Analysis of leaf morphology and photosynthesis in deletion mutants of rice (Oryza sativa L.)

• Main Author: Smillie, Ian R. A.
• Published: University of Nottingham 2012
• Subjects: 633.18; SB Plant culture
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.559617

As a plant operating the C3 photosynthetic pathway and commonly grown under tropical conditions of high light intensity and temperature, rice (Oryza sativa) displays high levels of photorespiration, to the detriment of photosynthetic efficiency. For this reason it is thought that improvements to net photosynthesis via an increased photosynthetic efficiency could provide significant gains in terms of grain yield. There is great interest in 1. Introducing CO2 concentrating mechanisms into C3 crop plants such as the C4 photosynthetic pathway in order to facilitate enhanced photosynthetic efficiency. This requires an understanding of C3 and C4 leaf development and establishing whether there is sufficient plasticity in the rice genome to produce plants with C4-like properties. 2. Improving existing C3 photosynthesis by means of increasing leaf thickness, vein density and investigation of the impacts of mesophyll cell size. It is in this context that a forward screen of approximately 100 mutant lines of the indica rice variety IR64 was developed at Nottingham to search for relevant changes in leaf morphology. Mutant seed produced using chemical mutagenesis (diepoxybutane and ethylmethanesulfonate) and irradiation (gamma and fast neutron) was supplied by the International Rice Research Institute (IRRI) in the Philippines. A rapid low resolution screen was devised using light microscopy of fresh, untreated hand cut leaf sections of plants at the leaf six stage. Seven mutant lines were identified as showing altered leaf morphologies and were termed alm mutants. alm1, alm5 and alm6 displayed a reduced interveinal distance between neighbouring veins, a common feature of C4 plants with Kranz anatomy, whilst alm1 and alm5 also displayed a reduction in the size of minor veins. alm3 and alm4 produced significantly thicker leaves than wild type plants, whilst the leaves of alm7 were significantly thinner. A detailed anatomical characterisation of leaf structure revealed that alm3, alm4 and alm5 plants all displayed a significant reduction in the size of mesophyll cells and that for all the mutant lines, the distance between veins was strongly correlated with mesophyll cell size rather than the number of mesophyll cells spanning the interveinal regions. Physiological properties of the alm lines were investigated using infra-red gas analysis (IRGA) measurements of gas exchange and chlorophyll fluorescence. It was shown that none of the mutant lines displayed an increase in photosynthetic capacity when compared to wild type plants, even in lines which were shown to possess what was thought to be a favourable leaf anatomy, quite possibly a result of widespread effects of the mutation process. The alm1 line was shown to display interesting physiological responses, with almost no transpiration and a severely reduced photosynthetic capacity, yet functioning stomata and an unimpaired stomatal conductance. In conclusion, the future success of photosynthetic improvement in rice will rely on the screen of much larger numbers of mutant lines of rice and C4 plants in order to identify the genes determining key conserved morphological features such as interveinal cell number, cell size and the degree to which rice mesophyll cells are lobed.