Roots

• plant roots evolved independently across several major clades durin the Devonian Period (416-360 mya)

  • oldest evidence comes from clubmosses and their closet relatives the exinct zosterophylls

Development

• radicle - embryonic root (first to emerge during seed germination)

  • features apical meristem

    • a.k.a root apical meristem (RAM)

• primary root forms from the radicle

  • may develop as tap roots or fibrous roots

• hypocotyl is a part of the stem where it transitions into a root

Tap Roots

• persistent (retained throughout life)

• act as a central axis for branch/lateral/secondary roots

  • grow in response to:

    • moisture and nutrient availbility

    • favorable soil structure

      • particle aggregation

      • pore space for gas exchange and moisture retention

  • tertiary roots emerge from the secondary roots

• provide strong anchorage and leverage to resist uproooting and hold the plant upright

Fibrous Roots

• start as tap roots

  • primary roots slow or stop growing

• roots start to form from underground shoot tissues (adventitous roots)

  • basal roots begin at the base of the stem and give rise to hypocotyl roots

    • stay close to the soil surface

  • rhizomes (stems) give rise to both shoot tissues and adventitious roots at their nodes

Root Apical Meristems (RAMs)

• RAMs are primary root tissues that give rise to:

  • protoderm develops into the root epidermis

  • ground meristem develps into the cortx

  • procambium develops into the cambium

    • vascular tissues arise from the cambium

Root Morphology

• stele - central part of root that houses vascular tissues

  • houses xylem and phloem

    • arranged in an X-shape in dicots

    • arranged in a ring around a central pith in monocots

  • pericycle - outermost part of the stele

    • gives rise to branch/lateral/secondary roots

      • cued by lipocalin (protein)

    • helps form the vascular cambium in plants that produce woody roots

    • some produce alkaloids

    • not always present

• cortex surrounds the stele

  • endodermis = innermost cell layer of the cortex and protective layer around the stele

    • contains the casparian strip,

      • waxy barrier

      • controls movement of water and minerals

• epidermis on the outside

• root tip epidermal cells secrete mucigel

  • polysaccharide secreted by the golgi apparatus

  • lubricates, prevents drying, and creates a favorable microbial environment

Moving In

• how do we get water and minerals into the roots?

• root hairs develop by stretching out epidermal cell wall and filling the space with cytoplasm

  • cell membranes become embedded with transport proteins to facilitate active transport of mineral (requires ATP)

  • water can pass through the cell membrane

    • aquaporins may get incorporated into cell membranes to facilitate faster water absorption

• symplastic route - epidermal root hairs uptake into the cytoplasm and pass through plasmodesmata

  • minerals dissolved in water typically enter this way

• apoplastic route - water moves through spaces between cells and within the cell walls

• transmembrane route - water crosses plasma membranes (enters and exts each cell) → both symplast and apoplast used

• all routes forced to enter the cytoplasm of endodermal cells

  • casparian strip filters apoplast before enter the stele

• eventually enter the xylem

• osmotic pressure created by trapped solutes in te casparian strip creates root pressure in some plants, which can push water into the vascular cylinger and up

  • not all plants develop root pressure

  • root pressure is higher in spring, but water moves more rapidly through the xylem in summer (due to transpiration)

• transpiration (water exiting leaves) is typically the driving force for moving water upward

Root Hormone Production

• roots produce cytokinins to promotes cell division and abscisic acid to inhibit cell division and cell expansion

• ethylene regulates root hair development and elongation

• jasmonis acid produced during development, espically in the pericycle for lateral root develoopment

• strigolactones help with root hair growth, especially in nutrient-deficient conditions

  • also function in signaling to symbiotic fungi

  • detected by some parasitic plants and used as a cue to germinate

Secondary Compound Production

• alkaloids:

  • nicotine protects from insects and herbivores

  • berberine protects from microbes/ fungi and is an antioxidant

    • used in medicine to improve insulin sensitivity, lower cholestrol, and improve gut health

  • reserpine protects from insects/pathogens (including parasitic nematodes) and helps acclimate during stress

    • used to treat high blood pressure and some psychiatric conditions

• phenolics:

  • flavonoids and tannins often released as defensive exudates

  • other aid in nutrient uptake or allepathy (inhibiting the growth of other plants by making soil unfavorable for germination or establishment)

• terpenoids:

  • saponins protect against pathogens, insectsm and herbivores

    • used to lower cholesterol, fight tumors, and calm inflammation

Storage Roots

• biennial plants store food in their roots for survival

• these develop through secondary growth

  • mediated by the cambium

  • xylem lacks woody cell and is comprised primarily of xylem parenchyma cells that contain strach granules

Roots That Hold Plants Up

• prop roots/brace roots - extend from vertically from the stem into the soil to provide extra mechanical support and increased absorption capacity

  • e.g. corn, figs, and banyan trees

• stilt roots - extend horizontally from the basal node at the maiin stem and may grow obliquely

  • e.g. mangroves

• buttress roots - wall-like roots that help support large trees in tropical regions with shallow soils

Mangroves

• pneumatorphones are aerial roots specialized for gas exchange

  • grow upward from primary root systems of a few plants

  • contain aerenchyma - cortex tissues containing air spaces

    • allows for ixygen-containing air to diffuse throughout the root

Epiphytic Plants

• epiphytes are plants that grow on other plants or objects for physical support

  • e.g. various orchids, bromeliads, some ferns and bryophytes

• not parasitic

• get water from rain and water vapor in the air

• minerals collected from debris on the supporting plant

Parasitic Plants

• parasitic plants penetrate other plants to steal their water and nutrients

• have specialized roots called haustoria to facilitate ths

  • form a vascular union with host to redirect nutrients

    • union formed via plasmodesmata

• some are still photosynthetic

  • e.g. mistletoe

• some are completely parasitic

  • little to no photosynthesis

  • e.g. dodder/strangleweed

Microbial Associations

• mycorrhizal fungi can help with absorption

• nitrogen-fixing bacteria help some plants

  • can be housed in root nodules made from modified lateral roots

    • e.g. leumes