Cyclopropene derivatives have been used as extremely reactive units in organic chemistry owing to their high ring-strain energy. They have become popular reagents both for bioorthogonal chemistry and for chemical biology because of their small size and ability to be genetically encoded. In this context, we conducted an exploratory study to identify the biologically active cyclopropenes that affect normal plant growth. We synthesized several cycloprop-2-ene-1-carboxylic acid derivatives and evaluated their effects on the early growth stage of Arabidopsis thaliana. Eventually, we identified the chemicals that affect apical hook development in Arabidopsis thaliana. Their mode of action is different from those of ethylene receptor inhibition and gibberellin biosynthesis inhibition. We expect that some of the chemicals reported here can be new tools in chemical biology to determine useful molecular targets for herbicides or plant growth regulators.

Plant growth and development are coordinated by multiple environmental and endogenous signals. Brassinosteroid (BR) and ethylene (ET) have overlapping functions in a wide range of developmental processes. However, the relationship between the BR and ET signalling pathways has remained unclear. Here, we show that BR and ET interdependently promote apical hook development and cell elongation through a direct interaction between BR-activated BRASSINOZALE-RESISTANT1 (BZR1) and ET-activated ETHYLENE INSENSITIVE3 (EIN3). Genetic analysis showed that BR signalling is required for ET promotion of apical hook development in the dark and cell elongation under light, and ET quantitatively enhances BR-potentiated growth. BZR1 interacts with EIN3 to co-operatively increase the expression of HOOKLESS1 and PACLOBUTRAZOL RESISTANCE FACTORs (PREs). Furthermore, we found that BR promotion of hook development requires gibberellin (GA), and GA restores the hookless phenotype of BR-deficient materials by activating EIN3/EIL1. Our findings shed light on the molecular mechanism underlying the regulation of plant development by BR, ET and GA signals through the direct interaction of master transcriptional regulators.

The apical hook is a crucial structure during seedling development in dicotyledonous plants. It protects the fragile shoot meristem during its journey toward the surface from constraints imposed by the surrounding soil, which safeguards seedling emergence. Emerging evidence sheds light on the regulation of hook development through mechanochemical constraints.

Dicotyledonous plants form an apical hook to protect the fragile apical meristem during upward protrusion from the soil. Etiolated pifq (pif1 pif3 pif4 pif5) seedlings display constitutive apical hook opening. Here, we show that PIF proteins control apical hook opening by regulating the expression of Budding Uninhibited by Benzimidazole 3.1 (BUB3.1) and affecting cytokinesis. Consistent with the major function of BUB3.1 in the organization of phragmoplasts during cytokinesis, the phragmoplasts are well formed in dark-grown pifq but not in wild type. DNA staining and flow cytometry analysis further demonstrate that cellular endoreduplication levels are dramatically reduced in pifq. Chemical treatment with caffeine, an inhibitor of phragmoplast-based cytokinesis, shows that cytokinesis is involved in the apical hook opening. Genetically, BUB3.1 is epistatic to PIFq in the regulation of cytokinesis. Our findings reveal an organ-specific role of PIF proteins in regulating cytokinesis by BUB3.1 during apical hook development.

Apical hook formation is essential for the emergence and stand establishment of cotton plants. Searching for agronomic measures to regulate apical hook formation and clarifying its mechanism are important for full stand establishment in cotton. In this study, cotton seeds were sown at varying seeding rates or depths in sand to determine if and how apical hook formation was regulated by seeding rates or depths. The results showed that deep seeding or low seeding rates increased mechanical pressure and then increased ethylene content by increasing GhACO1 and GhACS2 expression to improve apical hook formation. Silencing of the GhACO1 and GhACS2 genes or exogenous application of 1-methylcyclopropene (1-MCP) decreased the ethylene content and inhibited apical hook formation in the cotton seedlings. Deep seeding, a low seeding rate, or 1-amino cyclopropane-1-carboxylic acid (ACC) treatment increased the expression of GhHLS1 and GhPIF3 genes, but their expression was decreased in theVIGS-ACO1 and VIGS-ACS2 seedlings. Silencing of the GhHLS1 and GhPIF3 genes inhibited apical hook formation, although the expression of GhACO1 and GhACS2 was unchanged. GhPIF3 may act upstream of GhHLS1, as the expression of GhPIF3 in the VIGS-HLS1 seedlings was unchanged, while the expression of GhHLS1 in the VIGS-PIF3 seedlings decreased. These results suggested that raised mechanical pressure could increase ethylene content by inducing GhACO1 and GhACS2 gene expression, which promoted apical hook formation by increasing the expression of GhHLS1. Therefore, adjusting the mechanical pressure through changing the seeding depth or seeding rate is an important means to regulate apical hook formation and emergence.

