Plant early evolution & invasion of land

Monophyletic group of land plants

embryophytes • Can be further classified into 11 major groups of land plants: liverworts, mosses, hornworts, lycophytes, horsetails, leptosporangiate ferns, cycads, ginkgo, gnetophytes, conifers and flowering plants.

Landplants (embryophytes)

Land plants are also called *embryophytes* because, unlike the rest of the plants, they enclose their eggs in a protective embryo. • Plants are thought to have "invaded" land about 450 million years ago, at about the barrier between the Ordovician and Silurian periods.

Green algae

A clade that consists of the chlorophytes, coleochaetophytes, charophytes, and other "green algae". (pg 21). • This grouping is *NOT* *monophyletic* because it does not include an ancestor and all of its descendants. • Specifically, it excludes land plants (land plants are a sister plant to green algae)

Streptophytes

A monophyletic subset of green plants that consists of two group of green algae (coleochaetophytes and charophytes) and land plants. • distinguish members of Chlorophyta ("true" green algae) from members of Streptophyta (green algae more closely related to land plants) The earliest lineages of streptophytes include two form of green algae: Coleochaete (coleochaetophytes) and Chara (charophytes). These group are the most closely related clades to land plants, and they contain many of the distinguishing features that allow plants to evolve and grow on land. • Research has shown that *charophytes* *are* *the* *closest* *relatives* *of* *land* *plants*

Charophytes

A streptophyte, one of the earliest lineages. Research has shown that charophytes are the closest relatives of land plants. This is based on: • Channels called *plasmodesmata* doing the cytoplasm of adjacent cells. Plasmodesmata are important because they allow cells to communicated with other naother making multicellularity possible • Branching growth from the apical membrane (from the top) • Similar peroxisome contents, chloroplast structure, and processes for mitosis and cytokinesis. • Parenchymal tissue consisting of isodiametric (roughly spherical) parenchyma cells (parenchyma is basic plant tissue consisting of cell walls linked together by plasmodesmata) The fact that charophytes are the closest relatives to land plants makes them a sister taxa

Tiktaalik

A transitional fossil that dates around 375 million years ago when vertebrate animals made their transition to land. • This was the "missing link" between fish-like ancestors and tetrapods.

Alternation of generations

A universal synapomorphy of all land plants • There is a multicellular diploid (2n) formed (the *sporophyte*) and a multicellular haploid (n) formed (the *gametophyte*) To reproduce, the *sporophyte* (ie. the plant) produces *spores* through meiosis (this occurs in a specialized structure called the sporangium). • These spores germinate and divide mitotically, forming haploid gametophytes. • The male gametophyte produces the sperm and the female gametophyte produces the egg, or the embryo sac. • Specifically, the gametes are produced in specialized protective structures called *gametangia*. The male gametangium is called the *antheridium* and the female gametanium is called the *archegonium* Then the gametes fuse by fertilization, a diploid zygote is formed.The zygote divides via mitosis, forming an embryo, which remains in the tissue of the female gametophyte. • Placental transfer cells mediate the transfer of nutrients from the parent to the embryo, and a sporophyte develops. Then the process continues.

Land plant embryos

An adaptation to life on land :Land plants have *embryos*, tiny diploid plants initially surrounded and nourished by the cells of the gametophyte (this is why land plants are also called embryophytes). • A diploid *zygote* forms when the sperm makes their way down the archegonium (the female part of the plant), either by swimming there or by being splashed there. This is a process called *syngamy*, the gametes fuse together and form a zygote that will eventually divide and become the embryo. • The embryo will initially be housed and nourished in the gametophyte, but it will eventually grow into a multicellular sporophyte. • This initial protection and nourishment of the embryo makes it more likely that the woung plant will develop fully and survive

Mutualistic associations with fungi

An adaptation to life on land. Scientists believe that mutualistic associations between plants and fungi may have helped the earlies plants acquire the water and minerals needed to survive on land • *Mycorrhizae* are mutualistic association between fungi and underground plant structures (usually roots). • The filamentous fungi gater minerals and water from the soil and bring it back to the plant in exchange for the products of photosynthesis (i.e. glucose) There are two types of mycoreehizae: 1. *Ectomycorrhizal fungi*: A kind of mycorrhizae in which the mycelium forms a mantle (a dense sheath) over the root's surface. Fungal hyphae extend into the soil from the mantle. This increases the surface area and promotes the uptake of water and minerals 2. *Arbuscular mycorrhizal fungi*: Endomycorrhizae is a kind of mycorrhizae without a mantle over the root's surface. Fungal hyphae extent into the soil from the root instead of the mantle. These hyphae are microscopic, generally smaller than the hyphae involved in ectomycorrhizae. They digest small patches in the walls of the root cell and extend into it, penetrating the cell walls in the cortex but not the plasma membrane or cytoplasm. Some hyphae from *arbuscules* ("little trees") branches that increase the hyphal surface area and promote nutrient transfer.

What were the first land plants?

Bryophytes (liverworts, hornworts and mosses) were the first plants to transition to life on land. • The most well-supported hypothesis is that tiny plants called *liverworts* were the earliest land plants. • Some of the earliest fossil spores from about 470 million years ago have a pattern of parallel ridging that matches those of modern liverworts. • Many liverworts have symbiotic relationships with fungi, which is one reason why scientists believe that mycorrhiza played an important role int he transition of life on land. • Scientists have found fossil spored from Glomalean fungi that dat to about the same time as the fossil spores of the land based liverworts. • One reason why many scientists believe that liverworts were the first land plants is because they lack stomata, while all of the other land plants have stomata. It is assumed that stomata developed after liverworts moved onto land.

