Abstract
Polar auxin transport (PAT) is facilitated by polar localization of PIN-FORMED (PIN) efflux carriers, which direct auxin flow and regulate developmental events. Brassinosteroids (BRs) and auxin work synergistically to promote growth, and in root geotropisms this cross-talk involves BR-directed polarization of PIN through the mobilization of F-actin. However, the role of BR in PAT during shoot growth, hair formation, and embryogenesis has not been well studied. Orchid seed are mature at a point in development that is analogous to the globular-stage of embryogenesis in typical angiosperms. Thus, this system provided a unique opportunity to study the effects of BR on PAT during embryogenesis-like events, including meristem/first leaf formation and protocorm/stem development, which is followed by protocorm hair formation. In this work, the degree to which BRs rescued embryo-like protocorms from the impact of PAT-disrupting agents, such as PAT inhibitors or high auxin levels, was determined based on growth responses. This study first established that auxin and BRs work together synergistically to promote seedling elongation in Spathoglottis. Repressed seedling growth caused by the PAT-disrupting agents was alleviated with eBL, suggesting that BRs enhance PAT in embryogenesis-like stages of young protocorms. However, similar responses were not evident in seed embryos. Results from this study also suggested that BRs may enhance orchid protocorm elongation by regulating auxin transport through an F-actin-mediated mechanism. With regard to protocorm hairs, increased eBL levels inhibited formation, whereas reduced BR biosynthesis altered hair patterning, and prevented outgrowth of auxin-stimulated hairs. Moreover, PAT inhibitors and repression of BR biosynthesis caused hair bud formation without hair outgrowth, suggesting a role for BR in PAT during protocorm hair development.
Highlights
Brassinosteriods (BRs) are an essential class of plant growth regulators, of which brassinolide is deemed the most bioactive
Epibrassinolide and Auxin Synergistically Promoted Elongation in Young Protocorms, and Orchid Seed Germination Was Repressed by Epibrassinolide and Brassinazole
Epibrassinolide alone promoted elongation of the protocorm, and to a lesser degree, the leaf, when compared to the control seedlings. Consistent with this result, inhibition of brassinosteroid biosynthesis with BRZ treatment had an inhibitory effect on protocorm and leaf development, causing severely stunted leaves and reduced protocorm widths and lengths (Figure 1B)
Summary
Brassinosteriods (BRs) are an essential class of plant growth regulators, of which brassinolide is deemed the most bioactive. BRs exhibit cross-talk with auxin in regulating plant development, often through a synergistic relationship (Tian et al, 2018). This interdependence is evident in common gene targets for BR and auxin (Nemhauser et al, 2006; Chung et al, 2011; Walcher and Nemhauser, 2012), the induction of auxin biosynthesis by brassinolide (Chung et al, 2011; Maharjan et al, 2014), as well as a role for BRs in the directional flow of auxin through PIN-FORMED (PIN) protein (Li et al, 2005). In addition to continuous cycling of PIN through endocytosis, PIN polarity is thought to occur by clustering of the PIN protein and varying phosphorylation states of PIN with AGC kinases, PINOID (PID), PID-related AGC3, and the phosphatase PP2A, but details of the mechanism are not entirely understood (Michniewicz et al, 2007; Baluska et al, 2008; Dhonukshe et al, 2010; Huang et al, 2010; Adamowski and Friml, 2015)
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