Gibberellin (GA), a diterpene hormone, plays diverse roles in plant growth and development, including seed germination, stem elongation, and flowering time. Although it is known that GA accelerates flowering through degradation of transcription repressors, DELLAs, the underlying mechanism is poorly understood. We show here that DELLA directly binds to microRNA156 (miR156)-targeted SQUAMOSA PROMOTER BINDING–LIKE (SPL) transcription factors, which promote flowering by activating miR172 and MADS box genes. The interaction between DELLA and SPL interferes with SPL transcriptional activity and consequently delays floral transition through inactivating miR172 in leaves and MADS box genes at shoot apex under long-day conditions or through repressing MADS box genes at the shoot apex under short-day conditions. Our results elucidate the molecular mechanism by which GA controls flowering and provide the missing link between DELLA and MADS box genes.

Seed dormancy is an important economic trait for agricultural production. Abscisic acid (ABA) and Gibberellins (GA) are the primary factors that regulate the transition from dormancy to germination, and they regulate this process antagonistically. The detailed regulatory mechanism involving crosstalk between ABA and GA, which underlies seed dormancy, requires further elucidation. Here, we report that ABI4 positively regulates primary seed dormancy, while negatively regulating cotyledon greening, by mediating the biogenesis of ABA and GA. Seeds of the Arabidopsis abi4 mutant that were subjected to short-term storage (one or two weeks) germinated significantly more quickly than Wild-Type (WT), and abi4 cotyledons greened markedly more quickly than WT, while the rates of germination and greening were comparable when the seeds were subjected to longer-term storage (six months). The ABA content of dry abi4 seeds was remarkably lower than that of WT, but the amounts were comparable after stratification. Consistently, the GA level of abi4 seeds was increased compared to WT. Further analysis showed that abi4 was resistant to treatment with paclobutrazol (PAC), a GA biosynthesis inhibitor, during germination, while OE-ABI4 was sensitive to PAC, and exogenous GA rescued the delayed germination phenotype of OE-ABI4. Analysis by qRT-PCR showed that the expression of genes involved in ABA and GA metabolism in dry and germinating seeds corresponded to hormonal measurements. Moreover, chromatin immunoprecipitation qPCR (ChIP-qPCR) and transient expression analysis showed that ABI4 repressed CYP707A1 and CYP707A2 expression by directly binding to those promoters, and the ABI4 binding elements are essential for this repression. Accordingly, further genetic analysis showed that abi4 recovered the delayed germination phenotype of cyp707a1 and cyp707a2 and further, rescued the non-germinating phenotype of ga1-t. Taken together, this study suggests that ABI4 is a key factor that regulates primary seed dormancy by mediating the balance between ABA and GA biogenesis. Seed dormancy prevents or delays germination in maturated seeds. The optimal level of seed dormancy is a valuable trait for agricultural production and post-harvest management. High ABA and low GA content in seeds promote seed dormancy. However, the precise molecular mechanisms controlling seed dormancy and germination remain unclear. We found that ABI4, the key transcription factor in the ABA signaling pathway, indeed controls primary seed dormancy. This result contradicts the previous conclusion that ABI4 is not involved in the control of seed dormancy. Several lines of evidence support our conclusion. For example, detailed physiological analysis of the germination of abi4 seeds that were harvested immediately and stored for various periods of time and subjected to various treatments allowed us to conclude that ABI4 negatively regulates primary seed dormancy. The molecular mechanism responsible for this control is as follows: ABI4 directly or indirectly regulates the key genes of the ABA and GA biogenesis pathways, which then regulates the ABA and GA contents in seeds. Importantly, further genetic interactions between CYP707A1, CYP707A2, GA1, and ABI4 also support our conclusion.

The phytohormone cytokinin (CK) positively regulates the activity and function of the shoot apical meristem (SAM), which is a major parameter determining seed production. The rice ( Oryza sativa L.) Gn1a / OsCKX2 ( Grain number 1a / Cytokinin oxidase 2 ) gene, which encodes a cytokinin oxidase, has been identified as a major quantitative trait locus contributing to grain number improvement in rice breeding practice. However, the molecular mechanism of how the expression of OsCKX2 is regulated in planta remains elusive. Here, we report that the zinc finger transcription factor DROUGHT AND SALT TOLERANCE (DST) directly regulates OsCKX2 expression in the reproductive meristem. DST-directed expression of OsCKX2 regulates CK accumulation in the SAM and, therefore, controls the number of the reproductive organs. We identify that DST ^reg1 , a semidominant allele of the DST gene, perturbs DST-directed regulation of OsCKX2 expression and elevates CK levels in the reproductive SAM, leading to increased meristem activity, enhanced panicle branching, and a consequent increase of grain number. Importantly, the DST ^reg1 allele provides an approach to pyramid the Gn1a -dependent and Gn1a -independent effects on grain production. Our study reveals that, as a unique regulator of reproductive meristem activity, DST may be explored to facilitate the genetic enhancement of grain production in rice and other small grain cereals.

Summary: Abscisic acid (ABA) and cytokinin are key hormones controlling plant development. How ABA and cytokinin interplay to control the transition from a dry seed into a young seedling remains elusive. Here we undertook a gain‐of‐function genetic screen to identify ABA‐insensitive mutants during seed germination in Arabidopsis using an estradiol‐inducible approach. In the presence of estradiol, one of these mutants gim1 (germination insensitive to ABA mutant 1) exhibited an elevated level of cytokinin that was attributed to the estradiol‐induced expression of AtIPT8 that encodes an isopentenyltransferase for the biosynthesis of cytokinins. Our data on OE‐2 and Com‐1 transgenic plants carrying the ectopically expressing AtIPT8 gene indicated that the elevation of cytokinin level was responsible for the ABA‐insensitivity of gim1 seed germination. Further analyses on alterations of gene transcriptomes in the gim1 mutant demonstrated that the expression of some ABA‐inducible genes, including ABI5, was reduced, and could not be restored by exogenous ABA treatment. Moreover, we also failed to observe the ABA‐mediated repression of a family of cytokinin signal transducers and transcription repressors called type‐A ARR4, ARR5 and ARR6 in the gim1 seedlings. Further analysis demonstrated that type‐A ARR4, ARR5 and ARR6 could negatively regulate ABI5 expression, and the physical interaction of ABI5 and type‐A ARR4, ARR5 and ARR6 proteins was detected. In summary, our study suggests that the interaction of ABA and cytokinin during seed germination and seedling growth can be mediated by the interplay of transcriptional regulators in Arabidopsis.

Significance: Insect pests represent a major constraint that reduces crop yield and quality globally. Host plant resistance is often used as a key tactic to control insect pests, but is frequently overcome by newly emerged insect populations. In nature, plants have developed various strategies for sustainable defense. In this work, we isolated a brown planthopper-resistance gene,BPH9, and show that alleles of this gene locus have been widely used in rice breeding and saved rice production from massive brown planthopper (BPH) damage. Allelic diversity in this gene locus has provided resistance to rice against different BPH populations. Manipulating allelic diversity of the gene may provide a strategy for developing resistant varieties to cope with evolving insect populations with new virulence variation.