Evaluating the abiotic stress tolerance of transgenic barley expressing an Arabidopsis vacuolar proton-pumping pyrophosphatase gene (AVP1)

Abstract

Commercially relevant barley varieties with improved abiotic stress tolerance are needed to increase crop productivity. Previously, transgenic barley with constitutive CaMV 35S expression of AVP1, a gene encoding the type I Arabidopsis vacuolar proton-pumping pyrophosphatase (H⁺-PPase), had a larger shoot biomass in non-saline and saline conditions compared to null segregants. However, the growth and grain yield of the transgenic AVP1 barley was yet to be evaluated in a saline field. It was also yet to be investigated whether the larger shoot biomass of transgenic AVP1 barley in both non-saline and saline conditions arose from a change in tissue solute accumulation, water use, plant nutrition, carbohydrate metabolism, heterotrophic growth or a combination of these traits. In addition, for this AVP1 technology to be applicable for barley grain growers, a commercially relevant transgenic AVP1 barley cultivar with well-regulated control of AVP1 expression was needed. The first focus of this project evaluated the growth and grain yield of 35S:AVP1 barley (cv. Golden Promise) in a low and high salinity field near Kunjin, Western Australia. Field trial results validated greenhouse-based findings of improved shoot biomass in transgenic AVP1 barley compared to wild-type. Furthermore, results demonstrated for the first time that transgenic AVP1 barley had increased grain yield per plant compared to wild-type in a field with high salinity. These findings suggest that transgenic AVP1 barley is a promising option to help increase the grain yield of cereal crops in a saline field. The second focus of this project investigated the abiotic stress tolerance and potential factors contributing to the larger shoot biomass of 35S:AVP1 barley. At low phosphorus (P) supply, 35S:AVP1 barley had a larger shoot biomass, greater root P uptake and increased rhizosphere acidification compared to wild-type. At low nitrate (NO₃⁻) supply, two 35S:AVP1 barley lines had increased shoot biomass but with no difference in NO₃⁻ uptake capacity compared to null segregants. The shoot biomass of 35S:AVP1 barley was also increased compared to null segregants under low water availability and low water availability concurrent with salinity. Furthermore, an increase in plant biomass from 6 days after seed imbibition, thus seedling vigour, was detectable in 35S:AVP1 barley compared to null segregants. Leaf metabolites involved in ascorbic acid synthesis were also significantly altered in the 35S:AVP1 barley compared to null segregants. Collectively, these findings suggest that a combination of traits is contributing to the improved growth of transgenic AVP1 barley. The third focus of this project evaluated the salt stress inducibility of the ZmRab17 promoter and investigated the salinity tolerance of commercially relevant barley (cv. WI4330) expressing AVP1 via the ZmRab17 and the constitutive ZmUbi1 promoter. The ZmRab17 promoter was salt-stress inducible in barley root stelar cells with basal transgene expression in non-saline conditions. However, the shoot and root biomass of ZmRab17:AVP1 and ZmUbi1:AVP1 barley did not differ to wild-type and null segregants in saline conditions. These findings suggest that the type of promoter driving AVP1 expression in transgenic barley is an important factor.