Ch. 7: Roots

Diffuse growth

-All parts of the body grow simultaneously -Observed in most animals

Prop roots

-Make contact with the soil and transport additional nutrients and water to the stem, also contract slightly and put some tension on the stem to act as stabilizers -Can become woody for extremely strong support

Mycorrhizae

-Associations (symbiotic) between soil fungi and seed plants 2 main types Ectomycorrhizal relationship: Hyphae of fungi penetrate between outermost root cortex cells but don't invade cells themselves. Nearly all woody forest plants Endomycorrhizal association: Hyphae penetrate root cortex as far as the endodermis and pass through the walls of the cortex cells but not the casparian strip. Invade cells but don't break plasma membrane or tonoplast.

Casparian strips

-Bands of altered walls in endodermis cells -Involved in controlling the types of minerals that enter xylem water stream -This control is possible because of the waterproofing of endodermis cells, so minerals can only cross into the xylem if those cells absorb them from the apoplast and secrete them into the xylem -Many glands also have these to prevent the glands product from seeping into surrounding tissue

Root cap (Development)

-Cells closest to the root meristem are meristematic, undergoing cell division with transverse walls to form files of cells that are pushed forward -As cells develop they develop starch grains and ER becomes displaced to the forward end of the cell, they detect gravity b/c of the starch grains settle to the lower end of the cell -As cells reach the edge of the [term] the ER becomes less conspicuous, the starch grains are digested, the the dictyosomes release lots of mucigel by exocytosis -4-5 days from cell formation to cell death at the cap

Passage cells

-Cells within the endodermis of mature roots that have only casparian strips because at one point we thought they let minerals diffuse into the xylem -We now think they are just slow to develop

Branch roots

-Come off the single large taproot in many plants

Lateral roots

-Come off the single large taproot in many plants

Mucigel

-Complex polysaccharide secreted by cells in the root cap that lubricates passage of the root through the soil -Also causes soil to release nutrient ions and permits the ions to diffuse more rapidly toward the root -Rich in carbohydrates and amino acids, which allow rapid growth of soil bacteria around the root tip, which is also believed to help release nutrients from the soil matrix.

Adventitous roots

-Don't arise from pre-existing roots -Increase the absorptive and transport capacity of the root system -Important for plants that reproduce vegetative via stolons

Radicle

-Embryonic root that develops into the taproot -Present in the seed, after germination usually grows and becomes largest root in the system

Root hairs

-Extend out as narrow trichomes from the root hair zone -Only occur in parts of the root that aren't elongating, otherwise they'd be shorn off -Greatly increase the root's surface area, and also are small enough to fit into small pores in the soil that a root could not -Also CO2 released by them reacts with soil water to make carbonic acid, which releases ions from the soil -Unicellular, never have thick walls, extremely transitory, die and regenerate in 4-5 days

Zone of elongation

-Few mm long behind the root apical meristem and root cap -Cells are actively dividing and expanding -Behind it is the root hair zone

Mature roots

-In mature roots the endodermis matures to make a watertight sheath (using suberin and lignin) around the vascular tissue -This sheath allows root pressure to build -In many plants the cortex is discarded after the root hairs die, making the endodermis the outer surface of the root until bark can be developed (mostly in perennial plants)

Root (Structure)

-Individual fairly simple, no leaves or leaf scars therefore no leaf axils or axillary buds -Grow from the tip at apical meristems -Apical meristem protected by thick layer of cells called root cap that secretes mucigel -Behind root cap and apical meristem is a zone of elongation (few mm long) where cells divide and expand -Behind zone of elongation is root hair zone where epidermal cells extend out as narrow trichomes -Root hairs form only in a part of the root that isn't elongating, otherwise they'd be shorn off -Behind root hair region is a zone where new lateral roots emerge, can occur in rows or seemingly random (often depends on soil microenvironment)

Endodermis

-Innermost layer of cortical cells differentiate into this cylinder -Tangential walls, closest to vascular tissue and cortex is thin primary walls, however radial (up, down, side to side) is encrusted with lignin and suberin to waterproof them

Pericycle

-Irregular region of parenchyma cells between the vascular tissue and the endodermis -Give rise to lateral roots

Quiescent center

-Mitotically inactive center of root apical meristems -Believed to act as a reserve of healthy cells, as being mitotically inactive makes them more resistant to radiation and toxic chemicals -If root apical meristem or root cap is damaged these cells become active to form a new apical meristem

Root apical meristem (Development)

-More orderly than the shoot b/c it experiences no disruptions because of leaf primordia, leaf traces, or axillary buds -Have mitotically inactive center called quiescent center

Aerial roots (orchids)

