Regulation of Floral Development (Part II)

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FLORAL EVOCATION: INTERNAL AND EXTERNAL CUES

Flowering is regulated by:

ØInternal factors controlling the switch to reproductive development:

•Age

•Size

•Number of leaves

Ø External cues controlling seasonal response:

•Photoperiodism (Light/dark cycle) and  light quality (wavelength and intensity)

•Vernalization (Low temperature)

•Total light radiation

•Water availability

•External cues and internal developmental factors together fine-tune flower evocation in most plants.

•When flowering occurs strictly in response to internal developmental factors and does not depend on any environmental conditions, it is referred to as autonomous regulation.

THE SHOOT APEX AND PHASE CHANGES

•In higher plants, developmental changes occur in a single, dynamic region, the shoot apical meristem.

•During postembryonic development, the shoot apical meristem passes through three well-defined developmental stages:

ØThe juvenile phase

ØThe adult vegetative phase

ØThe adult reproductive phase

•The transition from one phase to another is called a phase change.

•Juvenile tissues are produced first and are located at the base of the shoot.

Competence and determination are two stages in floral evocation

•To initiate floral development, the cells of the meristem must first become competent i.e. capable of responding to floral stimulus (induction).

•A competent vegetative meristem responds to a floral stimulus (induction) to becoming florally determined (committed to producing a flower).

•Once determined, it will flower even after removal of the floral stimulus.

•Competent vegetative shoot (scion) grafted onto a flowering stock will flower.

•Reason: It is capable of responding to floral stimulus present in the stock.

•The grafted scion will fail to flower before attaining competence.

•Reason: Its shoot apical meristem is not yet able to respond to the floral stimulus present in the stock.

•A bud is said to be determined if it progresses to the next developmental stage (flowering) even after being removed from its normal context.

•Thus a florally determined bud will produce flowers even if it is grafted onto a vegetative plant that is not producing any floral stimulus

Demonstration of the determined state of axillary buds in tobacco

•In a day-neutral tobacco, plants typically flower after producing about 41 leaves or nodes.

•If a flowering tobacco plant is decapitated just above the 34th leaf, the axillary bud of the 34th leaf grows out, and flowers after producing 7 more leaves (for a total of 41).

•If the 34th bud is excised from the plant and either rooted or grafted onto a stock without leaves near the base, it produces a complete set of 41 leaves before flowering.

              •Reason: The 34th bud was not yet florally determined.

•If a flowering tobacco plant is decapitated just above the 37th leaf, the 37th axillary bud flowered after producing 4 more leaves in all three situations.

             •Reason:

           ØThe 37th bud was already florally determined.

     ØThe number of nodes a meristem produces before flowering is a function of two factors:

         •Strength of the floral stimulus from the leaves

         •Competence of the meristem to respond to the signal

PHOTOPERIODISM: MONITORING DAY LENGTH

•Photoperiodism is the ability of an organism to detect day length which makes it possible for an event to occur at a particular time of year, thus allowing for a seasonal response.

•Circadian rhythms (Biological clock) and photoperiodism have the common property of responding to cycles of light and darkness.

Plants can be classified by their photoperiodic responses

•The classification of plants according to their photoperiodic responses is usually based on flowering.

•The two main photoperiodic response categories are:

ØLong-day plant (LDPs)

ØShort-day plants (SDPs)

Critical day length

•Flowering in LDPs is promoted only when the day length exceeds a certain duration, called the critical day length, in every 24-hour cycle.

•Promotion of flowering in SDPs requires a day length that is less than the critical day length.

•The absolute value of the critical day length widely varies among species.

                 LDPs flower: Day length > critical day length

                 SDPs flower: Day length < critical day length

Both Xanthium (a SDP) and Arabidopsis (a LDP) flowers under 12 hours of photoperiod.

•Critical day length of Xanthium is 15 hours and it flowers when photoperiod is less than 15 hours).

•Critical day length of Arabidopsis is 11 hours and it flowers when photoperiod is greater than 11 hours.

Since, 12 hours of photoperiod is less than 15 hours and greater than 11 hours, both Xanthium and Arabidopsis can flower.

ØLong-day plants (LDPs):

•Flower only in long days (qualitative LDPs) or flowering is accelerated by long days (quantitative LDPs).

•Flowering promoted only when the day length exceeds critical day length, in every 24-hour cycle.

•LDPs  measure the lengthening days of spring or early summer and delay flowering until the critical day length is reached.

•Ex: Triticum aestivum

ØShort-day plants (SDPs):

•Flower only in short days (qualitative SDPs) or flowering is accelerated by short days (quantitative SDPs)

•Promotion of flowering in SDPs requires a day length that is less than the critical day length.

•SDPs flower in fall, when the days shorten below the critical day length.

