Abstract
Strigolactones (SLs) and karrikins (KARs) are both butenolide molecules that play essential roles in plant growth and development. SLs are phytohormones, with SLs having known functions within the plant they are produced in, while KARs are found in smoke emitted from burning plant matter and affect seeds and seedlings in areas of wildfire. It has been suggested that SL and KAR signaling may share similar mechanisms. The α/β hydrolases DWARF14 (D14) and KARRIKIN INSENSITIVE 2 (KAI2), which act as receptors of SL and KAR, respectively, both interact with the F-box protein MORE AXILLARY GROWTH 2 (MAX2) in order to target SUPPRESSOR OF MAX2 1 (SMAX1)-LIKE/D53 family members for degradation via the 26S proteasome. Recent reports suggest that SLs and/or KARs are also involved in regulating plant responses and adaptation to various abiotic stresses, particularly nutrient deficiency, drought, salinity, and chilling. There is also crosstalk with other hormone signaling pathways, including auxin, gibberellic acid (GA), abscisic acid (ABA), cytokinin (CK), and ethylene (ET), under normal and abiotic stress conditions. This review briefly covers the biosynthetic and signaling pathways of SLs and KARs, compares their functions in plant growth and development, and reviews the effects of any crosstalk between SLs or KARs and other plant hormones at various stages of plant development. We also focus on the distinct responses, adaptations, and regulatory mechanisms related to SLs and/or KARs in response to various abiotic stresses. The review closes with discussion on ways to gain additional insights into the SL and KAR pathways and the crosstalk between these related phytohormones.
Highlights
Environmental constraints, both biotic and abiotic, can deliver deleterious effects to both plant survival and crop productivity [1,2,3]
In the ethylene signaling-deficient ein2 and etr1 mutants, the root hair elongation response to SLs is reduced. These results indicated that the synthesis of ethylene was necessary for the promotion of root hair elongation by SLs, while it is possible that auxin signaling could be integrated with SL and ethylene signaling to regulate the elongation of root hairs [145]
Significant progress has been made towards an understanding of how the SL and KAR signaling pathways influence plant developmental and environmental responses
Summary
Environmental constraints, both biotic and abiotic, can deliver deleterious effects to both plant survival and crop productivity [1,2,3]. The biosynthesis and signaling of SLs are regulated by various abiotic stress factors [37,38,39,40], including the recently reported SL involvement in responding to nutrient deprivation, drought, chilling and salinity [38,40,41,42,43,44,45,46,47,48,49,50] Such studies provide new insights into the novel roles SL signaling plays in the regulation of plant adaptation to adverse environmental conditions [51,52,53,54]. The crosstalk between SLs, KARs and other phytohormones under adverse conditions are discussed
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