Root Exudates

1. Root Exudates and their
Role in Nutrition
PABOLU TEJASREE
M.Sc 1 st year
BAM-20-27
Dept of GPBR
Agricultural College , Bapatla

2. LearningObjectives
* Root exudates
* Types of root exudates
* Detection
* Transport
* Functions

3. Rate of exudates increases by the presence of microbes in
the rhizosphere becoz soil microbes release compounds
such as 2,4-diacetylphloroglucinaol & Zearalenone that
stimulate root exudation of primary metabolites ex:-
Amino acids

4. ⃰ Carbohydrates : Arabinose, glucose, galactose, fructose, sucrose, pentose, rhamnose,
raffinose, ribose, xylose and mannitol
⃰ Amino acids : All 20 proteinogenic amino acids, l-hydroxyproline, homoserine, mugineic
acid, aminobutyric acid
⃰ Organic acids : Acetic acid, succinic acid, l-aspartic acid, malic acid, l-glutamic acid,
salicylic acid, shikimic acid, isocitric acid , caffeic acid, p-hydroxybenzoic acid , tartaric
acid, protocatacheuic acid, p-coumaric acid, mugineic acid, oxalic acid, citric acid
⃰ Flavonols : Naringenin, kaempferol, quercitin, myricetin, naringin, rutin, genistein,
strigolactone and their substitutes with sugars
⃰ Lignins : Catechol, benzoic acid, nicotinic acid, phloroglucinol, cinnamic acid, gallic acid,
syringic acid, sinapoyl aldehyde, chlorogenic acid, coumaric acid , sinapyl alcohol .

5. ⃰Coumarins : Umbelliferone
⃰Aurones : Benzyl aurones synapates, sinapoyl choline
⃰Glucosinolates:Cyclobrassinone,desuphoguconapin,desulphoprogoitrin,,desulphoglucoalyssin
⃰Anthocyanins : Cyanidin, delphinidin, pelargonidin
⃰Indole compounds :Indole-3-acetic acid, brassitin, sinalexin, brassilexin, methyl indole
carboxylate, camalexin glucoside
⃰Fatty acids : Linoleic acid, oleic acid, palmitic acid, stearic acid
⃰Sterols : Campestrol, sitosterol, stigmasterol
⃰Allomones: Jugulone, sorgoleone, 5′-dimethoxyflavone, DIMBOA, DIBOA
⃰Proteins and enzymes : PR proteins, lectins, proteases, acid phosphatases, peroxidases,
hydrolases, lipase
List of components presented in this table is reported mainly from the model plant Arabidopsis (see Narasimhan et al. 2003)

7. COMPONENT RHIZOSPHERE FUNCTION
PHENOLICS
Chemoattractant signals to microbes
Nod gene inducers in rhizobia
Detoxifiers of Al
Phytoalexins against soil pathogens
ORGANIC ACIDS
Chemoattractant signals to microbes
Nod gene inducers in rhizobia
Chelators of poorly soluble mineral nutrients
Detoxifiers of Al
AMINOACIDS AND
PHYTOSIDEROPHORES
Chelators of poorly soluble mineral nutrients
Chemoattractant signals to microbes
VITAMINS Promoters of plant and microbial growth
ENZYMES Catalysts for P release from organic molecules
ROOT BORDER CELLS
Stimulate microbial growth
Release chemo attractants
Release mucilage and proteins
PURINES Nutrient source

8. Techniques to study chemistry of Root
Exudates
* Infrared Spectroscopy (IR)
* Solid State Nuclear Magnetic Resonance Spectroscopy
using the Carbon 13 isotope (C-13 SSNMR) and proton
(H-1) NMR
* Gas chromotography with Mass spectroscopy (GS/MS)
* Cine-photomicrography
* Cryo scanning electron and transmission electron
microscopy
* Patch clamp technique

9. Introduction
...
 Van Egeraat (1975) demonstrated that the root tips of the primary and lateral roots were sites of exudation by
spraying ninhydrin on the filter paper where the plant roots grew.
 Ninhydrin is a chemical compound that interacts specifically with amine groups to produce a purple colour.
 Using ninhydrin is still considered an important tool to identify the sites of exudation on roots, but the
limitation of using it is that it is able to detect only amino acids or ninhydrin-positive compounds in the
exudates.
 McDougall & Rovira (1970) used 14C-labelled compounds to identify the sites of exudation from wheat
roots, and noticed that diffusible material released from the whole length of roots
 Frenzel (1957, 1960) reported that different compounds were released from different parts of root system.
 Asparagine and threonine from the meristem and root elongation zone; glutamic acid, valine, leucine and
phenylalanine from root hair zone; and aspartic acid from the whole root.

