Table 1.
Summary of cytokinin (CK) modification-related studies and their reports on corresponding drought-tolerant phenotypes.
| CK Metabolic Gene and Source of Isolation | Genetic Engineering Approach | Promoter Controlling Transgene Expression | Transgenic Species | Phenotype Alterations | References |
|---|---|---|---|---|---|
| AtCKX1 (Arabidopsis thaliana) | overexpression | Beta-glucosidase (bGLU) from maize | Barley (Hordeum vulgare) | maintain higher water content; enhance growth and yield; increase root growth; alter drought-responsive gene expression; improve drought stress tolerance | [74,75] |
| overexpression | root-specific promoter WRKY6 and constitutive promoter 35S | Tobacco (Nicotiana tabacum) | maintain higher expression levels of genes encoding antioxidant enzymes and improve drought stress tolerance | [76] | |
| IPT (Agrobacterium tumefaciens) | inducible expression | stress- or senescence-activated promoter SAG12 | Creeping bentgrass (Agrostis stolonifera) |
alter transcriptional factor-encoding genes involved in stress signaling, oxidative protection and protein modification; enhance drought tolerance | [125,126] |
| IPT (Agrobacterium tumefaciens) | inducible expression | stress- and maturation-induced promoter (SARK) | Rice (Oryza sativa) | enhance sink strength; improve drought tolerance and increase grain yield | [56] |
| IPT (Agrobacterium tumefaciens) | inducible expression | stress- and maturation-induced promoter (SARK) | Peanut (Arachis hypogaea) | maintain higher photosynthetic rates, stomatal conductance and transpiration; improve drought tolerance and increase yield under field conditions. | [38] |
| IPT (Agrobacterium tumefaciens) | overexpression | modified developmentally regulated transcription factor AtMYB32 (AT4G34990) promoter from Arabidopsis | Canola (Brassica napus) | increase higher chlorophyll levels; delay leaf senescence; enhance yield under rain-fed and irrigated conditions | [127] |
| IPT (Agrobacterium tumefaciens) | inducible expression | stress- and maturation-induced promoter (SARK) | Cotton (Gossypium hirsutum) | delay senescence; enhance root and shoot biomass; maintain higher chlorophyll content and photosynthetic rates under water deficit conditions | [109] |
| IPT (Agrobacterium tumefaciens) | inducible expression | stress- and maturation-induced promoter (SARK) | Rice (Oryza sativa) | increase drought tolerance through the coordinated regulation of carbon and nitrogen assimilation | [39] |
| IPT (Agrobacterium tumefaciens) | overexpression | modified developmentally regulated transcription factor AtMYB32 (AT4G34990) promoter from Arabidopsis | Wheat (Triticum aestivum) | increase grain yield under water deficit | [44] |