arbuscular, ecto, ericoid mycorrhiza, mycoheterotrophy (lecture 2)

Question 1

what is the classical view of mycorrhizal symbioses?

Answer 1

• plants photosynthesise, fix C and translocate it to their root systems where C is shared with fungal partner
• fungus possesses different suite of chemicals and hyphae much finer than plant roots, enabling access to pools of nutrients in soil inaccessible to plants and increase area over which plants can gather nutrients
• formed by 80% extant plants, probably around same of all ever
• important and ancient, possibly facilitated move onto land

Question 2

what are arbuscular mycorrhiza?

Answer 2

• fungi that penetrate plant cells with arbuscules (“little trees”).
• 3D, finely branched structures ensheathed in membrane with huge surface area within cell
• exchanges nutrients with plant through interfacial apoplast via embedded transporter proteins

Question 3

what is the history of arbuscular mycorrhiza?

Answer 3

• evolved 420-480 million years ago, 45-90MY prior to roots, originally directly penetrating photosynthetic tissue of non-vascular bryophytes
• struck up relationship immediately upon move to land or it existed before & helped facilitate the move
• essential in formation of primeval soils along with the bacteria they associated with
• formed arbuscular mycorrhizal symbioses with ancient bryophytes: liverworts & hornworts. probably mosses too but modern mosses have lost the ability

Question 4

why don’t plants/fungi cheat by pumping only out of apoplast?

Answer 4

functional reasons:

• mycorrhiza obligate symbionts, cannot live freely
• plants often cannot get nutrients they need without fungus

physiological:

• (kiers et al, 2011)
• plants allocated more C to cooperative fungal strains which were supplying more P
• flipped: both aggressive and cooperative fungi allocated more P when recieving more sugar
• BIOLOGICAL MARKETPLACE HYPOTHESIS OF RECIPROCAL EXCHANGE

Question 5

What did Walder and van der Heijden (2015) find?

Answer 5

• Kiers (2011) only works when one fungus in root
• v highly responsive mycorrhizal plants reciprocally exchange C & nutrients with fungi
• most (partially & non responsive) plants no reciprocity… SOURCE SINK DYNAMICS
• physical imperative that more C must flow to uptake more resource, less for less

Question 6

what are mycorrhizal networks?

Answer 6

• plants connected to dozens of A.M fungus, and fungus to plants
• common mycelial networks
• increases competitive ability of plants in CMN as greater access to nutrients and non CMN plants experience antagonism
• improved plant defence
• e.g. against eyespot disease

Question 7

how to common mycelial networks affect non-mycorrhizal plants?

Answer 7

• arbuscular mycorrhizal antagonism: metabolic toxin released by fungus inhibits root hair production on non-mycorrhizal plants
• usually species w ruderal ecologies. pioneers, invaders… e.g. cruciferae, the cabbage fam
• rinaudo (2010)
• non-mycorrhizal weed biomass suppressed by mycorrhiza

Question 8

what are ectomycorrhizas?

Answer 8

• basidiomycota, more recent or “higher” fungi
• associate with trees and large woody shrubs as need LOTS of C
• in soil form thick cordlike foraging structures, branch into finer hyphae

Question 9

how do ectomycorrhizas associate with plants?

Answer 9

• cause plant root tips to bifocate and ensheathe them so reliant on fungus
• grows BETWEEN cells to form hartig net, a proliferation of hyphae between cortex and the site of exchange
• between cells but don’t penetrate hence ECTO

Question 10

what are ericoid mycorrhiza?

Answer 10

• the ascomycota, more recent “higher” fungi
• coils and vesicles that form in host cells
• e.g vaccinium and heathers

Question 11

what are mycoheterotrophs?

Answer 11

• at least 400 plant spp evolved to cheat mutualism
• derive all C and nutrients from parasitising, give nothing
• lack chlorophyll
• do not form haustorial connections to other plants, gain nourishment through parasitising fungi (Leake, 1995)

Question 12

how do mycoheterotrophs derive nutrients?

Answer 12

mycoheterotrophy (rare): parasitise saprophytic fungi that get C from organic matter

epiparasite (more common): a kind of mycoheterotroph, indirectly gains C from other plants by parasitising fungi in the common mycelial network

• tripartite symbiosis
• all epiparasites are mycoheterotrophs, not all (but most) mycoheterotrophs are epiparasites

Question 13

how do we know plants are mycoheterotrophs and not saprophytic?

Answer 13

• mckendrick et al (2000)

- showed flow of C from autotrophic plant through mycorrhiza to epiparasitic orchids

Question 14

when did mycoheterotrophy evolve?

Answer 14

• independently evolved a number of times across taxonomically diverse range of plants
• liverworts e.g. ghostwort
• 5 times in dicots
• vast majority of diversity in 3 families of monocots: triuridaceae, corsiaceae, orchidaceae
• stark contrast bw monocot mycoheterotroph abundance & dicot-exclusive haustorial parasitism (Kuijt, 1969)

Question 15

what are some visual features of mycoheterotrophs? why?

Answer 15

-all small, lack leaves, essentially just flowering stems

structural simplification to prevent wasting energy on unneeded structures

• leaves reduced to scales or absent
• reduced roots
• minute seeds dependent on symbiotic germination (orchids and gentians)
• fungus triggered germination (orchids)
• little or no chlorophyll (all epiparasites_