THE ROLE OF MYCORRHIZAE AND ROOT EXUDATES IN

PLANT NUTRITION.

AaaAAsThe

fungus-root symbiotic

relationship is called

“Mycorrhizae”

Mycorrhizae:

• In mycorrhizal SYMBIOSIS fungus provides to the host plant with nutrients, such as phosphate and nitrogen,and increases the abiotic (drought, salinity, heavy metals) and biotic (root pathogens) stress resistance of the host.

• In return,it gets photosynthetically fixed carbon from the host.

Root exudates;

•Growing roots release an appreciable amount of organic carbon in to rhizosphere.such as sugars and organic acids.

TYPES OF MYCORRHIZA;

1. ECTO MYCORRHIZA; The fungus forms a sheath around the rootlets and also enters roots,forming in the outer cortex,and also form as an inter cellular net of hyphae.(harting net)

2. ENDO MYCORRHIZA; the fungus forms finely devided haustorical branches inside the cells of root cortex.

Arbuscular mycorrhizal interactions; Arbuscular mycorrhizas are the most common form of mycorrhizal interactions. They are formed by a wide variety of host plants (approximately 65% of all known land plant species) including many agricultural important crop species, such as soybean, corn, rice, and wheat.

• . Structural characteristics of arbuscular mycorrhizae

 • AM fungi are obligate biotrophs and rely on their autotrophic host to

complete their life cycle and to produce the next generation of spores. The spores are able to germinate without the presence of a host, but the spores respond with an increase in hyphal branching and metabolic activity to root exudates.

• AM fungi are coenocytic hyphae

• The polymorphic nature of these nuclei and the relatively large genome of these fungi has made genome sequencing , but recently the first transcriptome of the AM fungus Glomus intraradices became available.

• They are asexual, but an exchange of genetic material between closely related fungi via anastomosis has been observed.

 

• On the host root surface, AM fungi form a specific appressorium – the hyphopodium.

• Fungal hyphae emerging from this hyphopodium penetrate into the root through the prepenetration apparatus, which guides the fungal hyphae through the root cells toward the cortex.

In the cortex the hyphae enter the apoplast, and grow laterally along the root axis, and penetrate into inner root cortical cells.

The fungus does not enter the plant symplast and is excluded from the host cytoplasm by the enlarged periarbuscular membrane (PAM) of the host. Some fungi also form vesicles, fungal storage organs in the root apoplast.

• The mycelium that is formed within the root, the intraradical mycelium (IRM) differs morphologically and functionally from the extraradical mycelium (ERM), the mycelium that grows into the soil.

• The ERM absorbs nutrients from the soil and transfers these nutrients to the host root.

• The IRM on the other hand releases nutrients into the interfacial apoplast and exchanges them against carbon from the host. 

•Mycorrhizal plants can take up nutrients from the soil via two pathways: the ‘plant pathway’ that involves the direct uptake of nutrients from the soil by the root epidermis and its root hairs.

• the ‘mycorrhizal pathway’ that involves the uptake of nutrients via the ERM of the fungus and the transport to the Hartig net in ECM interactions or to the IRM in AM interactions, and the uptake by the plant from the interfacial apoplast.

. Nutrient uptake pathways in arbuscular mycorrhizal or ectomycorrhizal roots

ROLE OF MYCORRHIZAE IN PLANT NUTRITION:

• MYCORRHIZAE absorbs P and N from the soil by ERM and it transports to the IRM.

• From IRM the nutrients transported to host plant. • There by the plants utilise these nutrients.• In return the plants supplies carbon source to the fungi.

• ERM mycelium absorbs the N in inorganic form NH4+ NO3

-

• ERM absorbs P as ortho phosphate.

• These inorganic nutrients converted into organic nutriens in the fungal hyphae.

• The plant takes nutrients in the organic form.

• CONCLUSION:

• Comparing with non-mycorrhizal plants,mycorrhizal plants shows more biomass.• Beacause the availability of nutrients increases at root zone.

• Helps in improved plant growth and developments, and enhanced plant tolerance to several diseases.

Root exudates

• Plant roots continuously exude compounds into the rhizosphere;

• this includes exudation or secretion of ions, free oxygen and water,enzymes, mucilage and a diverse array of carbon containing primary and secondary metabolites

Root exudates are often divided into two different types of compounds:

1. Low molecular weight compounds such as amino acids, organic acids, sugars, phenolics and other secondary metabolites which account for much of the diversity of root exudates (ROUGIER, 1981).

