SCREENING OF INDIGENOUS RICE (ORYZA SATIVA L.) GERMPLASMS OF ASSAM FOR TOLERANCE TO ANAEROBIC CONDITION DURING GERMINATION
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Date
2017-07
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AAU, Jorhat
Abstract
Rice is the staple food for the people of India and the major source of livelihood to the farmers. One of the most serious problems that adversely affect rice production in Assam is the recurrence of devastating floods. Even though more than 60% of summer rice is planted by the month of June, flooding after transplanting could completely devastate the crop. Furthermore, in the case of direct seeded rice, occurrence of flood delays sowing of seeds. Rice seeds can germinate under hypoxic conditions, but may fail to extend their coleoptiles and develop roots and leaves. Thus, there is an urgent need to provide farmers with rice varieties that besides being highly adoptive to local environment, also have the additional trait of tolerance to hypoxic condition. Breeding for hypoxic tolerance for germination was attempted but with limited success, because highly tolerant donors were unavailable and knowledge of the physiological and molecular basis of tolerance was inadequate.
The precise and stringent control of physiological responses of deep water germplasms to flooding indicates that plant must possess sensible oxygen sensing mechanisms. Despite the wealth of molecular and phenotypic data on plant responses to water logging, very less information about how declining oxygen levels are sensed, and how the complex and extensive expression changes are controlled. So, in this study deepwater germplasms from Assam were screened against the proven tolerant lines KHO, MZ Red and Khaiyan (obtained from IRRI).Phenotypic characterization of 160 germplasm from Assam, based on a set of physiological parameters identified Rangadhar Kekua Bao (RKB) with highest frequency of germination (78%) in anaerobic condition. Physiological basis of tolerance involved uninterrupted supply of energy through catabolism of starch offset in sugar homeostasis by increasing the sugar sink towards coleoptiles elongation. Biochemical analyses revealed, enzymes involved in starch catabolism, alcohol fermentation and ATP and PPi dependent substrate level energy generation were significantly higher under hypoxia in tolerant germplasm. Transcript studies conducted on tolerant rice germplasms using GADPH as stable endogenous gene revealed that genes involved in sugar signaling such as TPP7, CIPK15 and SnRK1A that regulate Ramy3D transcription involving the transcription factor, MYBS1 under hypoxia were significantly upregulated in the RKB compared to proven tolerant line, KHO and susceptible line IR-64.
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Group-VII members of Ethylene response factor family (ERFs) in rice namely ERF71 and ERF63 that are substrates for N-end rule of proteolysis also showed significant up regulation indicating towards an ethylene or low oxygen based hypoxia sensor that is yet to be identified in rice. Thus it could be inferred that hypoxia during germination of RKB is regulated by O2 sensing mechanisms involving ERFs that in turn activates sugar signalling pathways involving TPP7 a master regulator of sugar homeostatic, thus providing uninterrupted supply of energy, increasing the sugar sink towards coleoptile elongation possibly through action of EXPA7 and EXPA12.
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