Summary: In early seedlings, the primary root adapts rapidly to environmental changes through the modulation of endogenous hormone levels. The phytohormone ethylene inhibits primary root elongation, but the underlying molecular mechanism of how ethylene‐reduced root growth is modulated in environmental changes remains poorly understood. Here, we show that a novel rice (Oryza sativa)DOFtranscription factor OsDOF15 positively regulates primary root elongation by regulating cell proliferation in the root meristem, via restricting ethylene biosynthesis. Loss‐of‐function ofOsDOF15impaired primary root elongation and cell proliferation in the root meristem, whereasOsDOF15overexpression enhanced these processes, indicating that OsDOF15 is a key regulator of primary root elongation. This regulation involves the direct interaction of OsDOF15 with the promoter ofOsACS1, resulting in the repression of ethylene biosynthesis. The control of ethylene biosynthesis by OsDOF15 in turn regulates cell proliferation in the root meristem.OsDOF15transcription is repressed by salt stress, and OsDOF15‐mediated ethylene biosynthesis plays a role in inhibition of primary root elongation by salt stress. Thus, our data reveal how the ethylene‐inhibited primary root elongation is finely controlled by OsDOF15 in response to environmental signal, a novel mechanism of plants responding to salt stress and transmitting the information to ethylene biosynthesis to restrict root elongation.

Rice (Oryza sativa) germination and seedling establishment, particularly in increasingly saline soils, are critical to ensure successful crop yields. Seed vigor, which determines germination and seedling growth, is a complex trait affected by exogenous (environmental) and endogenous (hormonal) factors. Here, we used genetic and biochemical analyses to uncover the role of an APETALA2-type transcription factor, SALT AND ABA RESPONSE ERF1 (OsSAE1), as a positive regulator of seed germination and salt tolerance in rice by repressing the expression of ABSCISIC ACID-INSENSITIVE5 (OsABI5). ossae1 knockout lines exhibited delayed seed germination, enhanced sensitivity to abscisic acid (ABA) during germination and in early seedling growth, and reduced seedling salt tolerance. OsSAE1 overexpression lines exhibited the converse phenotype, with increased seed germination and salt tolerance. In vivo and in vitro assays indicated that OsSAE1 binds directly to the promoter of OsABI5, a major downstream component of the ABA signaling pathway and acts as a major regulator of seed germination and stress response. Genetic analyses revealed that OsABI5-mediated ABA signaling functions downstream of OsSAE1. This study provides important insights into OsSAE1 regulation of seed vigor and salt tolerance and facilitates the practical use of OsSAE1 in breeding salt-tolerant varieties suitable for direct seeding cultivation.

During plant growth and development, ethylene and abscisic acid (ABA) play important roles and exert synergistic or antagonistic effects on various biological processes, but the detailed mechanism underlying the interaction of the two phytohormones, especially in the regulation of the accumulation of reactive oxygen species (ROS), is largely unclear. Here, we report that ethylene inhibits but ABA promotes the accumulation of ROS in Arabidopsis (Arabidopsis thaliana) seedlings. Furthermore, changes in the biosynthesis of ascorbic acid (AsA) act as a key factor in integrating the interaction of ethylene and ABA in the regulation of ROS levels. We found that ethylene and ABA antagonistically regulate AsA biosynthesis via ETHYLENE-INSENSITIVE3 (EIN3) and ABA INSENSITIVE4 (ABI4), which are key factors in the ethylene and ABA signaling pathways, respectively. In addition, ABI4 is transcriptionally repressed by EIN3 in ethylene-regulated AsA biosynthesis. Via transcriptome analysis and molecular and genetic experiments, we identified VITAMIN C DEFECTIVE2as the direct target of ABI4 in the regulation of AsA biosynthesis and ROS accumulation. Thus, the EIN3-ABI4- VITAMIN C DEFECTIVE2 transcriptional cascade involves a mechanism by which ethylene and ABA antagonistically regulate AsA biosynthesis and ROS accumulation in response to complex environmental stimuli.

Primary root growth in cereal crops is fundamental for early establishment of the seedling and grain yield. In young rice (Oryza sativa) seedlings, the primary root grows rapidly for 7–10 days after germination and then stops; however, the underlying mechanism determining primary root growth is unclear. Here, we report that the interplay of ethylene and gibberellin (GA) controls the orchestrated development of the primary root in young rice seedlings. Our analyses advance the knowledge that primary root growth is maintained by higher ethylene production, which lowers bioactive GA contents. Further investigations unraveled that ethylene signaling transcription factor ETHYLENE INSENSITIVE3-LIKE 1 (OsEIL1) activates the expression of the GA metabolism genes GIBBERELLIN 2-OXIDASE 1 (OsGA2ox1), OsGA2ox2, OsGA2ox3, and OsGA2ox5, thereby deactivating GA activity, inhibiting cell proliferation in the root meristem, and ultimately gradually inhibiting primary root growth. Mutation in OsGA2ox3 weakened ethylene-induced GA inactivation and reduced the ethylene sensitivity of the root. Genetic analysis revealed that OsGA2ox3 functions downstream of OsEIL1. Taken together, we identify a molecular pathway impacted by ethylene during primary root elongation in rice and provide insight into the coordination of ethylene and GA signals during root development and seedling establishment.

Seed germination, a critical stage initiating the life cycle of a plant, is severely affected by salt stress. However, the underlying mechanism of salt inhibition of seed germination (SSG) is unclear. Here, we report that the Arabidopsis (Arabidopsis thaliana) CONSTITUTIVE PHOTOMORPHOGENESIS1 (COP1) counteracts SSG. Genetic assays provide evidence that SSG in loss of function of the COP1 mutant was stronger than this in the wild type. A GUS-COP1 fusion was constitutively localized to the nucleus in radicle cells. Salt treatment caused COP1 to be retained in the cytosol, but the addition of ethylene precursor 1-aminocyclopropane-1-carboxylate had the reverse effect on the translocation of COP1 to the nucleus, revealing that ethylene and salt exert opposite regulatory effects on the localization of COP1 in germinating seeds. However, loss of function of the ETHYLENE INSENSITIVE3 (EIN3) mutant impaired the ethylene-mediated rescue of the salt restriction of COP1 to the nucleus. Further research showed that the interaction between COP1 and LONG HYPOCOTYL5 (HY5) had a role in SSG. Correspondingly, SSG in loss of function of HY5 was suppressed. Biochemical detection showed that salt promoted the stabilization of HY5, whereas ethylene restricted its accumulation. Furthermore, salt treatment stimulated and ethylene suppressed transcription of ABA INSENSITIVE5 (ABI5), which was directly transcriptionally regulated by HY5. Together, our results reveal that salt stress and ethylene antagonistically regulate nucleocytoplasmic partitioning of COP1, thereby controlling Arabidopsis seed germination via the COP1-mediated down-regulation of HY5 and ABI5. These findings enhance our understanding of the stress response and have great potential for application in agricultural production.

The ubiquitin-binding protein OsDSK2a modulates the level of the gibberellin metabolism regulator EUI to coordinate plant growth and salt-stress responses in ri