Executive Summary : | A combination of factors renders arid, semi-arid and coastal agricultural areas of the world vulnerable to climate change impacts on soil salinity. Salinity tolerance has evolved independently as well as multiple times over millions of years in various grass lineages. More than 350 naturally halophytic grass species are known. Most cereal crops are grass species; however most cultivated cereal crops are not salt tolerant. Based on photosynthetic characteristics (C3 versus C4), BEP (C3) and PACMAD (C4) clades of grasses are recognized. C4 species, in general, differ from C3 species in more efficient resource use strategies (Water and Nitrogen Use Efficiency), more efficient hydraulic architecture that are derived primarily from the more efficient carbon fixation processes seen in C4 species. Also, within grass clades, a significant number of salt tolerant grass lineages occur in the PACMAD clade i.e. C4 species. Salinity tolerance is a polygenic trait and mechanisms of salinity tolerance have been relatively well examined for members of the BEP grasses (eg. rice, wheat, barley). However, salinity tolerance has been examined to a limited extent in C4 crop species. Within C4 species, salt tolerant accessions for millet species have been reported. Roots are the first site of salt perception and are highly plastic in their response to salinity. Plant roots have apoplastic barriers controlling ion movement that are altered by salinity. In the first component of this project, root function (architecture, morphology, anatomy, development of apoplastic barriers), ion content will be correlated with expression of transporter genes involved in ion homeostasis as well as development of apoplastic barriers in salt tolerant and sensitive accessions of C4 millet species: Pennisetum glaucum (pearl millet), Eleusine coracana (finger millet) and Echinochloa frumentacea (barnyard millet). This data will be compared to salt sensitive and tolerant accessions of rice and barley. Plants respond to salinity in a biphasic manner: an immediate, ‘osmotic response’ followed by an ‘ionic response’ in which growth inhibition occurs due to Na+ accumulation. Toxicity of Na+ ions in the longer term ‘ionic phase’ is resolved by a combination of avoidance and tolerance strategies, involving a range of transporters expressed at the plasma membrane and/or tonoplast. One such transporter, is the sodium specific HKT1;5 that confers salinity tolerance by ‘shoot exclusion of sodium’ via salinity induced root specific expression. While the important ‘genetic association’ of HKT1;5 with salinity tolerance has been well established for C3 species such as wheat, rice and barley by QTL/GWAS mapping, the function of HKT1;5 in C4 species has not been examined in detail except in maize. The second component of the project will compare HKT1;5 function in C4 and C3 species mentioned above through cloning, heterologous expression and site-directed mutagenesis approaches. |