PIF4 interacts with ABI4 to serve as a transcriptional activator complex to promote seed dormancy by enhancing ABA biosynthesis and signaling.

Transcriptional regulation plays a key role in the control of seed dormancy, and many transcription factors (TFs) have been documented. However, the mechanisms underlying the interactions between different TFs within a transcriptional complex regulating seed dormancy remain largely unknown. Here, we showed that TF PHYTOCHROME-INTERACTING FACTOR4 (PIF4) physically interacted with the abscisic acid (ABA) signaling responsive TF ABSCISIC ACID INSENSITIVE4 (ABI4) to act as a transcriptional complex to promote ABA biosynthesis and signaling, finally deepening primary seed dormancy. Both pif4 and abi4 single mutants exhibited a decreased primary seed dormancy phenotype, with a synergistic effect in the pif4/abi4 double mutant. PIF4 binds to ABI4 to form a heterodimer, and ABI4 stabilizes PIF4 at the protein level, whereas PIF4 does not affect the protein stabilization of ABI4. Subsequently, both TFs independently and synergistically promoted the expression of ABI4 and NCED6, a key gene for ABA anabolism. The genetic evidence is also consistent with the phenotypic, physiological and biochemical analysis results. Altogether, this study revealed a transcriptional regulatory cascade in which the PIF4-ABI4 transcriptional activator complex synergistically enhanced seed dormancy by facilitating ABA biosynthesis and signaling.

The ABI4-RGL2 module serves as a double agent to mediate the antagonistic crosstalk between ABA and GA signals.

Abscisic acid (ABA) and gibberellins (GA) antagonistically mediate several biological processes, including seed germination, but the molecular mechanisms underlying ABA/GA antagonism need further investigation, particularly any role mediated by a transcription factors module. Here, we report that the DELLA protein RGL2, a repressor of GA signaling, specifically interacts with ABI4, an ABA signaling enhancer, to act as a transcription factor complex to mediate ABA/GA antagonism. The rgl2, abi3, abi4 and abi5 mutants rescue the non-germination phenotype of the ga1-t. Further, we demonstrate that RGL2 specifically interacts with ABI4 to form a heterodimer. RGL2 and ABI4 stabilize one another, and GA increases the ABI4-RGL2 module turnover, whereas ABA decreases it. At the transcriptional level, ABI4 enhances the RGL2 expression by directly binding to its promoter via the CCAC cis-element, and RGL2 significantly upregulates the transcriptional activation ability of ABI4 toward its target genes, including ABI5 and RGL2. Abscisic acid promotes whereas GA inhibits the ability of ABI4-RGL2 module to activate transcription, and ultimately ABA and GA antagonize each other. Genetic analysis demonstrated that both ABI4 and RGL2 are essential for the activity of this transcription factor module. These results suggest that the ABI4-RGL2 module mediates ABA/GA antagonism by functioning as a double agent.

Seedling establishment: The neglected trait in the seed longevity field.

Seed longevity is a central actor in plant germplasm resource conservation, species reproduction, geographical distribution, crop yield and quality and food processing and safety. Seed longevity and vigor decrease gradually during storage, which directly influences seed germination and post-germination seedling establishment. It is noted that seedling establishment is a key shift from heterotropism to autotropism and is fueled by the energy reserved in the seeds per se. Numerous studies have demonstrated that expedited catabolism of triacylglycerols, fatty acid and sugars during seed storage is closely related to seed longevity. Storage of farm-saved seeds of elite cultivars for use in subsequent years is a common practice and it is recognized that aged seed (especially those stored under less-than-ideal conditions) can lead to poor seed germination, but the significance of poor seedling establishment as a separate factor capable of influencing crop yield has been overlooked. This review article summarizes the relationship between seed germination and seedling establishment and the effect of different seed reserves on seed longevity. Based on this, we emphasize the importance of simultaneous scoring of seedling establishment and germination percentage from aged seeds and discuss the reasons.

Seed Dormancy and Longevity: A Mutual Dependence or a Trade-Off?

Seed dormancy is an important agronomic trait in cereals and leguminous crops as low levels of seed dormancy during harvest season, coupled with high humidity, can cause preharvest sprouting. Seed longevity is another critical trait for commercial crop propagation and production, directly influencing seed germination and early seedling establishment. Both traits are precisely regulated by the integration of genetic and environmental cues. Despite the significance of these two traits in crop production, the relationship between them at the molecular level is still elusive, even with contradictory conclusions being reported. Some studies have proposed a positive correlation between seed dormancy and longevity in association with differences in seed coat permeability or seed reserve accumulation, whereas an increasing number of studies have highlighted a negative relationship, largely with respect to phytohormone-dependent pathways. In this review paper, we try to provide some insights into the interactions between regulatory mechanisms of genetic and environmental cues, which result in positive or negative relationships between seed dormancy and longevity. Finally, we conclude that further dissection of the molecular mechanism responsible for this apparently contradictory relationship between them is needed.

Repressors: the gatekeepers of phytohormone signaling cascades.

Coordinated phytohormone signal transduction, in which repressors are the key players, is essential to balance plant development and stress response. In the absence of phytohormones, repressors interplay to terminate the transcription of phytohormone-responsive genes. For phytohormone signal transduction, degradation or inactivation of the repressors is a prerequisite, a process in which proteasomal degradation or protein modifications, such as phosphorylation, are involved. In this review, we summarize the various repressor proteins and their methods of regulation. In addition, we also shed light on other post-transcriptional modifications, including protein sumoylation, acetylation, methylation, and S-nitrosylation, which might be involved in repressor regulation. We conclude that repressors are the gatekeepers of phytohormone signaling, allowing transcription of phytohormone-responsive genes only when required and thus serving as a universal mechanism to conserve energy in plants. Finally, we strongly recommend that plant research should be focused further on elucidating the mechanisms regulating repressor abundance or activity, to improve our understanding of phytohormone signal transduction.

The R2R3-MYB transcription factor BnaMYB111L from rapeseed modulates reactive oxygen species accumulation and hypersensitive-like cell death.

As one of the largest families of transcription factors in plants, the R2R3-MYB proteins play important roles in diverse biological processes including growth and development, primary and secondary metabolism such as flavonoid and anthocyanin biosynthesis as well as abiotic and biotic stress responses. However, functions of R2R3-MYB genes in rapeseed (Brassica napus L.) remain elusive. Here, we characterized BnaMYB111L, which is homologous to Arabidopsis MYB111 and encodes an R2R3-MYB protein in rapeseed. BnaMYB111L is responsive to abscisic acid (ABA), heat, cold, hydrogen peroxide and fungal pathogen Sclerotinia scelerotiorum treatments through quantitative RT-PCR assay. BnaMYB111L encodes a transcriptional activator and is localized exclusively to nuclei. Interestingly, overexpression of BnaMYB111L in tobacco (Nicotiana benthamiana) and rapeseed protoplasts promoted reactive oxygen species (ROS) production and hypersensitive response-like cell death, accumulation of malondialdehyde (MDA) as well as degradation of chlorophyll. Furthermore, BnaMYB111L expression evoked the alterations of transcript levels of genes encoding ROS-producing enzyme, vacuolar processing enzymes and genes implicated in defense responses. A further dual luciferase reporter assay indicated that BnaMYB111L activated the expression of RbohB, MC4 and ACRE132, which are involved in ROS generation, cell death as well as defense responses. Taken together, this study characterized the function of rapeseed MYB111L and identified its putative target genes involved in ROS production and cell death.