Abstract
Plant hormones are small molecules derived from various metabolic pathways and are important regulators of plant development. The most recently discovered phytohormone class comprises the carotenoid-derived strigolactones (SLs). For a long time these compounds were only known to be secreted into the rhizosphere where they act as signaling compounds, but now we know they are also active as endogenous plant hormones and they have been in the spotlight ever since. The initial discovery that SLs are involved in the inhibition of axillary bud outgrowth, initiated a multitude of other studies showing that SLs also play a role in defining root architecture, secondary growth, hypocotyl elongation, and seed germination, mostly in interaction with other hormones. Their coordinated action enables the plant to respond in an appropriate manner to environmental factors such as temperature, shading, day length, and nutrient availability. Here, we will review the current knowledge on the crosstalk between SLs and other plant hormones—such as auxin, cytokinin, abscisic acid (ABA), ethylene (ET), and gibberellins (GA)—during different physiological processes. We will furthermore take a bird's eye view of how this hormonal crosstalk enables plants to respond to their ever changing environments.
Highlights
Plant hormones are small molecules derived from various essential metabolic pathways
Several mutants with increased shoot branching phenotype have been identified in several plant species, including more axillary growth in Arabidopsis (Arabidopsis thaliana), ramosus in pea (Pisum sativum), dwarf (d) or high-tillering dwarf in rice (Oryza sativa), and decreased apical dominance in petunia (Petunia hybrida)
Basal application of the synthetic SL GR24 reduced basipetal auxin transport and PIN1 accumulation in the plasma membrane of xylem parenchyma cells in wild type and biosynthetic max mutants but not in max2 (Crawford et al, 2010). These results suggest that SLs dampen the polar auxin transport (PAT) stream in a MAX2-dependent manner (Crawford et al, 2010)
Summary
Plant hormones are small molecules derived from various essential metabolic pathways. SLs were found to play a key role in shoot branching inhibition and were identified as a new group of plant hormones (Gomez-Roldan et al, 2008; Umehara et al, 2008) Their biological functions were further explored and it was discovered that they exert their effects on different developmental processes including root development, seed germination, hypocotyl elongation, and secondary growth. Several mutants with increased shoot branching phenotype have been identified in several plant species, including more axillary growth (max) in Arabidopsis (Arabidopsis thaliana), ramosus (rms) in pea (Pisum sativum), dwarf (d) or high-tillering dwarf (htd) in rice (Oryza sativa), and decreased apical dominance (dad) in petunia (Petunia hybrida) All these mutants are defective in SL biosynthesis or signaling. More aspects about SLs biosynthesis, perception, and signaling as well as structure-function relationships have been nicely addressed and updated in several recent reviews
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