2. High molecular weight exudates such as mucilage (polysaccharides)and proteins are less diverse but often comprise to a large proportion of the root exudates

Enhancement of nutrient supply by root exudate effects on symbiotic microbes • The components of plant root exudates are complex and serve not

only as a source of carbon substrate for microbial growth, but also chemical molecules that promote chemotaxis of soil microbes to the rhizosphere.

• the root exudates of N2-fixing legumes are known generally for their

capacity to attract micro-organisms into the rhizosphere. Besides their direct effects on rhizobia, root exudates can also attract pathogenic microbes and promote the growth of plants.

Conclusions

• Through diverse mechanisms, root exudates play a fundamental role

in the mineral nutrition of plants. They either contain signals that act as regulators of microbial growth and function, or they possess molecules which directly control the rhizosphere processes that enhance nutrient uptake and assimilation.

• Root excretion of inorganic ions (e.g. HCO3−, OH−, H+) is also important in the mineral nutrition of plants. Plant uptake of anions in

excess of cations often causes the roots to secrete HCO3− in order to maintain electrical neutrality, a process that leads to increased

rhizosphere pH. Conversely, the uptake of cations in excess of anions can cause roots to exude H+ and lower the rhizosphere pH. The change in pH with HCO3

− extrusion tends to increase nutrient sup-ply in acidic soils, as happens with H+ exudation in calcareous soils.• • H+ extrusion is largely governed by cation/anion balance and is thus very much influenced by the N source. Plants growing on nitrate

generally maintain electronic neutrality by releasing excess anions, in-cluding OH−, which cause an increase in rhizosphere pH and an enhanced nutrient (P, Mo, etc.) availabil-ity in acidic soils (Marschner, 1995). An exception is the study by Gahoonia et al. (1992) which showed no effect on P mobilization in a luvisol soil following nitrate-induced increase in rhizosphere pH. However, when ammonium is supplied, there is a large excess of cations and a resulting enhanced H+ extrusion and rhizosphere acidification, thereby making P, Mn, Fe, Cu and Zn more toxic in the acidic range, or readily available in the alkaline range (Aguilar and Van Diest, 1981; Gahoonia et al., 1992; Gardner et al., 1983; Gillespie and Pope, 1990; Runge and Rode, 1991). Sometimes, however, acidification from ammonium nutrition does not result in increased P mobilization, especially in acidic oxisols (Gahoonia et al., 1992). Furthermore, H+ extrusion occurs during N2 fixation by symbiotic legumes (Raven et al., 1990), and this can lead to rhizosphere acidification and increased availability of limiting nutrient elements like P, Mo and Fe (Aguilar and Van Diest, 1981; Gahoonia et al., 1992; Gillespie and Pope, 1990; Runge and Rode, 1991) which are much needed in diazotrophy. Ad-ditionally, there are many reports of enhanced H+ extrusion under Fe defiency and P deficiency, both leading to acidification of localized areas around the root tips (Bienfait, 1985, 1988; Gardner et al., 1983; Hoffland et al., 1989; Römheld and Marschner, 1986) and a consequent improvement in the availability of

• Root exudate effects on rhizosphere pH and nutrient availability• • Another nutritional effect that organic acids have in root exudates is acidification of the rhizosphere.

Root exudation of high concentrations of organic acid anions as a result of P deficiency does lower rhizosphere pH, making P and micronutrients such as Mn, Fe and Zn to be more available in cal-careous soils.

• However, the relationship between organic acid exudation and rhizo-sphere acidification is not that simple as the extrusion of H+ would depend on the amounts of anions ab-sorbed by roots relative to cations. Whatever the case, acidific-ation below pH 5.5 can cause even major macronutri-ents to become limiting. Because micronutrients such as Mn, Fe and Al occur in high concentrations below pH 5.5 any further acidification

• by organic acids below this level can result in phyto-toxic effects on plant roots and beneficial

microbes. Intriguingly, the white lupin can mobilize P from both acid and alkaline soils by using citric acid in its pro-teoid root exudates to acidify even the alkaline soil, and thus solubilize P as well as Fe, Mn, Cu and Zn for uptake by roots.