Incorporation of heat tolerance into jyothi rice (Oryza sativa L.) veriety through marker assisted breeding

Abstract

Rice is the staple food of Kerala. Among the different rice varieties in Kerala, Jyothi is very popular with high consumer preference. But it is highly susceptible to heat stress especially during the panicle initiation and flowering stages. In major rice growing tracts of Kerala viz., Kole, Kuttanad and Palakkad, the temperature rises to 40ºC or even more which results in spikelet sterility leading to severe reduction in yield. Most of the Kerala's popular high-yielding rice varieties are highly susceptible to heat stress. In the current context of climate change and global warming, it is the urgent need of the hour to incorporate heat tolerance into high yielding popular rice varieties like Jyothi. Marker assisted selection (MAS) has been established as a reliable, reproducible, and time-saving method for confirming the presence of desired genes. It is important to study the negative effects of heat stress on various morphological, physiological and yield responses for the development of effective mitigation strategies. Hence, the present research work was undertaken to incorporate heat tolerance from the donor variety Nagina 22 (N22) into Jyothi (PTB 39) by hybridisation and marker assisted selection. In experiment 1, phenotyping of Jyothi (PTB 39) and Nagina 22 (N22) rice varieties in heat chamber and control conditions and genotyping of parents with SSR markers were carried out. Morphological characters evaluated were plant height (cm), days for flowering, pollen viability (%), tillers per plant, panicles per plant, panicle length (cm), spikelets per panicle, fertile grains per panicle, partially fertile grains per panicle, sterile grains per panicle, spikelet fertility (%), 100-seed weight (g) and grain yield per plant (g), along with three physiological traits viz., proline content, membrane stability and canopy temperature depression. Increased plant height and number of tillers were observed for both the varieties under heat stress condition. Highly significant reduction in pollen viability and spikelet fertility were observed in Jyothi under heat stress condition. High levels of proline were accumulated by Jyothi under the high temperature condition as compared to N22. Exposure of the parents to very high temperature (>40ºC) resulted in the reduction in panicle length, number of panicles, fertile grains per panicle, reduction in the 100-grain weight which ultimately resulted in the lower grain yield per plant. Jyothi exhibited higher amount of cell injury which resulted in very low membrane thermostability under heat stress condition. Canopy temperature was observed to be higher for Jyothi which exhibited lower canopy temperature depression values. The physiological characters viz., membrane thermostability and canopy temperature depression were high in N22 compared to Jyothi under heat stress. N22 maintained the panicle length, pollen viability, number of panicles, 100-seed weight and fertile grains under heat stress condition and thereby succeeded in maintaining the yield under very high temperature. Foreground and background SSR markers were used for detecting the polymorphism between parents at molecular level. Among the 42 SSR markers used, 30 markers exhibited polymorphism. Ten polymorphic foreground markers were RM5757, RM431, RM3735, RM5749, RM26212, RM7076, RM447, RM3586, RM3340 and RM6836. The background markers which exhibited polymorphism were RM336, RM85, RM9, RM169, RM473A, RM5926, RM225, RM256, RM280, RM19, RM251, RM7555, RM332, RM242, RM212, RM100, RM3701, RM445, RM166 and RM552. The hybridity of Jyothi x N22 was confirmed with the foreground marker RM26212. As the experiment 2, hybrid of Jyothi x N22 was developed. In experiment 3, evaluation of the hybrid along with parents was carried out. Apart from all the morphological and physiological traits observed for parental evaluation, grain length (mm) and seed kernel length (mm) were also recorded for hybrid and parents. Highly significant difference between parents and hybrids for eleven characters indicated substantial amount of genetic variability. Heterobeltiosis was exhibited by the hybrid for plant height, pollen viability, tillers per plant, spikelet fertility, 100 seed weight and grain yield per plant. In experiment 4, F1 hybrid was backcrossed to Jyothi to develop BC1F1 seeds. The backcrossed seeds and F2 seeds were harvested separately. As the future line of work, the backcross generations (BC2F1, BC3F1 and BC4F1) may developed. F2 seeds may be used to develop segregating populations through subsequent selfing.