PhD

Management of water stress in tomato (Solanum lycopersicum L.)
through beneficial root endophytic fungus, Piriformospora indica
The study entitled ‘Management of water stress in tomato (Solanum
lycopersicum L.) through beneficial root endophytic fungus, Piriformospora
indica’ was conducted at College of Agriculture, Vellayani during 2019- 2022 with
the objective to to enhance water stress tolerance in tomato plants through
colonization with the beneficial root-endophytic fungus, Piriformospora indica,
and to explore the underlying physiological and molecular mechanisms driving this
tolerance.
The comprehensive study comprised five distinct experiments wherein P.
indica-colonized and non-colonized tomato plants of the variety Vellayani Vijai
were meticulously evaluated under varying water stress conditions. The cocultivation of tomato seedlings with the fungus was carried out according to the
established standard protocol (Johnson et al., 2011). Following the co-cultivation
phase, both colonized and non-colonized tomato seedlings were transplanted into
pots under protected conditions at 30 days after sowing (DAS). Subsequently, water
stress was induced by adapting different methods to simulate water stress (limiting
irrigation, applying polyethylene glycol (PEG), mannitol, abscisic acid (ABA) and
flooding) at 45 DAS, persisting for a duration of seven days.
In all the experiments, the results consistently demonstrated the positive
impact of P. indica colonization on various vegetative traits, such as plant height,
stem girth, primary branches per plant, leaf length, leaf width, and leaf area.
Conversely, with the escalation of water stress levels, these vital vegetative
characteristics exhibited noticeable declines. Moreover, P. indica colonized tomato
plants consistently exhibited significant advancements in achieving first and fifty
percent flowering compared to their non-colonized counterparts. This consistent
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trend held true across diverse water stress conditions, whether induced by restricted
irrigation, the application of PEG, mannitol, ABA, or even submergence.
The impact of P. indica colonization was evident in various aspects of
flowering and fruiting. The number of flower clusters per plant, flowers per cluster,
fruit set percentage, fruits per truss, and fruits per plant were all significantly
enhanced. P. indica colonization consistently resulted in approximately 6-14%
higher fruit set. Additionally, colonized plants showed an impressive increase up to
36% in the number of fruits formed per cluster. For instance, under mannitolinduced stress, colonized plants demonstrated a remarkable 58% increase in the
number of fruits per plant.
Colonized plants consistently yielded larger tomatoes, as evidenced by their
greater fruit length and diameter, even across varying stress levels, when compared
to non-colonized plants. The fruit weight in colonized plants consistently exceeded
that of non-colonized plants in all experiments, with a pronounced 16-28%
enhancement. Colonized plants demonstrated significantly higher yields, producing
up to double the yield obtained from non-colonized plants (401.11g vs. 280.03g as
observed in first experiment). Similarly, in experiments involving the application
of PEG, mannitol, ABA, and submergence, colonized plants consistently achieved
superior yields. As the stress levels increased the effect of P. indica became more
evident. For instance, when comparing the yield of plants that were colonized with
those that were not, there was a noticeable enhancement in yield by 14%, 28%,
146%, and 376% under the conditions of normal irrigation, 75% field capacity (FC),
50% FC, and 25% FC, respectively.
Across all experiments, the qualitative attributes of tomato fruits, including
ascorbic acid, lycopene, and TSS, displayed an upward trend with rising stress
levels. Importantly, P. indica colonization consistently led to improved fruit
quality, manifesting as higher levels of ascorbic acid, lycopene, and TSS in all
experiments, regardless of whether the plants were subjected to stress or not.
In all experiments, physiological parameters such as relative water content
(RWC), cell membrane stability (CMS), and chlorophyll stability index (CSI)
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exhibited a notable decline as water stress levels increased, with reductions of up
to 50% observed in RWC under severe stress conditions. However, the presence of
P. indica colonization consistently mitigated this decline, enhancing RWC by 6-
12%, CMS by 13-27% and CSI by 16-35% compared to non-colonized plants under
severe stress conditions in all conducted experiments.
Tomato plants colonized by P. indica exhibited significantly elevated
proline accumulation when compared to the control plants under conditions of
water stress. During instances of moderate stress, characterized by 50% field
capacity (FC), -7 bar PEG, 7% mannitol, and 6µmol ABA, proline accumulation in
the colonized tomato plants was notably enhanced by a range of 41-69%.
Furthermore, when subjected to more severe stress conditions, such as 25% FC,
-10 bar PEG, 10% mannitol, and 10µmol ABA, the proline accumulation witnessed
an even more substantial increase, ranging from 55 to 116%.
The colonization of P. indica consistently amplified anti-oxidant activities
(superoxide dismutase, catalase and peroxidase) in response to escalating water
stress levels. As the intensity of stress heightened, in all the conducted experiments,
the colonized plants consistently exhibited significantly higher anti-oxidant
activities in comparison to the non-colonized plants subjected to equivalent levels
of water stress. The increased anti-oxidant activities in P. indica-colonized plants
suggests an enhanced ability to scavenge superoxide radicals and protect against
oxidative stress caused by drought.
P. indica colonization in tomato plants significantly enhanced the
expression of the stress responsive transcription factor-SlAREB1, particularly under
drought stress conditions. When both colonized and non-colonized tomato plants
were subjected to drought stress (50% FC), non-colonized plants exhibited only
1.75-fold increase in gene expression compared to the control (normally irrigated),
whereas P. indica-colonized plants displayed a significantly higher 2.76-fold
increase. This highlights the potential synergy between P. indica colonization and
gene expression in response to drought stress.
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In conclusion, colonization with P. indica had a consistently positive impact
on multiple facets of tomato plant growth and development, even under varying
water stress conditions. Across all experimental conditions, the colonized tomato
plants consistently outperformed their non-colonized counterparts, highlighting the
potential of P. indica as a valuable ally in sustainable agriculture, particularly in
mitigating the adverse effects of water stress on tomato crops. This symbiotic
relationship provides a promising avenue for optimizing water usage in tomato
cultivation. Further studies should be conducted to elucidate the role of other
antioxidants, and molecular mechanisms involved in this host-endophyte
interactions. Field and multi-locational studies should also be conducted for
confirmation of the results







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