Control of OsARF3a by OsKANADI1 contributes to lemma development in rice

Enigmatic role of auxin response factors in plant growth and stress tolerance

Abiotic and biotic stresses globally constrain plant growth and impede the optimization of crop productivity. The phytohormone auxin is involved in nearly every aspect of plant development. Auxin acts as a chemical messenger that influences gene expression through a short nuclear pathway, mediated by a family of specific DNA-binding transcription factors known as Auxin Response Factors (ARFs). ARFs thus act as effectors of auxin response and translate chemical signals into the regulation of auxin responsive genes. Since the initial discovery of the first ARF in Arabidopsis, advancements in genetics, biochemistry, genomics, and structural biology have facilitated the development of models elucidating ARF action and their contributions to generating specific auxin responses. Yet, significant gaps persist in our understanding of ARF transcription factors despite these endeavors. Unraveling the functional roles of ARFs in regulating stress response, alongside elucidating their genetic and molecular mechanisms, is still in its nascent phase. Here, we review recent research outcomes on ARFs, detailing their involvement in regulating leaf, flower, and root organogenesis and development, as well as stress responses and their corresponding regulatory mechanisms: including gene expression patterns, functional characterization, transcriptional, post-transcriptional and post- translational regulation across diverse stress conditions. Furthermore, we delineate unresolved questions and forthcoming challenges in ARF research.

Acyl carrier protein OsMTACP2 confers rice cold tolerance at the booting stage

Low temperatures occurring at the booting stage in rice (Oryza sativa L.) often result in yield loss by impeding male reproductive development. However, the underlying mechanisms by which rice responds to cold at this stage remain largely unknown. Here, we identified MITOCHONDRIAL ACYL CARRIER PROTEIN 2 (OsMTACP2), the encoded protein of which mediates lipid metabolism involved in the cold response at the booting stage. Loss of OsMTACP2 function compromised cold tolerance, hindering anther cuticle and pollen wall development, resulting in abnormal anther morphology, lower pollen fertility, and seed setting. OsMTACP2 was highly expressed in tapetal cells and microspores during anther development, with the encoded protein localizing to both mitochondria and the cytoplasm. Comparative transcriptomic analysis revealed differential expression of genes related to lipid metabolism between the wild type and the Osmtacp2-1 mutant in response to cold. Through a lipidomic analysis, we demonstrated that wax esters, which are the primary lipid components of the anther cuticle and pollen walls, function as cold-responsive lipids. Their levels increased dramatically in the wild type but not in Osmtacp2-1 when exposed to cold. Additionally, mutants of two cold-induced genes of wax ester biosynthesis, ECERIFERUM1 and WAX CRYSTAL-SPARSE LEAF2, showed decreased cold tolerance. These results suggest that OsMTACP2-mediated wax ester biosynthesis is essential for cold tolerance in rice at the booting stage.

The GaKAN2, a KANADI transcription factor, modulates stem trichomes in Gossypium arboreum

KEY MESSAGE

GaKAN2, a member of the KANADI family, was found to be widely expressed in the cotton tissues and regulates trichome development through complex pathways. Cotton trichomes are believed to be the defense barrier against insect pests. Cotton fiber and trichomes are single-cell epidermal extensions with shared regulatory mechanisms. Despite several studies underlying mechanism of trichome development remains elusive. The KANADI is one of the key transcription factors (TFs) family, regulating Arabidopsis trichomes growth. However, the function of KANADI genes in cotton remains unknown. In the current study genome-wide scanning, transcriptomic analysis, gene silencing, subcellular localization, and yeast two-hybrid techniques were employed to decipher the function of KANADI TFs family genes in cotton crop. A total of 7 GaKAN genes were found in the Gossypium arboreum. Transcriptomic data revealed that these genes were significantly expressed in stem and root. Moreover, GaKAN2 was widely expressed in other tissues also. Subsequently, we selected GaKAN2 to validate the function of KANADI genes. Silencing of GaKAN2 resulted in a 24.99% decrease in single-cell trichomes and an 11.33% reduction in internodal distance, indicating its potential role in regulating trichomes and plant growth. RNA-Seq analysis elucidated that GaSuS and GaERS were the downstream genes of GaKAN2. The transcriptional activation and similarity in silencing phenotype between GaKAN2 and GaERS suggested that GaKAN2 regulates trichomes development through GaERS. Moreover, KEGG analysis revealed that a significant number of genes were enriched in the biosynthesis of secondary metabolites and plant hormone signal transduction pathways, thereby suggesting that GaKAN2 regulates the stem trichomes and plant growth. The GFP subcellular localization and yeast transcriptional activation analysis elucidated that GaKAN2 was located in the nucleus and capable of regulating the transcription of downstream genes. This study elucidated the function and characteristics of the KANADI gene family in cotton, providing a fundamental basis for further research on GaKAN2 gene in cotton plant trichomes and plant developmental processes.