•Ex: Chrysanthemum morifolium

•However, day length alone is an ambiguous signal because it cannot distinguish between spring and fall.

•For avoiding the ambiguity of day length signal, plants often couple a temperature requirement to a photoperiodic response.

•Other plants avoid seasonal ambiguity by distinguishing between shortening and lengthening days and are called “dual–day length plants”.

The “dual–day length plants” fall into two categories:

•Long-short-day plants (LSDPs): flower only after a sequence of long days followed by short days. LSDPs,

•Ex: Bryophyllum, Kalanchoe, and Cestrum nocturnum flower in the late summer and fall, when the days are shortening

•Short-long-day plants (SLDPs): flower only after a sequence of short days followed by long days. SLDPs

•Ex: Trifolium repens, Campanula medium, and Echeveria harmsii, flower in the early spring in response to lengthening days.

•Day neutral plants (DNPs): Species that flower under any photoperiodic condition and are insensitive to day length.

   •Flowering in DNPs is typically under autonomous regulation—that is, internal developmental control.

    •Ex: Phaseolus vulgaris, Castilleja chromosa, and Abronia villosa

Plants Monitor Day Length by Measuring the Length of the Night

Night breaks can cancel the effect of the dark period

•The dark period can be made ineffective by interruption with a short exposure to light, called a night break.

•But interrupting a long day with a brief dark period does not cancel the effect of the long day.

•When given during a long dark period, a night break promotes flowering in LDPs and inhibits flowering in SDPs.

•A night break was found to be most effective when given near the middle of a dark period.

Leaf Is the Site of Perception of the Photoperiodic Stimulus

•Treatment of a single leaf of a SDP with short photoperiods are sufficient to cause flowering when the rest of the plant is exposed to long days.

•But, treatment of the shoot apex with short photoperiods don’t induce flowering if the rest of the plant is exposed to long days

•In response to photoperiod, leaf transmits a signal that regulates the transition to flowering at the shoot apex. The photoperiod-regulated processes that occur in the leaves resulting in the transmission of a floral stimulus to the shoot apex is referred to collectively as photoperiodic induction.

Floral stimulus is transported via the phloem

•Once produced, the floral stimulus is transported to the meristem via phloem, and it appears to be chemical in nature.

•Treatments that block phloem transport, such as girdling or localized heat-killing prevent movement of the floral signal.

•The floral stimulus is translocated along with sugars in the phloem and it is subject to source-sink relations.

•An induced leaf positioned close to the shoot apex is more likely to cause flowering than an induced leaf at the base of a stem, which normally feeds the roots.

                                (Images kindly shared by Oxford University Press and are being used for teaching purpose only)

Chromosomal Aberration (Part III)

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EUPLOIDS

•  The changes in the number of chromosomes are usually described as variations in the ploidy of the organism.

• Organisms with one or more complete sets of chromosomes are said to be euploid.

• Organisms that carry more than two sets of chromosomes are said to be polyploid and the level of polyploidy is described by referring to a basic chromosome number, usually denoted as n.

•      Haploids: carry a single set of chromosomes (n)

•      Diploids: carry two chromosome sets (2n)

•      Triploids: carry three chromosome sets (3n)

•      Tetraploids: carry four chromosome sets (4n)

• An individual of a normally diploid species that has only one chromosome set is called a monoploid to distinguish it from an individual of a normally haploid species (n).

MONOPLOIDS

•  An individual of a normally diploid species that have only one chromosome set is called a monoploid.

•  Male bees, wasps, and ants are monoploid. They develop by parthenogenesis (the development of an unfertilized egg into an embryo).

•   In most other species, monoploid zygotes fail to develop.

•  The reason is that virtually all individuals in a diploid species carry a number of deleterious recessive mutations, together called a “genetic load.”

•  The deleterious recessive alleles are masked by wild-type alleles in the diploid condition but are automatically expressed in a monoploid derived from a diploid.

•  Monoploids that do develop to advanced stages are abnormal. If they survive to adulthood, their germ cells cannot proceed through meiosis normally because the chromosomes have no pairing partners. Thus, monoploids are characteristically sterile.

POLYPLOIDS

•  Organisms that carry more than two sets of chromosomes are said to be polyploid.

•  Polyploid plants are often larger and have larger component parts than their diploid relatives.

• Polyploids with odd numbers of chromosome sets, such as triploids, are sterile or highly infertile because their gametes and offspring are aneuploid.

AUTOPOLYPLOIDS AND ALLOPOLYPLOIDS

• Autopolyploids: They have multiple chromosome sets originating from within one species

•  Allopolyploids: They have sets of chromosomes from two or more different species.

• Allopolyploids form only between closely related species; however, the different chromosome sets are only homeologous (partially homologous), not fully homologous as they are in autopolyploids.

•  Generally plants seem to be much more tolerant of polyploidy than animals.