10.  Generally, the apical meristem root tip wends its way through the soil . It has been proposed that these
sloughed-off cap cells play a significant of plant roots is covered by a group of cells arranged in layers called
the root cap, which sloughs off as the role in determining the rhizosphere ecology, and therefore the term
‘border cells’ was proposed (Hawes 1990).
 Border cells are involved in several functions: they decrease frictional resistance experienced by root tips ,
they regulate microbial interactions through avoidance of harmful microbes (pathogens) and favouring
associations with beneficial microbes (PGPR) and they protect against heavy metal toxicity such as aluminium
 A mucilaginous layer has been observed on the surface of roots, particularly at the root tip, where it can form a
droplet in the presence of water . This secretion may derive from the root cap or from the degradation of
epidermal cell walls

11. Mechanism of
uptake of
nutrients
Through
chelation and
desorption

13. TRANSPORT
Root exudation is a complex phenomenon encompassing processes that drive C transport to roots and
exudation from roots to soil. The long distance transport of C produced in source organs takes place in the
phloem, through the widely accepted Munch’s pressure-driven mechanism of phloem flow .
According to this mechanism, phloem metabolites are transported by a difference in turgor between sink
and source organs generated by concentration gradients, which are determined by source-sink
activities .
RELEASE OF PRIMARY METABOLITES AT THE ROOT TIP

15. Release Of Primary Metabolites At The Root Tip…
⃰ Phloem unloading occurs through plasmodesmata (combination of mass flow and diffusion). During
unloading, low-molecular-weight solutes and proteins are diverted into the phloem-pole pericycle, a tissue
connected to the protophloem by a unique class of “funnel plasmodesmata” . While proteins are
released in discrete pulses (referred to as “batch unloading”)
⃰ The discovery made by Ross-Elliott et al. (2017) is very important in connection to root exudation at the
root tip . The principal route for all solutes to be unloaded and that they will move toward the surrounding
cells through diffusion because of the high degree of plasmodesmatal connections in this area

16. ROOT STRUCTURE AND AREAS OF EXUDATION

17. Movement Of Primary Metabolites Outside
The Root Tip
⃰ While metabolites can move quite freely through the symplastic pathway, in
order to be excreted to the soil environment, they need to pass through at least one
plasma membrane to reach the apoplast .
⃰ Therefore, these molecules only transit the plasma membranes through specific
transmembrane proteins, which form small pores through the lipid bilayer,
allowing polar or charged molecules to cross the membrane without interacting
with the hydrophobic fatty acid chains of the membrane phospholipids

18.  Solutes move through symplast and apoplast , but when
are retaken up casparian strip limit the apoplast pathway.
cortex and epidermis are responsible for apoplast .
 Both symplast and apoplast pathways are used
becoz of lack of casparian strip
Differentiated region Undifferentiated region

19. Transport………..
 Traditionally, root exudation has been suspected to be a passive process mediated by diffusion, channels,
and vesicle transport.
 However, recent studies elucidated a pivotal role of tightly regulated primary and secondary active
transport processes across the root plasma membrane in the export and accumulation of defense
phytochemicals in the rhizosphere.
 Two protein families involved in mediating the transport of a wide array of organic substances, namely
multidrug and toxic compound extrusion (MATE) and ATP-binding cassette (ABC) transporters
 In the case of MATE transporter proteins, a subclade that can be found in all plants analyzed so far is
implicated in the release of citrate into the rhizosphere to confer aluminum resistance to plants.
 Recently, a MATE transporter in the stele of rice roots was found to facilitate efflux of phenolic
compounds into the xylem.

20.  ABC transporters are involved in diverse cellular processes, such as the excretion of potential toxic
compounds, lipid translocation, heavy metal tolerance, nutrient transport, salt stress and in disease
resistance
 Study reported that the secretion of genistein , a signal flavonoid involved in rhizobium symbiosis secreted
from soy bean roots, was mediated by an ABC transporter by an ATP-dependent manner, which was
demonstrated by using the specific ABC transporter inhibitor sodium orthovanadate
 Plant roots secrete citric, oxalic and malic acids to detoxify aluminium in the soils , and the secretions of
these organic acids are highly specific to aluminium stress.
 Phosphorus deficiency also results in enhanced root secretion of phenolic compounds in certain tree and
legume species, and the specificity of organic acid secretion in response to P deficiency varies with plant
species
 While P deficient in rape typically releases malic acid near its root tips or at sites in contact with insoluble
rock phosphate, P-starved hedge mustard does not secrete organic acids

21. – Catechin , a potential allelotoxin produced by spotted knapweed through increased secretion of oxalic
acid, which protects the roots against damage incurred by reactive oxygen species (ROS) resulting from
interactions with the allelochemical .
– Benzoxazinoids , found in the root exudates of maize attract plant-beneficial microbe interaction .
– Prescence of PGPR B. subtilis invokes abscisic acid and salicylic acid signaling pathways in A.thaliana ,
resulting in the closure of stomata and thus restrict the pathogen entry .
– Flavanoids present in the root exudates of legumes that activate Rhizobium meliloti genes responsible for
the nodulation process .
– High concentrations of Organic acids in root exudates lead to P def and this lowers the pH making Mn , Fe
and Zn to be more available in calcareous soil.
– As micronutrient def occur in high concentration below 5.5 pH can cause some major macronutrients to
become limiting . However , Organic acids are able to solubilize unavailable soil macronutrients – Ca .