Plant cell elongation and expansion require the biosynthesis and remodeling of cell wall composition. Recently, Aryal et al. reported how feedback between the cell wall and the auxin response controls differential growth in apical hook development.

Tissue bending is vital to plant development, as exemplified by apical hook formation during seedling emergence by bending of the hypocotyl. How tissue bending is coordinated during development remains poorly understood, especially in plants where cells are attached via rigid cell walls. Asymmetric distribution of the plant hormone auxin underlies differential cell elongation during apical hook formation. Yet the underlying mechanism remains unclear. Here, we demonstrate spatial correlation between asymmetric auxin distribution, methylesterified homogalacturonan (HG) pectin, and mechanical properties of the epidermal layer of the hypocotyl in Arabidopsis. Genetic and cell biological approaches show that this mechanochemical asymmetry is essential for differential cell elongation. We show that asymmetric auxin distribution underlies differential HG methylesterification, and conversely changes in HG methylesterification impact the auxin response domain. Our results suggest that a positive feedback loop between auxin distribution and HG methylesterification underpins asymmetric cell wall mechanochemical properties to promote tissue bending and seedling emergence.

Ethylene is a gaseous phytohormone that is perceived by two-component histidine kinase-type receptors. Recent studies identified choline transporter-like 1 (CTL1) essential for Arabidopsis growth and development, including apical hook development in the etiolated seedlings. Here, we report that CTL1 contributes to apical hook development by enhancing ethylene response. The expression of CTL1 was highly correlated with the intensity of ethylene response and was enriched in the apical hook, cotyledon tip and hypocotyl. Genetic analysis showed that the dark-grown ctl1 mutant displayed a defect in ethylene-induced apical hook development as compared with the wild type. Accordingly, the expression of ethylene signaling reporter EBS::GUS in ctl1 mutant was greatly reduced in leaves, apical hook, hypocotyl and root, suggesting that the disruption of CTL1 impairs the ethylene signaling. Furthermore, protein-protein interaction assays demonstrated that CTL1 may interact with ethylene receptors, including ETR1, ETR2, ERS1, ERS2. Importantly, the abundance of CTL1 was diminished when ETR1 was disrupted upon ethylene response. Taken together, our results suggest that CTL1 functions as a positive regulator in ethylene signaling which in turn contributes to apical hook development of etiolated plant seedlings.

Apical hook curvature is crucial for buried seedling survival and a superb model for dissecting differential cell growth. HOOKLESS1 (HLS1) is essential for apical hook formation, acting as a hub integrating various external and internal signals. However, its functional mechanism remains unclear. Here, we demonstrate that HLS1 protein is present as an oligomer in the nucleus of dark-grown seedlings. Oligomerization is required for HLS1 activation, as the mutated HLS1 protein abolishing self-association exists as nonfunctional monomers. Upon light exposure, photoreceptor phyB translocates into the nucleus and interacts with HLS1, disrupting the self-association and oligomerization of HLS1 to initiate hook unfolding. Remarkably, genetic expression of nuclear-localized phyB is sufficient to inactivate HLS1, resulting in compromised hook curvature in etiolated seedlings. Together, we conclude that HLS1 protein is active as oligomeric form in darkness and achieves allosteric photo-deactivation upon light, providing intriguing mechanistic insight into the molecular switch for developmental transition.

The apical hook is a transiently formed structure that plays a protective role when the germinating seedling penetrates through the soil towards the surface. Crucial for proper bending is the local auxin maxima, which defines the concave (inner) side of the hook curvature. As no sign of asymmetric auxin distribution has been reported in embryonic hypocotyls prior to hook formation, the question of how auxin asymmetry is established in the early phases of seedling germination remains largely unanswered. Here, we analyzed the auxin distribution and expression of PIN auxin efflux carriers from early phases of germination, and show that bending of the root in response to gravity is the crucial initial cue that governs the hypocotyl bending required for apical hook formation. Importantly, polar auxin transport machinery is established gradually after germination starts as a result of tight root-hypocotyl interaction and a proper balance between abscisic acid and gibberellins.This article has an associated \'The people behind the papers\' interview.