Novelties of the Flowering Plant (140)

7/31/2019 Novelties of the Flowering Plant (140)

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Joseph H. Williams

Dept. of Ecology and Evolution, Univ. ofTennessee, Knoxville


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Progamic Phase interval betweenpollination and fertilization

Plesiomorphy - an ancestral or primitive

character Evolutionary novelty - any newly acquired

structure or property that permits the

performance of a new function, which, in

turn, will open a new adaptive zone


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Gymnosperms Angiosperms

non-flowering plant flowering plant

naked seed enclosed seed

have strobili or cones have flowers

male and female cones male pollen and female

ovules

haploid endosperm (n) triploid endosperm (3n)

mostly wind pollination mostly insect pollination

occupy land and water dwell in land only

of ancient evolutionary

origin

or more recent

evolutionary origin

include 600-750 species include 250,000 species(80% of all plant species)


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Angiosperms > GymnospermsFlower

Closed carpel

Highly reduced male and female gametophytes

Double Fertilization

Sexually formed polyploid endosperm

Long styles

Multi-seeded ovariesMuch faster pollen tube growth rates


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Parameters Gymnosperms Angiosperms

tip cellulose esterified pectin

wall cellulose microfibril-

based

amorphous callose-

based

plug esterified pectin callose

pollen tube growthrates


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Amorphous callose-based wall is faster and more

energy efficient than biosynthesis of a cellulose

microfibril-based wall.

Silencing of genes involved in callose synthesis

reduces pollen competitive ability.

Properties of Callose Walls and Plugs:

Walls: Impermeable

Plugs: Seal off the pollen tube

Reduce the risk of damage and allow pollen tubes

to grow longer distances


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Allows faster growth rates because

i. The pectic tip is more plastic and rapidly

extensible

ii. The mature tube cell wall has greater

resistance to tension stress due to secretion of

callose

iii. Callose plugs help maintain positive turgorpressure in the growing tip

Angiosperms pollen tube tips: esterified pectins

Lateral tube wall: with callose secondary wall

A f 900 di i di h h i


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A survey of >900 studies indicates that the timeinterval between pollination and fertilization(fertilization interval) ranges from:

10 hours to >12 months - Gymnosperms15 minutes to >12months - Angiosperms


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Early Cretaceous angiosperms and

ephedroids (Gnetales) were both diversifying

in similar habitats

Today 65 species ofEphedra are confined tosemiarid habitats whereas angiosperms have

radiated into virtually every environment on

earth


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Gymnosperm reproductive evolution isconstrained because their slow pollen tube

growth rates impose a trade-off between

pollen tube developmental time and pollen

tube pathway length.

pollen tube developmental time = pollentube pathways = pollen tube growth rate

pollen tube developmental time = pollen

tube pathways = pollen tube growth rate


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Gymnosperm pollen tube growth rate

evolution is thus severely constrained by the

tight linkage of developmental time and

pollen tube pathway length.


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A critical step in early angiosperm prehistory

was the origin of their unique pollen tube

wall growth pattern.

The callose wall and plug preceded the originof novelties such as true closed carpels, solid

styles, and deep multi-ovulate ovaries, as

well as the evolution of extreme traits such

as fertilization intervals as short as 15 min,pollen tube pathway lengths as longs as 500

mm, and pollen tube growth rates >1,000-

fold faster than those of gymnosperms.

Angiosperm pollen tube wall innovations gavepollen tubes the capacity for rapid and long-

distance growth, increasin the evolutionary

potential of both pollen tubes and the tissues

they interact with.


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Because angiosperms lack the gymnosperm

developmental constraint of slow and staticpollen tube growth rates, developmental

time and pollen tube pathway length became

evolutionary dissociated in derived groups.

Angiosperm fertilization biology is

distinguished not only by many novelties and

extreme traits, but also by much greater

independence (modularity) of their

developmental processes.


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Elevated reproductive trait diversity, and

perhaps increased modularity as well, are

strongly linked to the elevated taxonomic

diversity of flowering plants.

Rapid reproduction, due in large part to

accelerated pollen tube growth rates, is a

fundamental life history strategy shared bymany of the most diverse herbaceous clades

such as grasses and asters.

Moreover, the great developmental flexibility

of angiosperm pollen tubes expanded thepotential for pollen competition and

maternal responses to its effects, in turn,

speeding the evolution of prezygotic forms of

mating systems and reproductive barriers.


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Gymnosperms, generally lack strong

prezygotic barriers.

Pollen tube growth rate innovations truly lie

at the heart of the tremendousreproductive flexibility and opportunism

that Stebbins and others have described as

the critical factor in angiosperm success.


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Th ll f i l t ll t f lfill t


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The walls of growing plant cells must fulfill two

simultaneous and seemingly contradictory requirements.

First, they must expand to accommodate cell growth,

which is anisotropic in many tissues and determines organ

morphology. Second, they must maintain their structuralintegrity, both to constrain the turgor pressure that drives

cell growth and to provide structural rigidity to the plant.

These requirements are met by constructing primary cell

walls that can expand along with growing cells, whereassecondary cell walls are deposited after cell growth has

ceased and serve the latter function.One of the major constituents of both types of cell walls is cellulose, which exists as microfibrils composed of parallel

-1,4-linked glucan chains that are held together laterally by hydrogen bonds (Somerville, 2006). Microfibrils are 2 to 5

nm in diameter, can extend to several micrometers in length, and exhibit high tensile strength that allows cell walls to

withstand turgor pressures of up to 1 MPa (Franks, 2003). In vascular plants, cellulose is synthesized by a multimeric

cellulose synthase (CESA) complex composed of at least three types of glycosyl transferases arranged into ahexameric rosette (Somerville, 2006). After delivery to the plasma membrane, CESA initially moves in alignment with

cortical microtubules (Paredez et al., 2006), but its trajectory can be maintained independently of microtubule

orientation. For example, in older epidermal cells of the root elongation zone in Arabidopsis (Arabidopsis thaliana),

cellulose microfibrils at the inner wall face are oriented transversely despite the fact that microtubules reorient from

transverse to longitudinal along the elongation zone (Sugimoto et al., 2000), suggesting that microtubule orientation

and cellulose deposition are independent in at least some cases.

Depending on species, cell type, and developmental stage, cellulose microfibrils may be surrounded by additional

networks of polymers, including hemicelluloses, pectins, lignin, and arabinogalactan proteins (Somerville et al., 2004).

Hemicelluloses are composed of -1,4-linked carbohydrate backbones with side branches and include xyloglucans,

mannans, and arabinoxylans. Xyloglucan is thought to interact with the surface of cellulose and form cross-links
http://www.ncbi.nlm.nih.gov/pubmed/16824006http://www.ncbi.nlm.nih.gov/pubmed/12730269http://www.ncbi.nlm.nih.gov/pubmed/16824006http://www.ncbi.nlm.nih.gov/pubmed/16627697http://www.ncbi.nlm.nih.gov/pubmed/11115865http://www.ncbi.nlm.nih.gov/pubmed/15618507http://www.ncbi.nlm.nih.gov/pubmed/15642717http://www.ncbi.nlm.nih.gov/pubmed/15618507http://www.ncbi.nlm.nih.gov/pubmed/11115865http://www.ncbi.nlm.nih.gov/pubmed/16627697http://www.ncbi.nlm.nih.gov/pubmed/16824006http://www.ncbi.nlm.nih.gov/pubmed/12730269http://www.ncbi.nlm.nih.gov/pubmed/16824006


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Novelties of the Flowering Plant (140)