MSc

Rice (Oryza sativa L.) is the staple food crop of Kerala. Rice production in
the state is decreasing owing to the declining trend in rice cultivation, due to various
stresses. In the present context of climate change, mainly in the form of global
warming, the high temperature is one of the most important environmental factor
influencing the growth, development and yield of rice. Rice is highly susceptible
to heat stress, particularly during the reproductive and ripening stages. Hence high
temperature induced sterility has become a serious problem. Since most of the
prevailing high yielding varieties are highly susceptible to heat stress, the heat
tolerance trait must be incorporated into them using potential donors. Marker-
Assisted Selection (MAS) has been identified as a dependable, reproducible and
time saving strategy to confirm the presence of the desirable gene and to quicken
the breeding cycle. The study entitled ‘Bulked segregant analysis for heat tolerance
in segregating generation of rice (Oryza sativa L.) using SSR markers’ was carried
out with the objective to identify the SSR markers linked to the gene/s for heat
tolerance in rice through bulked segregant analysis.
Two rice varieties, heat sensitive high yielding variety Uma and heat
tolerant variety N22 and their hybrid (Uma × N22, developed at RARS, Pattambi
during 2014) were used in this study. Initially, parents were grown to assess the
parental polymorphism by SSR markers. DNA was isolated by CTAB method with
some modification. Total 197 SSR markers were used to assess the parental
polymorphism.
SSR
markers
were
obtained
from
Gramene
database
(www.gramene.org). The 41 SSR markers were polymorphic among 197 SSR
markers between Uma and N22 indicating 20.81 per cent parental polymorphism.
After this, F3 generation along with parents were kept for germination
during last week of December 2016 and transplanted during last week of January
2017. Heat tolerance screening was done in polyhouse during March and April
2017. The temperature inside the polyhouse ranged from 37.1 to 42.7 ̊C during
these two months. High-temperature stress was given at booting stage of the plants
for fifteen days. After 15 days of heat treatment, all the treated plants were taken
outside of the polyhouse to ambient conditions. High-temperature stress directly
affects spikelet fertility. Hence spikelet fertility was the major trait for identification
and selection of F3 plants for BSA. During heat stress in this study, N22 (heat
tolerant parent) exhibited the highest mean spikelet fertility (90.92 %) and Uma
(heat susceptible parent) exhibited the lowest mean spikelet fertility (2.62 %)
whereas, F3 plants showed 44.78 per cent mean spikelet fertility. DNA from seven
most tolerant plants and seven most susceptible F3 plants were used to constitute
the tolerant and susceptible bulk, respectively.
The identified 41 parental polymorphic markers were used for the
genotyping of tolerant bulk, susceptible bulk, tolerant parent (N22) and susceptible
parent (Uma). Among these, one RM marker (RM5749 on chromosome number 4)
showed polymorphism between tolerant bulk and susceptible bulk during BSA.
This marker, which was found to co-segregate with the trait, was again used to
screen with the individuals of tolerant and susceptible bulks. The LOD value
obtained during Single marker analysis (SMA) for the linkage between marker and
spikelet fertility was 6.86, indicating that RM5749 on chromosome number 4 is
tightly linked to the spikelet fertility trait under heat stress.
The identified 41 polymorphic markers could be further used for the
identification of QTLs and linkage analysis. RM5749 on chromosome number 4
could be used for the marker-assisted selection for heat tolerance in rice. SSR
markers near to RM5749 on chromosome number 4 could be screened for heat
tolerance. High-level heat tolerance in N22 could be transferred to high yielding
heat susceptible variety Uma, through marker assisted breeding using RM5749.