MADS1-regulated lemma and awn development benefits barley yield

Floral organ shape and size in cereal crops can affect grain size and yield, so genes that regulate their development are promising breeding targets. The lemma, which protects inner floral organs, can physically constrain grain growth; while the awn, a needle-like extension of the lemma, creates photosynthate to developing grain. Although several genes and modules controlling grain size and awn/lemma growth in rice have been characterized, these processes, and the relationships between them, are not well understood for barley and wheat. Here, we demonstrate that the barley E-class gene HvMADS1 positively regulates awn length and lemma width, affecting grain size and weight. Cytological data indicates that HvMADS1 promotes awn and lemma growth by promoting cell proliferation, while multi-omics data reveals that HvMADS1 target genes are associated with cell cycle, phytohormone signaling, and developmental processes. We define two potential targets of HvMADS1 regulation, HvSHI and HvDL, whose knockout mutants mimic awn and/or lemma phenotypes of mads1 mutants. Additionally, we demonstrate that HvMADS1 interacts with APETALA2 (A-class) to synergistically activate downstream genes in awn/lemma development in barley. Notably, we find that MADS1 function remains conserved in wheat, promoting cell proliferation to increase awn length. These findings extend our understanding of MADS1 function in floral organ development and provide insights for Triticeae crop improvement strategies.

The OsSGS3-tasiRNA-OsARF3 module orchestrates abiotic-biotic stress response trade-off in rice

Recurrent heat stress and pathogen invasion seriously threaten crop production, and abiotic stress often antagonizes biotic stress response against pathogens. However, the molecular mechanisms of trade-offs between thermotolerance and defense remain obscure. Here, we identify a rice thermo-sensitive mutant that displays a defect in floret development under high temperature with a mutation in SUPPRESSOR OF GENE SILENCING 3a (OsSGS3a). OsSGS3a interacts with its homolog OsSGS3b and modulates the biogenesis of trans-acting small interfering RNA (tasiRNA) targeting AUXIN RESPONSE FACTORS (ARFs). We find that OsSGS3a/b positively, while OsARF3a/b and OsARF3la/lb negatively modulate thermotolerance. Moreover, OsSGS3a negatively, while OsARF3a/b and OsARF3la/lb positively regulate disease resistance to the bacterial pathogen Xanthomonas oryzae pv. oryzae (Xoo) and the fungal pathogen Magnaporthe oryzae (M. oryzae). Taken together, our study uncovers a previously unknown trade-off mechanism that regulates distinct immunity and thermotolerance through the OsSGS3-tasiRNA-OsARF3 module, highlighting the regulation of abiotic-biotic stress response trade-off in plants.

Roles of auxin response factors in rice development and stress responses

Auxin signalling plays a key role in various developmental processes ranging from embryogenesis to senescence in plants. Auxin response factor (ARF), a key component of auxin signalling, functions by binding to auxin response element within promoter of auxin response genes, activating or repressing the target genes. Increasing evidences show that ARFs are crucial for plant response to stresses. This review summarises the recent advance on the functions and their regulatory pathways of rice ARFs in development and responding to stresses. The importance of OsARFs is demonstrated by their roles in triggering various physiological, biochemical and molecular reactions to resist adverse environmental conditions. We also describe the transcriptional and post-transcriptional regulation of OsARFs, and discuss the major challenges in this area.