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  • From: Lawrence London <lfljvenaura@gmail.com>
  • To: permaculture <permaculture@lists.ibiblio.org>
  • Subject: Re: [permaculture] red clover extract that stimulates vesicular arbuscular mycorrhizae - Google Search
  • Date: Sat, 23 Mar 2013 14:57:17 -0400

Attn: Alia -
<>

This seems to have to do with Alia's information about available phosphate
levels in soil affecting a
host plant's need to allow colonization of mycorrhizal fungi on its roots
for phosphate uptake;
i.e. phosphate is a regulator of this process.
The article quoted below goes into this and is fascinating:
The regulation of arbuscular mycorrhizal symbiosis by phosphate in pea
involves early and systemic signalling events
http://jxb.oxfordjournals.org/content/62/3/1049.full

<>
http://aem.asm.org/content/57/2/434

Isolation and Identification of Vesicular-Arbuscular Mycorrhiza-Stimulatory
Compounds from Clover (Trifolium repens) Roots

Muraleedharan G. Nair1*,
Gene R. Safir2 and
Jose O. Siqueira2†

+ Author Affiliations

1Bioactive Natural Products Laboratory, Department of Horticulture and
Pesticide Research Center, and Department of Botany and Plant Pathology, 2
Michigan State University, East Lansing, Michigan 48824

ABSTRACT

Two isoflavonoids isolated from clover roots grown under phosphate stress
were characterized as formononetin (7-hydroxy,4′-methoxy isoflavone) and
biochanin A (5,7-dihydroxy,4′-methoxy isoflavone). At 5 ppm, these
compounds stimulated hyphal growth in vitro and root colonization of an
undescribed vesicular-arbuscular mycorrhiza, a Glomus sp. (INVAM-112). The
permethylated products of the two compounds were inactive. These findings
suggest that the isoflavonoids studied may act as signal molecules in
vesicular-arbuscular mycorrhiza symbiosis.

Articles citing this article

==================

The regulation of arbuscular mycorrhizal symbiosis by phosphate in pea
involves early and systemic signalling events J Exp Bot January 2011 62:3
1049-1060

Abstract
Full Text
PDF

The regulation of arbuscular mycorrhizal symbiosis by phosphate in pea
involves early and systemic signalling events
http://jxb.oxfordjournals.org/content/62/3/1049.full

Abstract

Most plants form root symbioses with arbuscular mycorrhizal (AM) fungi,
which provide them with phosphate and other nutrients. High soil phosphate
levels are known to affect AM symbiosis negatively, but the underlying
mechanisms are not understood. This report describes experimental
conditions which triggered a novel mycorrhizal phenotype under high
phosphate supply: the interaction between pea and two different AM fungi
was almost completely abolished at a very early stage, prior to the
formation of hyphopodia. As demonstrated by split-root experiments,
down-regulation of AM symbiosis occurred at least partly in response to
plant-derived signals. Early signalling events were examined with a focus
on strigolactones, compounds which stimulate pre-symbiotic fungal growth
and metabolism. Strigolactones were also recently identified as novel plant
hormones contributing to the control of shoot branching. Root exudates of
plants grown under high phosphate lost their ability to stimulate AM fungi
and lacked strigolactones. In addition, a systemic down-regulation of
strigolactone release by high phosphate supply was demonstrated using
split-root systems. Nevertheless, supplementation with exogenous
strigolactones failed to restore root colonization under high phosphate.
This observation does not exclude a contribution of strigolactones to the
regulation of AM symbiosis by phosphate, but indicates that they are not
the only factor involved. Together, the results suggest the existence of
additional early signals that may control the differentiation of hyphopodia.
Key words

Phosphorus
arbuscular mycorrhiza
strigolactone
symbiosis
hyphopodium

Previous SectionNext Section
Introduction

Roots of the vast majority of plant species develop symbiotic associations
with arbuscular mycorrhizal (AM) soil fungi. Fungal hyphae develop in the
root cortex where they form intracellular highly branched structures called
arbuscules, and simultaneously in the soil where they form a dense mycelial
network. Within the root the plant supplies the fungus with hexoses, at a
cost of up to 20% of the carbon fixed by photosynthesis (Smith and Read,
2008). In return, it obtains water and minerals taken up from soil by the
mycelial network. The main benefit of the symbiosis for the plant is an
enhanced acquisition of phosphorus (P), a frequent limiting factor in plant
growth due to its poor solubility and mobility in soils.

Despite the importance of AM symbiosis, cellular and molecular events
underlying this interaction are only beginning to be unravelled (Parniske,
2008). Direct genetic screens to identify mycorrhizal (myc−) mutants are
extremely cumbersome. As a result, most myc− mutants in fact belong to a
subset of mutants initially isolated as deficient in nitrogen-fixing
symbiosis, this latter interaction being easier to examine. A consequence
of this bias is the relative scarcity of mutants affected in events unique
to the AM symbiosis, including pre-colonization signalling and arbuscule
development and function (Marsh and Schultze, 2001). Nonetheless, several
specific myc− mutants have been identified in the past few years. They can
be affected in different stages of the interaction as summarized in Pumplin
et al. (2009): pre-symbiotic fungal growth, formation of hyphopodia (root
attachment and penetration structures, formerly referred to as
appressoria), epidermal penetration, and arbuscule development (see also
Zhang et al., 2010).

