An evaluation of the effects of over-production of ABA on whole plant water use, growth and productivity

Abstract

Predicted climate change and increasing global populations suggest that water will become an increasingly scarce and valuable commodity. Breeding plants which produce equivalent yields with reduced water input is therefore vital to sustain and increase crop production in the future. The phytohormone, abscisic acid (ABA), is important in controlling plant responses to water stress. It may be possible to improve water use efficiency (WUE) by genetically modifying tomato and other species to maintain elevated levels of ABA under optimal (unstressed) conditions, thereby manipulating an intrinsic signalling mechanism which is known to mediate drought-induced alterations of stomatal behaviour. ABA is synthesised via the oxidative cleavage of C40 epoxycarotenoid precursors, a reaction catalysed by the key enzyme 9-cis-epoxycarotenoid dioxygenase (NCED). Pure breeding transgenic tomato lines constitutively over-expressing LeNCED1, known as sp5 and sp12, both have elevated ABA concentrations, which reduce stomatal conductance under optimal (unstressed) growth conditions, thus conserving soil water during periods when corresponding wild type (WT) control plants were inefficient in its use. Under well-watered conditions, whole plant transpiration efficiency (TEp) was significantly greater in both 'high ABA' lines (sp12 and sp5) than in WT plants. The over-expression of LeNCED1 was combined with over-expression of a gene (LeBCH2) encoding β-carotene hydroxylase (BCH), an enzyme acting earlier in the ABA biosynthetic pathway. These 'double transgene' lines (G28 and G29) consistently exhibited further improvements in ABA accumulation and TEp relative to corresponding 'single transgene' parental lines. Lines G28 and G29 respectively exhibited 37 and 54 % improvements in TEp relative to WT controls. When evaluated as a potential 'high ABA' rootstock, it was found that the 'double transgene' G29 line did not provide a sufficiently strong root-sourced signal to affect the stomatal behaviour of scions. To increase ABA biosynthesis in the roots further, a programme designed to combine the over-expression of three ABA biosynthetic genes (LeNCED1; LeBCH2; LePSY1) was initiated with the objective of obtaining a rootstock which produced sufficient ABA to affect stomatal behaviour when grafted onto WT scions. Unfortunately, there was insufficient time to complete this work by the end of the period reported in this thesis, although the programme is ongoing