AUTOPOLYPLOIDS: TRIPLOIDS

•  Triploids are usually autopolyploids.

•  They may arise spontaneously in nature.

•  They can also be constructed by geneticists from the cross of a 4n (tetraploid) and a 2n (diploid). The 2n and the n gametes produced by the tetraploid and the diploid, respectively, unite to form a 3n triploid.

•  Triploids are characteristically sterile because of their problem in synapsis during meiosis.

•  The three homologous chromosomes of a triploid may pair in two ways at meiosis:

a trivalent 

a bivalent plus a univalent

• It is unlikely that a gamete will receive two for every chromosomal type, or that it will receive one for every chromosomal type. Hence, they will be aneuploids.

AUTOPOLYPLOIDS: TETRAPLOIDS

•  Autotetraploids arise by the doubling of a 2n complement to 4n.

• This doubling can occur spontaneously, but it can also be induced artificially by applying chemical agents that disrupt microtubule polymerization (ex: colchicine).

• In colchicine-treated cells, the S phase of the cell cycle occurs, but chromosome segregation or cell division does not. As the treated cell enters telophase, a nuclear membrane forms around the entire doubled set of chromosomes.

•  Therefore, if a diploid cell of genotype A/a; B/b is doubled, the resulting autotetraploid will be of genotype A/A/a/a ; B/B/b/b.

•  Because 4 is an even number, autotetraploids can mostly have regular meiosis.

• Chromosomes may pair as:

a pair of bivalents (chromosomes segregate normally producing diploid gametes)

a quadrivalent (chromosomes segregate normally producing diploid gametes)

a trivalent plus a univalent (nonfunctional aneuploid gametes)

ALLOPOLYPLOIDS

• An allopolyploid is a plant that is a hybrid of two or more species, containing two or more copies of each of the input genomes.

• Allopolyploid plants can be synthesized by crossing related species and doubling the chromosomes of the hybrid or by fusing diploid cells.

Raphanobrassica

•  An allotetraploid synthesized by G. Karpechenko in 1928.

• He wanted to make a fertile hybrid that would have the leaves of the cabbage (Brassica) and the roots of the radish (Raphanus).

• Each of these two species has 18 chromosomes.

                                           2n₁ = 2n₂ = 18

•  The species are related closely enough to allow intercrossing.

• Fusion of an n₁ and an n₂ gamete produced a viable hybrid progeny but it was sterile.

•  Eventually, one part of the hybrid plant produced some seeds due to accidental chromosome doubling which enabled synapsis during meiosis.

•  This resulted in an allotetraploid individual (2n₁ + 2n₂) which would produce n₁ + n₂  gametes.  These gametes fuse to form 2n₁ + 2n₂ progeny again.

•  This kind of allopolyploid is sometimes called an amphidiploid or doubled diploid.

•  Unfortunately, the hybrid exhibited leaves like radish and roots like cabbage!

Bread wheat

• A particularly interesting natural allopolyploid is bread wheat, Triticum aestivum (6n = 42). 

• Bread wheat is an allohexaploid (6x), which regularly forms 21 pairs of chromosomes (2n = 42) during meiosis.

• These chromosomes are subdivided into 3 closely related (homoeologous) groups of chromosomes, the A, B, and D genomes.

• In the first hybridization event, the A genome progenitor combined with the B genome progenitor to form primitive tetraploid wheat (2n=28, AABB). This hybrid occurred in the cytoplasm of the B genome.

• The second event involved hybridization between the tetraploid (AABB) form and the D genome progenitor to form the basic hexaploid configuration, AABBDD, again in the B genome cytoplasm.

• At meiosis, pairing is always between homologs from the same ancestral genome.

• Hence, in bread wheat meiosis, there are always 21 bivalents.

• The domestication of diploid and tetraploid wheat is thought to have occurred in the fertile crescent of the Middle East.

• Domestication of the diploid and tetraploid wheat is thought to have occurred at least nine thousand years ago with the hybridization event producing hexaploid wheat occurring more than six thousand years ago.

APPLICATION OF POLYPLOIDY IN AGRICULTURE

• Monoploids: Monoploids help breeders to select recessive characters in hybrids.

•  Autotriploids: Commercially available seedless bananas are sterile triploids (3n = 33). Seedless watermelons are another example.

• Autotetraploids: Many autotetraploid plants have been developed as commercial crops to take advantage of their increased size. Large fruits and flowers are particularly favored.

• Allopolyploids: Best example is bread wheat. New World cotton is a natural allopolyploid that occurred spontaneously. A synthetic amphidiploid, Triticale, has been widely used commercially. This is an amphidiploid between wheat (Triticum, 6n=42) and rye (Secale, 2n=14). Hence, for Triticale, 2n=2x(21+7)  56. This novel plant combines the high yields of wheat with the ruggedness of rye.