide the direct entry of pollen tube into it. Three cells present at chalazal end of embryo sac are called as antipodals which makes options C and D incorrect as antipodals and synergids are present at opposite ends of embryo sac. 

Siphonogamy is the condition in plants in which pollen tubes are developed for the transfer of the male cells to the eggs. The seed plants are termed as siphonogamous. As a consequence, the spermatophytes are called as siphonogamous by some botanists. Syngamy is the fusion of gametes producing in the formation of a zygote, which develops into a new organism. 

Porogamy is the entrance of the pollen tube in the seed plants through the micropyle. Whereas Chalazogamy is the process of fertilization in which the pollen tube penetrates to the embryo sac.

Therefore, the correct answer is option C.

The entry of the pollen tube into the ovule through the chalaza is known as the chalazogamy. This was discovered by Treub. One such example of xerophyte is Casuarina, which is a plant which can withstand prolonged periods of drought. A flower is a bracteate, bracteolate, sessile, incomplete and unisexual.

Thus, the correct option is D.

Chemotropism is the directional growth movement in response to external chemical stimuli. Movement of pollen tube towards ovule is chemotropic movement. Haptotropism is the growth movement of plant part in response to the touch stimuli. Thigmotropism is the growth movement in response to the touch. Phototropism is the growth movement of plant part in response to the light.

Amoeboid or Invasive is also known as Periplasmodial Tapetum. It is the most common type of tapetum in the angiosperms. This type of tapetum can be seen in Alisma, Butomus, Tradescantia and Typha, etc. The tapetal cells fuse among themselves to make the tapetal periplasmodium. The protoplast of the fused tapetal cells then moves into the locule. This protoplast movement into the locule, may take place during meiotic prophase.

Thus the correct option is B.

After the germination of pollen grains on stigma pollen tube is produced through a germ pore . Pollen tube grows chemotropically through the tissue of stigma and style in response to sugary fluid.

The pollen tube wall is primarily composed of pectocellulose, which provides the necessary flexibility for growth.

The growth of the pollen tube towards the embryo sac is directed by chemical signals released by the synergids, a process known as chemotaxis.

A mature male gametophyte in angiosperms typically consists of three cells: one tube cell and two male gametes.

The process fertilization accomplished by means of pollen tube in which the transfer of male gametes to the female gametes takes place.

The pollen tube releases its male gametes into one of the two synergids, specifically the one that is degenerating.

An unfertilized ovule contains the embryo sac (female gametophyte), which is a generation within the sporophytic tissue of the ovule.

Mesogamy refers to the entry of the pollen tube into the ovule through the integuments, rather than through the micropyle (porogamy) or the chalaza (chalazogamy). This mode of entry is commonly seen in plants like Cucurbita.

Each seed requires one male gamete and one female gamete. One pollen mother cell (PMC) produces 4 microspores, each forming a pollen grain with 2 male gametes. However, only one pollen grain is needed per seed. Thus, 400 seeds require 400 pollen grains. Since 1 PMC produces 4 microspores, 400/4 = 100 PMCs are needed.

The pollen tube enters the embryo sac through the micropylar end. It can enter directly through the synergid or can enter from between the egg cell and synergid.

Calcium ions play a crucial role in establishing and maintaining the tip growth polarity of pollen tubes by regulating vesicle fusion and actin cytoskeleton dynamics at the apex.

Porogamy is the term for pollen tube entry through the micropyle (the small opening in the ovule integuments). Chalazogamy is entry through the chalaza, and mesogamy is through the integument. Apogamy refers to asexual reproduction without fertilization. Note: the question has a typo - it says 'microphyte' but means 'micropyle'.

Length of pollen tube depends upon distance of pollen on stigma and ovule because after pollination pollen germinates and form pollen tube that reaches upto ovule in ovary and release male gamete to fuse with female gamete.

There are various ways by which the pollen tube can reach embryo sac for pollination.

The function of filiform apparatus in an angiospermic embryo sac is to guide the entry of pollen tube from synergid to egg. The synergid cells present within the female gametophyte play a vital role in the reproduction process in angiosperm. The cell wall of synergid develops an extremely thick structure called the filiform apparatus at the micropylar end of the ovule.

Porogamy is the most common type in which the pollen tube enters the ovule through the micropyle. An example includes lily, etc. 

So the correct option is D.

Siphonogamy is the process of fertilization by means of a pollen tube. This is a characteristic feature of seed plants, which includes gymnosperms, monocots, and dicots.

In sexual reproduction of plants, the sperm is transferred to the egg through a long, polarized tube of a pollen cell called as the pollen tube. The growth of the pollen tube is apical. It grows at its tip at very rapid rate. The rapid growth is due to a highly polarized fusion of apical vesicles. These transport cell wall components to the growing tip via cytoplasmic streaming and are incorporated into a zone of elongation in the apical region of the pollen tube. Hence, the pollen tube extends its length by apical growth in a relatively small region.

After reaching ovary, the pollen tube enters the ovule. When the pollen tube enters the ovule through micropyle, it is called porogamy. It is the most common type. e.g., Lily. 

Apical zone of pollen tube contains abundant vesicles but lacks cell organelles. It is hemisphere in shape and transparent in nature. This transparent apical growing region of pollen tube is called as cap block. It exists only until the tube is growing. It disappears as soon as the growth ceases. 

Callose is a beta-1,3-glucan. It is the major component of the pollen tube cell wall. It is also deposited within the cytoplasm as distinct callose grains. This leads to the formation of callose plugs. Callose plugs separate the living, growing and functional part of the pollen tube in the tip from the remaining degenerating part.