Expansion and Functional Diversification of TFIIB-Like Factors in Plants

A critical role of a plant-specific TFIIB-related protein, BRP1, in salicylic acid-mediated immune response

An integral part of plant immunity is transcription reprogramming by concerted action of specific transcription factors that activate or repress genes through recruitment or release of RNA polymerase II (Pol II). Pol II is assembled into Pol II holoenzyme at the promoters through association with a group of general transcription factors including transcription factor IIB (TFIIB) to activate transcription. Unlike other eukaryotic organisms, plants have a large family of TFIIB-related proteins with 15 members in Arabidopsis including several plant-specific TFIIB-related proteins (BRPs). Molecular genetic analysis has revealed important roles of some BRPs in plant reproductive processes. In this study, we report that Arabidopsis knockout mutants for BRP1, the founding member of the BRP protein family, were normal in growth and development, but were hypersusceptible to the bacterial pathogen Psuedomonas syringae. The enhanced susceptibility of the brp1 mutants was associated with reduced expression of salicylic acid (SA) biosynthetic gene ISOCHORISMATE SYNTHASE 1 (ICS1) and SA-responsive PATHOGENESIS-RELATED (PR) genes. Pathogen-induced SA accumulation was reduced in the brp1 mutants and exogenous SA rescued the brp1 mutants for resistance to the bacterial pathogen. In uninfected plants, BRP1 was primarily associated with the plastids but pathogen infection induced its accumulation in the nucleus. BRP1 acted as a transcription activator in plant cells and binded to the promoter of ICS1. These results collectively indicate that BRP1 is a functionally specialized transcription factor that increasingly accumulates in the nucleus in response to pathogen infection to promote defense gene expression.

Transcription Factors and Their Regulatory Roles in the Male Gametophyte Development of Flowering Plants

Male gametophyte development in plants relies on the functions of numerous genes, whose expression is regulated by transcription factors (TFs), non-coding RNAs, hormones, and diverse environmental stresses. Several excellent reviews are available that address the genes and enzymes associated with male gametophyte development, especially pollen wall formation. Growing evidence from genetic studies, transcriptome analysis, and gene-by-gene studies suggests that TFs coordinate with epigenetic machinery to regulate the expression of these genes and enzymes for the sequential male gametophyte development. However, very little summarization has been performed to comprehensively review their intricate regulatory roles and discuss their downstream targets and upstream regulators in this unique process. In the present review, we highlight the research progress on the regulatory roles of TF families in the male gametophyte development of flowering plants. The transcriptional regulation, epigenetic control, and other regulators of TFs involved in male gametophyte development are also addressed.

TFIIB-Related Protein BRP5/PTF2 Is Required for Both Male and Female Gametogenesis and for Grain Formation in Rice

Transcription factor IIB (TFIIB) is a general transcription factor for RNA polymerase II, exerting its influence across various biological contexts. In the majority of eukaryotes, TFIIB typically has two homologs, serving as general transcription factors for RNA polymerase I and III. In plants, however, the TFIIB-related protein family has expanded greatly, with 14 and 9 members in Arabidopsis and rice, respectively. BRP5/pollen-expressed transcription factor 2 (PTF2) proteins belong to a subfamily of TFIIB-related proteins found only in plants and algae. The prior analysis of an Arabidopsis atbrp5 mutant, characterized by a T-DNA insertion at the 5' untranslated region, demonstrated the essential role of BRP5/PTF2 during the process of pollen germination and embryogenesis in Arabidopsis. Using a rice transformation system based on CRISPR/Cas9 technology, we have generated transgenic rice plants containing loss-of-function frameshift mutations in the BRP5/PTF2 gene. Unlike in the Arabidopsis atbrp5 mutant, the brp5/ptf2 frameshift mutations were not transmitted to progeny in rice, indicating an essential role of BRP5/PTF2 in both male and female gamete development or viability. The silencing of rice BRP5/PTF2 expression through RNA interference (RNAi) had little effect on vegetative growth and panicle formation but strongly affected pollen development and grain formation. Genetic analysis revealed that strong RNAi silencing of rice BRP5/PTF2 was still transmissible to progeny almost exclusively through female gametes, as found in the Arabidopsis atbrp5 knockdown mutant. Thus, reduced rice BRP5/PTF2 expression impacted pollen preferentially by interfering with male gamete development or viability. Drawing upon these findings, we posit that BRP5/PTF2 assumes a distinct and imperative function in the realm of plant sexual reproduction.

New Insights into the Mechanism of Trichoderma virens-Induced Developmental Effects on Agrostis stolonifera Disease Resistance against Dollar Spot Infection

Trichoderma is internationally recognized as a biocontrol fungus for its broad-spectrum antimicrobial activity. Intriguingly, the crosstalk mechanism between the plant and Trichoderma is dynamic, depending on the Trichoderma strains and the plant species. In our previous study, the Trichoderma virens 192-45 strain showed better pathogen inhibition through the secretive non-volatile and volatile substrates. Therefore, we studied transcriptional and metabolic responses altered in creeping bentgrass (Agrostis stolonifera L.) with T. virens colonization prior to a challenge with Clarireedia homoeocarpa. This fungal pathogen causes dollar spot on various turfgrasses. When the pathogen is deficient, the importance of T. virens to the enhancement of plant growth can be seen in hormonal production and microbe signaling, such as indole-3-acrylic acid. Therefore, these substrates secreted by T. virens and induced genes related to plant growth can be the ‘pre-defense’ for ensuing pathogen attacks. During C. homoeocarpa infection, the Trichoderma–plant interaction activates defense responses through the SA- and/or JA-dependent pathway, induced by T. virens and its respective exudates, such as oleic, citric, and stearic acid. Thus, we will anticipate a combination of genetic engineering and exogenous application targeting these genes and metabolites, which could make creeping bentgrass more resistant to dollar spot and other pathogens.

The Mediator Complex: A Central Coordinator of Plant Adaptive Responses to Environmental Stresses

As sessile organisms, plants are constantly exposed to a variety of environmental stresses and have evolved adaptive mechanisms, including transcriptional reprogramming, in order to survive or acclimate under adverse conditions. Over the past several decades, a large number of gene-specific transcription factors have been identified in the transcriptional regulation of plant adaptive responses. The Mediator complex plays a key role in transducing signals from gene-specific transcription factors to the transcription machinery to activate or repress target gene expression. Since its first purification about 15 years ago, plant Mediator complex has been extensively analyzed for its composition and biological functions. Mutants of many plant Mediator subunits are not lethal but are compromised in growth, development and response to biotic and abiotic stress, underscoring a particularly important role in plant adaptive responses. Plant Mediator subunits also interact with partners other than transcription factors and components of the transcription machinery, indicating the complexity of the regulation of gene expression by plant Mediator complex. Here, we present a comprehensive discussion of recent analyses of the structure and function of plant Mediator complex, with a particular focus on its roles in plant adaptive responses to a wide spectrum of environmental stresses and associated biological processes.