The 'C4 rice project' aims to introduce the C4 photosynthetic mechanism into C3 rice plant in order to boost biomass and grain yield to cope with the predicted food demand and adverse environmental conditions in the future. As a first step, identification of rice mutants with novel photosynthetic traits conferring increased photosynthetic capacities is an utmost requirement in order to improve biomass production. One approach is to screen large numbers of rice mutants to achieve this goal. This would confirm whether the rice genome possesses the required 'plasticity' to introduce C4-like traits. Two traits are ubiquitous in the C4 syndrome and can be introduced into high throughput screens: anatomically they have higher leaf vein densities and physiologically they have lower CO2 compensation points (r). As a first step, a systematic screening experiment was conducted to identify C4 properties in a gamma-induced, EMS-induced mutant populations of IR64. The major objectives are to develop reliable rapid screening techniques and to identify C4-like photosynthetic traits in rice. A mass-screening chamber with low-C02 treatment (1000 seedlings per week) was used. Rice seedlings at 5th leaf stage were exposed to 60 ppm CO2 for the first day and 30 ppm for the remaining three days. Following this, putative candidates were identified by measuring chlorophyll using SPAD method by a SPAD depletion % (SPD%) calculation . Phenotypic responses of the mutants were compared with wild type IR64 and with the Cr C4 intermediate Panicum milioides. Initial SPD% results showed a distinct gap between parental wild type IR64 and P. milioides, means ranging from 65% to 11% respectively, whereas in the mutant lines SPD% spanned 0 to 100%, suggesting considerable phenotypic variation in response to low CO2 in the mutant population. Mutants with lower SPD% «10%) were identified as 'Chlorophyll retention mutant' (CRM) variants under low CO2 stress. Up to now, 1909 mutant lines have been screened for low SPD% under 30 ppm CO2 and 28 CRM lines from M4 families were identified as putative candidates with tolerance to low CO2 were self seeded and produced Ms generation lines to determine the genetic control of the altered response to low CO2. Ms putative mutants were subject to physiological measurements. Leaf gas exchange analyses with the U-6400 XT portable photosynthetic system were performed for CO2 compensation point (r) carboxylation efficiencies (CE) and CO2 assimilation rates at ambient levels (A400). A comprehensive profile of r in wild type rice plants were revealed, In early growth stages (8st to 11th leaf) rice displayed an increased r of above 50 J.!mol mol-1 , whilst leaves produced in the reproductive stages (lih ,13th , 14th and Flag leaves, FL) showed a sharp decline in r and reached below 50 J.!mol mor1 with the lowest r seen in FL. With the analysis of rice CRM lines, it was shown that three mutant lines had significantly declined r in early growth stages ( CRM 27, CRM 29 and CRM 65) than the WT, whilst further analysis with CRM 29 showed a sharp decline in r (43 J.!mol mol-1 ) in FLs. Additional photosynthetic analysis indicated CE and A400 were superior in these lines compared to WT. Photosynthetic correlation studies was shown a negative correlation between rand CE and A400 suggesting lowering r in rice is beneficial in terms of improving rice photosynthetic efficiencies in CRM lines. Detailed anatomical investigations were carried out in CRM 29 FLs which displayed significantly increased mesophyll cell plan area and number of lobes per cell than in WT. Biomass analysis suggested these three mutant lines had superior ability in producing AGDM, panicle weight than the control plants, suggests moderately lowering r with improved anatomical traits in rice may be associated with greater biomass and grain yield in CRM lines. In conclusion, it is optimistic to produce an intermediate C4-photosynthetic rice plant that can out-yield current wild types before successfully incorporating a complete functioning C4 pathway into rice. Significant changes may be achieved by targeting existing metabolic processes and leaf anatomy in rice. There is a need to search the genes which determine these novel photosynthetic and anatomical features in leaves of rice. Thus a co-ordinated attempt of plant physiologist, geneticists, and the molecular biologists is required. ii
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:606807 |
| Date | January 2013 |
| Creators | Nagoor, Mohamed Mubarak Ahamadeen |
| Publisher | University of Nottingham |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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