The HYR (Higher Yield Rice) transcription factor gene regulates photosynthetic carbon metabolism (PCM) in rice. Overexpression of HYR (Os03g02650) in rice lines enhances photosynthetic capacity, and contributes to higher grain yield under normal conditions as well as under drought and high temperature stress. Since HYR is involved in regulation of PCM and response to environmental stress signals we were interested in studying the genetic basis of this response. To study the intrinsic role of HYR in regulating PCM in diverse rice genotypes, the expression profile of HYR was characterized in diverse genotypes at different times of the day. Rice genotypes belonging to the Oryza sativa s.sp japonica (Nipponbare, Kaybonnet, Bengal), and O. sativa s.sp indica (Vandana, Nagina, Aochiu) and a stress tolerant African rice O. glaberrima accession were used for this analysis. Circadian rhythm is an endogenous entrainable oscillation or cycle of around 24 hours, synchronized to the light/dark diurnal cycles that regulates the expression of multiple genes, and closely relevant to photosynthetic activities. The set of diverse rice genotypes were grown in 500-ml pots in growth chambers (14h light/10h dark cycle with light intensity 580 µmol m^-2s^-1, with light period from 7am-9pm at 25°C and 65% RH) under well-watered conditions. At the vegetative stage, 48-day-old plants were used for sampling the youngest fully expanded leaves that were collected at 12am, 3am, 7am, 9am, 12pm, 3pm, 6pm and 9pm. RNA was isolated and used to generate cDNA. qRT-PCR experiments were carried out using a CFX-96 Bio-Rad thermocycler. The comparative threshold cycle (Ct) method of quantitation with ubiquitin as reference were used for qRT-PCR analysis. The results show that the expression of HYR was strongly affected by circadian rhythm. The variation of the HYR expression among rice genotypes indicate HYR has different internal time in each rice genotype.

Keywords: Circadian rhythm, HYR gene expression, Rice, Photosynthetic.