-Orchids are epiphytic, attached to the branches of trees -Roots spread along the surface of the bark and often dangle freely -Orchids, which live in rainforests, are adapted to drought conditions to prevent drying out in the few hours that it does not rain -Root's epidermis (called velamen in these orchids) is made of a few layers of large dead cells that are white, this acts as waterproof barrier

Lateral roots (Development)

-Originate by cell divisions in the pericycle, some cells become more densely cytoplasmic with smaller vacuoles and resume mitotic activity -Localized to just a few cells, making root primordium that organizes itself into a root apical meristem and pushes outward, this ultimately breaks the endodermis

Fibrous root system

-Present in most monocots and some eudicots, it is a mass of many similarly sized roots -Arises b/c the radicle dies during or immediately after germination -Arise from root primordia at the base of the radicle and go on to form more, roots from the second round of primordia are adventitous roots b/c they did not arise from pre-existing roots

Root apical meristem

-Protected by a thick layer of cells referred to as the root cap

Storage roots

-Provide long-term storage of carbohydrates accumulated during summer photosynthesis -Preferred for energy storage because they are below ground so less available as food than aerial stems, also less change in temperature and humidity

Root hair zone (Zone of maturation)

-Region behind the zone of elongation in which many epidermal zones branch off as narrow trichomes (root hairs)

Root pressure

-Results from the continued maturation of endodermis into a watertight sheath around vascular tissues, but water is still absorbed in the root hair zone, building up this water pressure

Zone of maturation

-Root hairs grow outward here, increasing absorption of water and minerals -In some a thin cuticle appears on root epidermal cells (could just be a layer of fats) -merges gradually with zone of elongation -Cortical cells often as aerenchyma to allow diffusion of oxygen -Have endodermis around vascular tissues that form casparian strips, these allow selective entry into the now fully differentiated metaxylem and metaphloem -Metaxylem and metaphloem arranged differently in roots, instead of collateral bundles the xylem makes a solid mass in the center surrounded by strands of phloem (Exception, some monocots have strands of xylem and phloem distributed in ground tissue). In the xylem metaxylem are on the outside and protoxylem in the middle (# protoxylem strands=# of phloem strands) -In phloem strands protophloem is on the outside and metaphloem on the inside -Pericycle forms between vascular tissue and endodermis, which give rise to lateral roots -No pith

Localized growth

-Roots and stems elongate only in small meristematic regions

Buttress roots

-Roots of certain tropical trees that become tall and plate-like -Brace the trunk from being blown over by the wind

Contractile roots

-Roots that, after extending through the soil and becoming firmly anchored, the uppermost portion slowly contracts and so the stem is pulled downward (how bulbs get underground)

Roots

-Shaped to maximize surface to volume ratio for max. absorption (many small cylinders), shape also allows it to push through soil 3 functions: 1. Anchoring the plant to a substrate 2. Absorbing water and minerals 3. Producing hormones

Zone of elongation

-Similar to the shoot's subapical meristem -Cell's begin to differentiate into visible pattern, none of the cells mature -Outermost cells are protoderm, center is provascular tissue -Tissues are all quite permeable, minerals penetrate deep into the root through apoplast by diffusing along the thin, fully hydrated young walls and intercellular spaces. Little actual absorption works here though, much of the nutrients is for the root's own growth

Taproots

-Site of carbohydrate storage in many plants -Most plants have a single large one of these, with many small lateral roots and branch roots coming off it

Arbuscule

-The fungal structure within cells that have an endomycorrhizal relationship -Fills with granules of phosphorus that later disappears as the phosphorus is absorbed by the plant

Root cap

-Thick layer of cells that protect the root apical meristem, similar idea to bud scales or young leaves -Must be constantly replenished in order to remain useful, as the soil constantly wears through it as the root pushes through the soil -Cells secrete complex polysaccharide called mucigel that lubricates passage of root through soil

Symplastic transport

-Transfer of minerals from the epidermis to the vascular tissue by absorption into cytoplasm of a cortical cell and transferring cell to cell, likely through plasmodesmata

Apoplastic transport

-Transfer of minerals from the epidermis to the vascular tissue by diffusion through the walls and intercellular spaces

Endomycorrhizal relationship

-Type of symbiotic mycorrhizae between fungi and plants -Hyphae penetrate root cortex as far as the endodermis -Hyphae penetrate through walls of the cortex cells but not the casparian strip. They invade these cells but don't break the plasma membrane or tonoplast

Ectomycorrhizal relationship

-Type of symbiotic mycorrhizae between fungi and seed plants -Hyphae of fungi penetrate between outermost root cortex cells but don't invade the cells themselves -Present in nearly all woody forest plants -Left in photo