Abscisic acid function in plant development Seed quality Stress tolerance Dormancy Germination Biotic stress response Stomatal aperture Gene expression Development Abscisic Acid abscisin II and dormin ABSCISIC ACID (ABA) controls many plant processes including stress responses, development and reproduction Guard cell responses Root growth Vegetative dehydration responses and osmoprotectants Seed development Biotic stress responses Drought-tolerant plants Adapted with permission from RIKEN

Cytokinins in plant growth Cytokinins (CKs): promote cell division in the shoot delay leaf senescence regulate nutrient allocation promote root nodule development contribute to environmental signaling and pathogen responses regulate auxin action and distribution In the 1950s, Folke Skoog’s research group identified a synthetic cytokinin, kinetin Control With kinetin + CK + auxin and CK Top row – the plant on the right is silenced in a CK receptor and deficient in root nodule formation. Bottom row: The figure on the left shows a rice variety with slightly increased CK levels leading to increased seed production. In the middle is the classic experiment showing the effects of CK, auxin or both on differentiation of tobacco leaf discs. On the right the AtCKX1 overexpression plant has an increased turnover of CK leading to increased root growth. Cytokinin was discovered through efforts to identify compounds that increase the growth of plant cells in culture

Brassinosteroids Growth-promoting natural products found at low levels in pollen, seeds, and young vegetative tissues  Brassinolide was the first isolated brassinosteroid in 1979, when pollen from Brassica napus was shown to promote stem elongation and cell divisions over 70 BR compounds have been isolated  Promotion of cell expansion and cell elongation; works with auxin to do so. It has an unclear role in cell division and cell wall regeneration. Promotion of vascular differentiation; BR signal transduction has been studied during vascular differentiation. Is necessary for pollen elongation for pollen tube formation. Acceleration of senescence in dying tissue cultured cells; delayed senescence in BR mutants supports that this action may be biologically relevant. Can provide some protection to plants during chilling and drought stress. BRs appear to be synthesized at their site of action; there is no evidence for long-distance transport of endogenous BRs BR synthesis is regulated by negative feedback, light and circadian cycles, and interactions with other hormones BR biosynthesis mutants are dwarfed; a family of cytochrome P450 genes is central to their synthesis