Green plants

Green plants are a monophyletic group that consists of green algae and land plants. • They include: chlorophytes, coleochaetophytes, charophytes, other green algae, and land plants. Two key synapomorphies that characterize members of this clade include: 1. *Chlorophyll b*: whereas red algae contains only chlorophyll a, green plants contain chlorophyll a AND chlorophyll b. They are both similar but they differ by a single menthyl group in the upper right carbon. 2. *Starch*: whereas red algae stores their sugars as floridean starch, green plants store their sugars as start in chloroplasts.

Glaucophytes

Microscopic, freshwater algae. • They form the sister group to the rest of Plantae • Glaucophytes have chloroplasts that have retained their peptiglycan walls. • Red algae and green plants, in contrast, have lost this cell wall. • This is the key feature that distinguished glaucophytes.

Red algae

Red algae are almost all multicellular, although there are some unicellular red algae. • Their characteristic red color comes from *phycoerythin*, an accessory pigment found in the chloroplasts of red algae that absorbs green light and extends the range of photosynthesis. • In addition to the accessory pigments, their chloroplasts contain carotenoids and chlorophyll a (not chlorophyll b). • These plants retained their cellulose cell walls. • They have no centrioles, lack flagellated cells, and store their sugars as floridean starch (more highly branched than normal starch) • They are not always red, their color depends on the ratio of two pigments in their chloroplasts: phycoerythin (which produces a red color) and chlorophyll a (which produces a green color). • The ratio of these pigments depends on the intensity of light where the algae live. The closer they are to the water's surface, the more chlorophyll a they will contain, making them more likely to be green rather than red. • Red algae is used to wrap sushi, as a thickening compound in ice cream and other food products, and in agar plates and electrophoretic gels in laboratories.

Drawbacks of the first land plants

The first land plants (bryophytes) had a number of drawbacks that prevented them from thriving on land: • They relied on osmosis to get water into their tissues • They have swimming sperm, so they required water for fertilization • Their gametophytes lacked strong supportive tissues.

Green plants can be defined as

We can define a green plant as a eukaryotic organism that satisfies the following criteria: • Their chloroplasts developed from primary endosymbiosis • Their chloroplast consist of both chlorophyll a and chlorophyll b • They store carbohydrates as actual starch (and NOT floridean starch)

Plant adaptions to life on land

• *Cuticle*: Land plants further protected themselves from water loss by developing a coating of waxy lipids called a cuticle (made out of cutin and other waxy polymers). The cuticle traps the water and protects it against evaporation. It is typically on both the bop and the bottom of the plants leaves. • *Stomata*: openings through which plants can exchange gases (oxygen and carbon dioxide) with the outside environment. Stomata are able to open and close based on the plan's needs and environmental conditions. Primarily located on the underside of the leaf. • *UV-absorptive compounds*: land plants evolved certain UV absorptive pigments that protect against ultraviolet radiation, one of the most significant challeneges to living on land. They absorb certain wavelengths of light and sdissipate excessive ratiation (sunscreen for plants). Collectively these pigments are called *accessory* *pigments* . • *Thick* *spore* *walls*: Land plants developed spores with thick walls containing *sporopollenin*, a polymer that resists decay and keeps spores from drying out. Spore swimming in aquatic environments, in contrast, do not have thick sporopollenin walls. •*Multicellular gametangia*: *Gametangia* are multicellular organs in which gametes are produced. *Archegonia* are the female gametangia, which produce eggs via mitosis and serve as the site of fertilization. *Antheridia* are the male gametangia, which produce sperm with two subapical whiplash flagella via mitosis (the sperm must swim to the archegonium to engage in fetilization). Gametangia are advantageous structures for land plants because they protect egg and sperm from drying out. Compare this strategy to that of chlorophytes, which rely only on a cellulose cell wall to avoid desiccation.

Evolution of plants: Vascular tissue

• Another evolutionary development by plants that helped them thrive on land: *vascular tissue that assist in the movement of water and sugars*: Vascular tissue includes the *xylem* and * phloem*, which are the "pipes" through which water and sugar flow throughout the plant. The xylem brings water and nurtients from the roots to the top of the plant, while the phloem take sugars from the photosynthetic parts of the pants (sources) to the parts of the plants that needed them.

Evolution of plants: Partnerships with land animals that promote growth and reproduction.

• Another evolutionary development by plants that helped them thrive on land: • *Partnerships with land animals that promote growth and reproduction*: Some plants developed *flowers* and *fruits* which attract animals. • The development of flowers or fruits are associated with the huge diversification of animal life. • Flowers have a nice fragrance and bright colors while fruits may have a sweet taste. All of which attract animals. When an animal eats a fruit, it takes the seeds with it and eventually releases them in its poop. This helps a plant species spread to new habitats.

Evolution of plants: Reproductive mechanisms

• Another evolutionary development by plants that helped them thrive on land: • *Reproductive mechanisms that don't depend as heavily on water for fertilization*: for example *pollen* are small male gametophytes that are compact enough that they can travel though the air. They are transferred to the female reproductive structures either through the air or by other animals.