From Arabidopsis model to green revolution cereal crop varieties : studies of the co-regulation of plant growth and nitrogen metabolism

Modern semi-dwarf "Green Revolution" varieties (GRVs) of wheat and rice dramatically increased global grain yields during the mid to late 20th Century. GRVs are dwarf because they accumulate growth-repressing DELLA proteins. GRVs are thus high yielding because they apportion more resource to grain filling than to stem elongation, and are less susceptible to the yield-reducing effect of "lodging". However, recent evidence suggests that GRVs have a low nitrogen-use efficiency (NUE) as a result of the negative pleiotropic effect of stabilised DELLAs. Therefore, the cultivation of GRVs requires environmentally unsustainable levels of inorganic fertiliser application. The aim of this study is to uncover the underlying mechanism of DELLA-mediated inhibition of the metabolism of nitrate (the major form of nitrogen in most agricultural soils), using the Arabidopsis semi-dwarf GRV model mutant ga insensitive (gai). It was first established that gai is more resistant to chlorate, the toxic chemical analogue of nitrate, suggesting that like GRVs, Arabidopsis gai also exhibits compromised nitrate metabolism. Chimeric analyses and enzymatic assays discovered an inhibitory effect of DELLA proteins on shoot nitrate reductase activity. A similar effect of DELLAs was found to be conserved in rice, providing a potential explanation for the low NUE observed in GRV crop plants. Moreover, as DELLA-interacting proteins (DIPs), PHYTOCHROME-INTERACTING FACTOR3 (PIF3) was identified as a positive regulator of nitrate assimilation, potentially acting downstream of DELLAs. Simultaneous forward screens for mutagenized gai isolated multiple mutants which genetically uncoupled gai-mediated inhibition of stem elongation and nitrate metabolism, with usually multiple acids move in and out transporter32 (umamit32) identified as a potential novel mutation conferring enhanced chlorate resistance. Meanwhile, several tall gai mutants were selected and initially characterised, which provided new insights into the GA-DELLA signalling pathway and how it relates to the regulation of plant growth. The ultimate goal of this research is to apply the knowledge obtained from Arabidopsis to modern crops, in the hope of improving their NUE and thereby contributing to the development of the next generation of GRVs of crops.