Symbiotic Fungi: Principles and Practice (Soil Biology)

24 A. Das and A. Varma
intracellular hyphae in the root cortex, often differentiating into dense hyphal coils
(arbuscule-like structures), spore and vesicle-like structures. As with AM fungi,
hyphae multiply within the host cortical tissues and never traverse through the
endodermis. Likewise, they also do not invade the aerial portion of the plant (stem
and leaves).
P. indica is a potential candidate to serve as biofertilizer, bioprotector, bioregulator,
bioherbicide/weedicide, and to combat environmental stresses (chemical,
thermal, and physical). The fungus also preserves soil fertility, and improves
plant health as well. Similarly to arbuscular mycorrhizal fungi, P. indica stimulates
nitrate assimilation in the roots and solubilizes insoluble phosphatic components in
the soil. Recent experiments have amply demonstrated that P. indica provides
resistance against heavy-metal contamination in the soil. Piriformospora indica,
promotes growth of Arabidopsis and tobacco seedlings, and stimulates nitrogen
accumulation and the expression of the genes for nitrate reductase and the starchdegrading
enzyme glucan–water dikinase (SEX1) in roots (Sherameti et al. 2005).
P. indica is been reported to induce resistance to fungal diseases in the monocotyledonous
plant barley, along with tolerance to salt stress. The beneficial effect
on the defense status is detected in distal leaves demonstrating a systemic induction
of resistance by a root-endophytic fungus. The systemically altered ‘‘defense
readiness’’ is associated with an elevated antioxidative capacity due to an activation
of the glutathione–ascorbate cycle and an overall increase in grain yield. Since
P. indica can be easily propagated in the absence of a host plant, we conclude that
the fungus could be exploited to increase disease resistance as well as yield in crop
plants (Waller et al. 2005). The axenically cultivable root endophyte Piriformospora
indica has also been described as a model organism to be used as a potential for
biocontrol strategies. It is able to increase biomass and grain yield of crop plants. In
barley, the endophyte induces local and systemic resistance to fungal diseases and
to abiotic stress. To elucidate the lifestyle of P. indica, scientists have analyzed its
symbiotic interaction and endophytic development in barley roots. It was found that
fungal colonization increases with root tissue maturation. The root tip meristem
showed no colonization, and the elongation zone showed mainly intercellular
colonization. In contrast, the differentiation zone was heavily infested by interand
intracellular hyphae and intracellular chlamydospores. The majority of hyphae
were present in dead rhizodermal and cortical cells that became completely filled
with chlamydospores. In some cases, hyphae penetrated cells and built a meshwork
around plasmolyzed protoplasts, suggesting that the fungus either actively kills
cells or senses cells undergoing endogenous programmed cell death. Seven
days after inoculation, expression of barley BAX inhibitor-1 (HvBI-1), a gene
capable of inhibiting plant cell death, was attenuated. Consistently, fungal proliferation
was strongly inhibited in transgenic barley overexpressing GFP-tagged
HvBI-1, which shows that P. indica requires host cell death for proliferation in
differentiated barley roots. It has been suggested that the endophyte interferes with
the host cell death program to form a mutualistic interaction with plants (Deshmukh
et al. 2006).
Piriformospora indica, a basidiomycete of the Sebacinaceae family, promotes
the growth, development, and seed production of a variety of plant species too.