Various physiological situations are known to affect the development of AM
symbiosis. For instance, plants control the extent to which AM fungi can
colonize their roots according to their own nutritional requirements. The
best known example of such regulations is the control of AM symbiosis
according to P availability. Roots can acquire P as inorganic
orthophosphate (Pi) through different pathways (Bucher, 2007). In certain
conditions the mycorrhizal uptake pathway, which involves specific Pi
transporters (Rausch et al., 2001; Harrison et al., 2002; Paszkowski et
al., 2002), can be the major route for P uptake (Smith et al., 2003). When
P is abundant, a direct, probably less costly uptake pathway is preferred
(Nagy et al., 2008), and a reduced root colonization by AM fungi is
observed. This down-regulation of the symbiosis by P has been known for a
long time (Graham et al., 1981; Thomson et al., 1986; Elias and Safir,
1987; Rausch et al., 2001; and many others). It seems to be a general
phenomenon, although its magnitude can vary (Javot et al. 2007; Smith and
Read, 2008). It has far-reaching consequences in natural ecosystems where
it modulates the effect of AM fungi on plant species diversity (Collins and
Foster, 2009), as well as in agriculture where strong P fertilization may
in the long term decrease the presence and richness of soil AM communities
(Johnson, 1993).

Little is known about mechanisms underlying the regulation of AM symbiosis
by P. A recent study (Branscheid et al., 2010) has documented this
down-regulation in Medicago truncatula, and investigated the identity of
the internal signal that triggers suppression of the interaction under high
P. Nonetheless, the downstream mechanisms that prevent or limit root
colonization by AM fungi remain largely unknown. Early studies led to
conflicting results and interpretations, partly due to the variety of
species combinations and experimental systems. Some of these early studies
interpreted the impact of high P on the fungus in terms of trophic effects:
high P would decrease the root secretion of metabolites used by the fungus,
such as amino acids or carbohydrates (e.g. Graham et al., 1981; Thomson et
al., 1986). An alternative proposition was that qualitative rather than
quantitative differences between root exudates of P-replete and P-deficient
plants could account for their differential effects on the fungus (Elias
and Safir, 1987). This led to the suggestion that P-deprived roots exuded
important flavonoid signals that triggered pre-symbiotic fungal growth and
activity (Nair et al., 1991). Advances made in the last 10 years have
indeed emphasized the importance of signalling events in mycorrhizal
interactions, and the recent identification of some signals may shed new
light on the regulation of AM symbiosis by P.

Plants and AM fungi are known to exchange molecular signals prior to
physical contact, at the so-called pre-symbiotic stage. Various lines of
evidence indicate that AM fungi produce diffusible compounds able to
modulate root gene expression (Kosuta et al., 2003; Weidmann et al., 2004),
intracellular signalling (Navazio et al., 2007; Kosuta et al., 2008),
development (Olah et al., 2005), and metabolism (Gutjahr et al., 2009).
Reciprocally, plant roots secrete compounds that stimulate the fungus
(Gianinazzi-Pearson et al., 1989; Siqueira et al., 1991; Tsai and Phillips,
1991; Giovannetti et al., 1996; Buée et al., 2000). A group of secondary
metabolites called strigolactones were identified as major contributors to
this effect (Akiyama et al., 2005; Besserer et al., 2006). Strigolactones
trigger morphological and developmental responses in the fungus such as
hyphal branching and spore germination, and enhance fungal mitochondrial
activity and respiration (Besserer et al., 2006, 2008).
Strigolactone-mediated signalling is necessary for a normal level of root
colonization, as demonstrated using strigolactone-deficient mutants
(Gomez-Roldan et al., 2008). Most interestingly, these root-exuded
compounds also play an important role in planta, acting as hormones that
contribute to the regulation of shoot branching (Gomez-Roldan et al., 2008;
Umehara et al., 2008).

Prior to the discovery of their roles in AM symbiosis and plant
development, strigolactones were known as germination stimulants for the
seeds of the parasitic plants Striga and Orobanche (Bouwmeester et al.,
2007). Damage caused to crops by these weeds is lower under strong nutrient
fertilization, which led to the investigation of whether P availability
influenced strigolactone release into the soil. Indeed, several studies
demonstrated a strong negative effect of high P supply on strigolactone
production and exudation in various species (Yoneyama et al., 2007a, b;
Lopez-Raez et al., 2008). A reasonable hypothesis is that high P
availability would decrease the extent of AM symbiosis by reducing
strigolactone production in roots (Bouwmeester et al., 2007; Yoneyama et
al., 2007b).

In this report, P fertilization conditions which lead to an almost complete
arrest of the first stages of the interaction between pea (Pisum sativum
L.) and two species of AM fungi are described. This strong effect is at
least partly linked to regulatory events occurring in the plant partner, as
shown by split-root experiments. Furthermore, it is demonstrated that like
root colonization, strigolactone production is controlled in a systemic
manner by P supply. Hence, strigolactones may contribute to the regulation
of AM symbiosis by P, but supplementation experiments indicate that they
are not the only factor involved.

==========================

Tissue-Specific Localization of Pea Root Infection by Nectria haematococca.
Mechanisms and Consequences Plant Physiol. April 2005 137:4 1363-1374

Abstract
Full Text
PDF

Intracellular pH in Arbuscular Mycorrhizal Fungi . A Symbiotic
Physiological Marker Plant Physiol. April 1998 116:4 1279-1288

Abstract
Full Text



  • Re: [permaculture] red clover extract that stimulates vesicular arbuscular mycorrhizae - Google Search, Lawrence London, 03/23/2013

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