Activation of HLS1 by Mechanical Stress via Ethylene-Stabilized EIN3 Is Crucial for Seedling Soil Emergence.

Xing Shen,Yanli Li,Shangwei Zhong,Ying Pan

doi:10.3389/fpls.2016.01571

Abstract

The seeds of terrestrial flowering plants often start their life cycle in subterranean darkness. To protect the fragile apical meristematic tissues and cotyledons from mechanical injuries during soil penetration, dicotyledonous seedlings form an elegant apical hook at the top of the hypocotyl. The apical hook has been considered as an adaption structure to the subterranean environment. However, the role of the apical hook in seedling emergence and the molecular mechanism of apical hook formation under real-life conditions remain highly speculative. Here, we find that HOOKLESS 1 (HLS1), a critical gene in apical hook formation in Arabidopsis thaliana, is required for seedling emergence from the soil. When grown under soil, hls1 mutant exhibits severe emergence defects. By contrast, HLS1 overexpression in the hls1 background fully restores emergence defects and displays better emergence capacity than that of WT. Our results indicate that HLS1 transcription is stimulated in response to the mechanical stress of soil cover, which is dependent on the function of the transcription factors ETHYLENE INSENSITIVE 3 (EIN3) and EIN3-LIKE 1 (EIL1). Soil-conferred mechanical stress activates the ethylene signaling pathway to stabilize EIN3 by repressing the activity of the F-box proteins EBF1 and EBF2. These combined results reveal a signaling pathway in which plant seedlings transduce the mechanical pressure of soil cover to correctly modulate apical hook formation during soil emergence.

Highlights

After seeds are released from the mother plant, they often become covered with leaves and soil, which protects them from adverse environmental conditions including cold temperature and seed predators
To determine whether the apical hook functionally promotes seedling soil emergence, we examined the emergence phenotypes of A. thaliana Col-0 (WT) and hls1 mutant seedlings when covered by a layer of soil
We showed that apical hook formation was regulated by soil cover through a linear signaling pathway, and played a pivotal role in mediating seedling emergence from the soil (Figure 7)

Summary

Introduction

After seeds are released from the mother plant, they often become covered with leaves and soil, which protects them from adverse environmental conditions including cold temperature and seed predators. When the environment becomes favorable for growth, seeds sense the changes and begin the process of germination (Penfield et al, 2005; Shi et al, 2013, 2015). Germinating seedlings in subterranean darkness undergo heterotrophic growth supported by seed reserves, and direct the apical tip toward the surface. Soil emergence is a vulnerable phase in the life cycle of terrestrial flowering plants, and is tightly controlled by both environmental signals and endogenous hormones (Zhong et al, 2014; de Wit et al, 2016). Seedlings grown in the dark display a characteristic phenotype named skotomorphogenesis, which is maintained by

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Conclusion