Greening immediately after etiolated- seedling's emergence from the soil is critical for plants to initiate their autotrophic life cycle through photosynthesis. The greening process relies on a complex transcriptional network that fine- tunes the biosynthesis of chlorophyll and prevents premature development of chloroplasts. In this study, we identified the Arabidopsis HOOKLESS1 (HLS1) as a key regulator of light- induced cotyledon greening. Our results demonstrated that HLS1 is essential for the proper expression of greening- related genes controlling chlorophyll biosynthesis and chloroplast development. Loss of HLS1 severely disrupts the Pchlide- to- Chlide transition and impairs reactive oxygen species (ROS) scavenging in etiolated seedlings upon light exposure, leading to catastrophic ROS burst and even photobleaching. Biochemical assays revealed that HLS1 is a histone acetyltransferase mediating the deposition of H3K9ac and H3K27ac marks at multiple greening- related genes, thereby promoting their transcriptional activation. Genetic analysis further confirmed that HLS1's promotive effect on the greening process is fully dependent on its histone acetyltransferase activity. Moreover, the loss of HLS1 also interrupts the promotive effect of ethylene signaling on the greening process by reducing the binding of ETHYLENE- INSENSITIVE 3 to the promoter region of POR genes, thus inhibiting the activation effect of ethylene signaling on the expression of PORs. Collectively, our study reveals that HLS1 acetylates histones to activate greening- related genes, optimizing chlorophyll biosynthesis and chloroplast development during dark- to- light transition in seedlings.

Background and Aims Soil salinization adversely threatens plant survival and food production globally. The mobilization of storage reserves in cotyledons and establishment of the hypocotyl/root axis (HRA) structure and function are crucial to the growth of dicotyledonous plants during the post-germination growth period. Here we report the adaptive mechanisms of wild and cultivated soybeans in response to alkali stress in soil during the post-germination growth period.Methods Differences in physiological parameters, microstructure, and the types, amounts and metabolic pathways of small-molecule metabolites and gene expression were compared and multi-omics integration analysis was performed between wild and cultivated soybean under sufficient and artificially simulated alkali stress during the post-germination growth period in this study.Key Results Structural analysis showed that the cell wall thickness of wild soybean under alkali stress increased, whereas cultivated soybeans were severely damaged. A comprehensive analysis of small-molecule metabolites and gene expression revealed that protein breakdown in wild soybean cotyledons under alkali stress was enhanced, and transport of amino acids and sucrose increased. Additionally, lignin and cellulose syntheses in wild soybean HRA under alkali stress were enhanced.Conclusions Overall, protein decomposition and transport of amino acids and sucrose increased in wild soybean cotyledons under alkali stress, which in turn promoted HRA growth. Similarly, alkali stress enhanced lignin and cellulose synthesis in the wild soybean HRA, which subsequently enhanced cell wall synthesis, thereby maintaining the stability and functionality of the HRA under alkali stress. This study presents important practical implications for the utilization of wild plant resources and sustainable development of agriculture.