22. Mechanisms of root exudation of compounds through the plant cell
membranes
Antimicrobial flavonoids
3-deoxyanthocyanidins
accumulate on sorghum
leaves at sites of fungal
infection

23. Exudation
mechanisms through
the plasma membrane
at the root tip
Top panel – active transport
mechanisms (primary or secondary ).
Against electrochemical gradient .
the bottom panel – passive transport
mechanism ( diffusion ) following
electrochemical gradient

24. Functions –Case studies
– N&P acquisition by root exudates
– Role of root exudates in plant to plant signalling
– The Influence of Root Exudates on Symbiotic Relationships in an
Intercropping System
– Role of root exudates in N cycle - Root Exudates as a Means To
Mitigate Agricultural Nitrogen Losses

26. Roles of root
exudates in plant-
to-plant signalling
 Allelopathic plants release allelochemicals to
inhibit the growth of neighbouring roots
(Asaduzzaman et al., 2016; Macías et al.,
2019).
 Neighbouring plants must then detect the
presence of allelopathic species and adjust root
placement to avoid these allelochemicals.
 If they do not alter root placement, their roots
should be inhibited by allelochemicals,
representing a benefit to the allelopathic plant
and a cost to the target.
 Accordingly, root-placement patterns may be
driven by both allelochemicals and signalling
chemicals released from interacting
neighbours' root exudates

27. * Root exudates that specifically inhibit
soil nitrification have been identified in
important crop species, including rice,
wheat, and sorghum, while others have
been shown to stimulate root
nodulation and N2 fixation, even in
neighboring plants.

28. Major N transformations in the soil - that are catalyzed by specific enzymes, including various nitrate
reductases (NAS, NAR, NAP, and EUK-NR), nitrite reductases (NIR, NRF), nitric oxide reductase (NOR), nitrous
oxide reductase (NOS), nitrogenase (NIF), ammonia monooxygenase (AMO), hydroxylamine oxidoreductase (HAO),
nitrite oxidoreductase (NXR), and hydrazine hydrolase (HH)
ROOT EXUDATES

29. Root Exudates as a
Means To Mitigate
Agricultural
Nitrogen Losses
Biological nitrification
inhibitors (BNIs) released
from root exudates
suppress nitrification via
AMO and HAO inhibition

30. Category Compound Source Comments
BNIs from
root exudates
Sorgoleone Sorghum bicolor Blocks AMO and HAO;
allelopathic compound
Sakuranetin Sorghum bicolor Blocks AMO and HAO;
noneffective BNI in soil assay;
phytoalexin
Methyl 3-(4-
hydroxyphenyl) Sorghum bicolor
propionate Blocks AMO; infuences root
system architect
Brachialactone Brachiaria humidicola Blocks AMO and HAO;
Reduces Field-level nitrication and
N2O emission
1,9-Decanediol Oryza sativa Blocks AMO; release correlated to NUE

31. The Influence of Root
Exudates on Symbiotic
Relationships in an
Intercropping System
 In a maize–faba bean intercropping
system, root exudates from maize
(e.g., flavonoids such as genistein)
can stimulate rhizobial Nod factors,
as well as nodulation and biological
N2 fixation (BNF) in faba bean roots,
thereby enhancing N nutrition,
biomass, and yield.
 In exchange, root exudates
containing fixed N (e.g., NH4 + ,
amino acids, etc.) can be transferred
from faba bean to maize, thus also
benefiting N nutrition, growth, and
yield of maize.

32. References
– Coskun, D., Britto, D.T., Shi, W and Kronzucker, H.J.2017. How plant root exudates
shape the nitrogen cycle. Trends in Plant Science. 22 (8): 661-673.
– Badri, D.V. and Vivanco, J.M., 2009. Regulation and function of root
exudates. Plant, cell & environment, 32(6), pp.666-681.
– Baetz, U. and Martinoia, E., 2014. Root exudates: the hidden part of plant
defense. Trends in plant science, 19(2), pp.90-98.
– Bais, H.P., Weir, T.L., Perry, L.G., Gilroy, S. and Vivanco, J.M., 2006. The role of
root exudates in rhizosphere interactions with plants and other organisms. Annu. Rev.
Plant Biol., 57, pp.233-266.
– Bais, H.P., Weir, T.L., Perry, L.G., Gilroy, S. and Vivanco, J.M., 2006. The role of
root exudates in rhizosphere interactions with plants and other organisms. Annu. Rev.
Plant Biol., 57, pp.233-266.

33. – Wang, N.Q., Kong, C.H., Wang, P. and Meiners, S.J., 2020. Root exudate signals
in plant–plant interactions. Plant, Cell & Environment.
– Dakora, F.D. and Phillips, D.A., 2002. Root exudates as mediators of mineral
acquisition in low-nutrient environments. Food security in nutrient-stressed
environments: Exploiting plants’genetic capabilities, pp.201-213.

34. PABOLU TEJASREE
BAM-20-27
M.Sc 1st Year
Dept of GPBR