Special Roots

Food-storage

• Roots that have special parenchyma cells for storing starches

• Sometimes called root tubers, and function similarly to stem tubers (see Session 5 Stems)

• Root tubers are modified lateral roots 

• Plants are usually biennial in nature producing, leaves and stems the first year, and then flowering the second year.

• Examples include sweet potato (Ipomoea batatas), Cassava (Manihot esculenta), Sagittaria, and Dahlia

What is the difference between a sweet potato and a yam?

• • Sweet potatoes are food-storage roots

  • Yams are food-storage stems

Water-storage

• Roots with abundant parenchyma cells modified for storing large amounts of water

• Common in arid areas

• Present in the gourd family (Cucurbitaceae)

Contractile Roots

• Roots that pull the plant down into the soil over time

• Found in monocot bulbs and corms, and some eudicot tap roots

• Parenchyma cells in the proximal portion of root expand radially and shorten longitudinally

• Common in bulbs that use stable ground temperature as protection from frost

• Present in the lily family (Liliaceae), as well as dandelions (Taraxacum officinale)

• Add'l Reading: Bluebells, Contractile Roots, and Mycorrhizae

Mycorrhizal roots

• Specialized roots that form a mutualistic relationship with fungi

• The plant makes / shares sugars with their fungal symbiont 

• The fungus absorbs / shares water and minerals with their plant symbiont 

  • This is considered a mutualistic symbiosis

• Most (¾) of all seed plants have an association with mycorrhizae

• Few roots hairs grow; fungus is more efficient at absorbing water / minerals

• Endomycorrhizae: fungus lives within the cells of the plant's root cortex

• Ectomycorrhizae: fungus surrounds roots, and invades epidermal layers and root hairs

• The co-evolution of plants and mycorrhizae may be as old as when plants evolved onto land (Brundrett 2002)

Root nodules

• Structures on plant roots that contain nitrogen-fixing bacteria (i.e. rhizobia)

• Rhizobial bacteria are able to uptake N2 gas from the atmosphere and convert it into a solid, usable form (e.g. ammonia, nitrates, nitrites)

• These are mainly found in the legume family (Fabaceae), such as peanuts, alfalfa, and clover.

• A different group of bacteria, the cyanobacteria, are also able to fix nitrogen but do not form root nodules, but can form symbiotic relationships with other plant groups (e.g. cycads, Azolla, hornworts)

  • Cycads have a similar mechanism, in which they create coralloid roots that house cyanobacteria which provide the plant with extra nitrogen

  • Alders (Alnus) also create nitrogen-fixing nodules, but this association is with actinomycete bacteria

Aerial roots

• Roots that appear above the soil surface

Adventitious roots

• Roots that originate from a plant organ other than the roots (i.e. radicle)

• Usually grow from the stem, such as a rhizome or aerial stem

• There are a few examples from leaves, especially in leaf succulents

Parasitic roots

• Adventitious (mostly aerial) roots that “tap into” the stems of a photosynthetic plant to take sugars and water

• Plants use haustoria (peg-like projections) to take nutrients

  • Some parasitic plants are rooted in the soil and connect to the host plant underground

• Found in holoparasitic plants which have completely lost the ability to photosynthesize and obtain all sugars from parasitism

  • e.g. Dodder (Cuscuta), Beech drops (Epifagus americana), Bear corn (Conopholis americana)

  • Parasitic dodder can communicate with host plant (Kim et al. 2014)

  • Lophophytum can steal mitochondrial genes (Smith 2017)

• Also found in hemiparasitic plants, which retain photosynthetic capacities as well as parasitizing other plants

  • e.g. Mistletoes (Phoradendron)

Buttress roots

• Adventitious roots that appear as large, wall-like projections from the trunk 

• Provide stability to tall, water-logged trees (e.g. kapok, fig)

• Some evidence suggests that they also shuttle oxygen to submerged roots for respiration

Root Suckers

• New upright shoots being produced from the root stock

• Many trees including apples (Malus), pears (Pyrus), aspens (Populus), and tree-of-heaven (Ailanthus) are known for suckering

• Earth's largest and heaviest, and probably oldest organism was created by root suckering

  • "Pando", a vast group of quaking aspen trees (Populus tremuloides), growing in Fishlake National Forest, south of Salt Lake City, UT

  • The organism comprises about 47,000 tree trunks, all connected underground as root offshoots, making this one large cloned organism.

  • Researchers have estimated that Pando originated between 80,000 to one million years ago.

  • Google Map of Pando's location

Pneumatophores

• Aerial roots that shuttle oxygen to water-logged roots for respiration

• Roots grow away from gravity, known as negatively gravitropic growth

• Found in black mangrove (Avicennia nitida), white mangrove (Laguncularia racemosa), and water weed (Ludwigia repens)

• N.b. there is no evidence that the "knees" of Taxodium (bald cypress) function as pneumatophores

Photosynthetic Roots

• Roots that grow above the ground (i.e. aerial), are green, and conduct photosynthesis 

• Photosynthetic roots are frequently found in epiphytic, tropical orchids

• Velamen: tissue prevents water-loss in roots (e.g. orchids)