Identification of differentially expressed genes under heat stress conditions in rice (Oryza sativa L.)

Rice production in recent years is highly affected by rapidly increasing temperatures in the tropical and sub-tropical countries, which threatens the sustainable production in near future. Hence, understanding the heat tolerance mechanism and evolving tolerant varieties is an immense need in the staple crop rice. An experiment has been conducted to identify differentially expressed genes in rice under heat stress conditions by employing a diverse set of 32 rice genotypes that includes reported heat tolerant genotypes Nagina 22 (N22) and Dular. Screening of the genotypes at field conditions during Summer-2018 for reproductive stage heat tolerance (wherein the mean minimum (29.8 degrees C) and maximum (38.4 degrees C) temperatures surpassed optimum temperatures (25 degrees C night/30 degrees C day) required for rice flowering and grain filling stages) and lab conditions employing thermal induction response (TIR) technique to know the genotype's acquired thermal tolerance revealed that the genotype FR13A (indica landrace) showed highest overall performance for multitude of traits viz., 95.29% of spikelet fertility (SF-%) at field level and 100% seedling survival percentage (SSP) at sub-lethal temperatures under laboratory conditions. The relative performance (under TIR) across all the three traits viz., relative shoot length (RSL) (4.91), relative root length (RRL) (equal to the control) and relative seedling dry weight (RSDW) (6.94) over control is high when compared to the other genotypes under study. However, the highly susceptible genotype PUSA1121 performed with 43.59 of SF%, 73.33% SSP, - 43.59 of RSL, - 36.02 of RRL over control. Hence, these contrasting genotypes were used for molecular analysis for identification of differentially expressed genes by employing 29 heat related gene specific primers. Five genes viz., OsGSK1, TT1, HSP70-OsEnS-45, OsHSP74.8 and OsHSP70 have shown differential expression between the two genotypes. Hence, the genotype FR13A, an 'indica' genotype, can be utilized in heat tolerance breeding programmes as donor parent in addition to the reported 'aus' genotypes, N22 and Dular. To our knowledge this is the first indica genotype identified for heat tolerance. The HSP70s, TT1 and OsGSK1 that proved with differential expression might be used for identification of gene specific InDels and thereby to develop functional markers that help in the marker assisted introgression breeding to develop heat tolerant varieties that can sustain production under dramatically changing climatic conditions.