The DELLA Proteins Influence the Expression of Cytokinin Biosynthesis and Response Genes During Nodulation

Symbiotic synergy: How Arbuscular Mycorrhizal Fungi enhance nutrient uptake, stress tolerance, and soil health through molecular mechanisms and hormonal regulation

Arbuscular Mycorrhizal (AM) symbiosis is integral to sustainable agriculture and enhances plant resilience to abiotic and biotic stressors. Through their symbiotic association with plant roots, AM improves nutrient and water uptake, activates antioxidant defenses, and facilitates hormonal regulation, contributing to improved plant health and productivity. Plants release strigolactones, which trigger AM spore germination and hyphal branching, a process regulated by genes, such as D27, CCD7, CCD8, and MAX1. AM recognition by plants is mediated by receptor-like kinases (RLKs) and LysM domains, leading to the formation of arbuscules that optimize nutrient exchange. Hormonal regulation plays a pivotal role in this symbiosis; cytokinins enhance AM colonization, auxins support arbuscule formation, and brassinosteroids regulate root growth. Other hormones, such as salicylic acid, gibberellins, ethylene, jasmonic acid, and abscisic acid, also influence AM colonization and stress responses, further bolstering plant resilience. In addition to plant health, AM enhances soil health by improving microbial diversity, soil structure, nutrient cycling, and carbon sequestration. This symbiosis supports soil pH regulation and pathogen suppression, offering a sustainable alternative to chemical fertilizers and improving soil fertility. To maximize AM 's potential of AM in agriculture, future research should focus on refining inoculation strategies, enhancing compatibility with different crops, and assessing the long-term ecological and economic benefits. Optimizing AM applications is critical for improving agricultural resilience, food security, and sustainable farming practices.

Dwarfs standing tall: breeding towards the 'Yellow revolution' through insights into plant height regulation

High oilseed production is an exigency due to the increasing edible oil consumption of the growing population. Rapeseed and mustard are cultivated worldwide and contribute significantly to the world's total oilseed production. Already a plateau is reached in terms of area and yield in most of the existing cultivars. Most of the commercially cultivated high yielding rapeseed and mustard varieties are tall, mainly due to a wider use of heterosis. However, they are susceptible to lodging and consequent yield losses. Plant yield is strongly dependent upon its architecture; therefore, 'ideotype breeding' is the key approach adopted to develop new varieties with enhanced yield potential, which is less explored in these crops. Dwarf/ semi dwarf plant type varieties has shown its improved yield potential over tall plant type in cereals which further leads to 'Green revolution' in Asian countries. Although, many induced dwarf mutants in rapeseed and mustard were isolated, unlike dwarf green-revolution varieties of cereals, most of them had undesirable plant types with defects including extreme dwarfism and sterility, leading to poor yield potential. Understanding the genetic and molecular mechanisms governing plant height and its correlation with yield and yield contributing characters is crucial. In this review, recent insights into genetic, molecular, and anatomical regulation of plant height have been discussed. The role of hormones, their crosstalk, and hormonal control for cell division and expansion have been delineated with respect to plant architecture. Many dwarfing genes are identified as being part of various phytohormone pathways. Parallelly, molecular links between plant height and flowering time have been explored. The overall synthesis of the review points out some key target pathways and genes that will be useful for plant breeders as well as biotechnologists for targeted genome editing for improving plant architecture without a yield penalty.

Right time, right place: The dynamic role of hormones in rhizobial infection and nodulation of legumes

Many legume plants form beneficial associations with rhizobial bacteria that are hosted in new plant root organs, nodules, in which atmospheric nitrogen is fixed. This association requires the precise coordination of two separate programs, infection in the epidermis and nodule organogenesis in the cortex. There is extensive literature indicating key roles for plant hormones during nodulation, but a detailed analysis of the spatial and temporal roles of plant hormones during the different stages of nodulation is required. This review analyses the current literature on hormone regulation of infection and organogenesis to reveal the differential roles and interactions of auxin, cytokinin, brassinosteroids, ethylene, and gibberellins during epidermal infection and cortical nodule initiation, development, and function. With the exception of auxin, all of these hormones suppress infection events. By contrast, there is evidence that all of these hormones promote nodule organogenesis, except ethylene, which suppresses nodule initiation. This differential role for many of the hormones between the epidermal and cortical programs is striking. Future work is required to fully examine hormone interactions and create a robust model that integrates this knowledge into our understanding of nodulation pathways.

BBX24 Interacts with DELLA to Regulate UV-B-Induced Photomorphogenesis in Arabidopsis thaliana

UV-B radiation, sensed by the photoreceptor UVR8, induces signal transduction for plant photomorphogenesis. UV-B radiation affects the concentration of the endogenous plant hormone gibberellin (GA), which in turn triggers DELLA protein degradation through the 26S proteasome pathway. DELLA is a negative regulator in GA signaling, partially relieving the inhibition of hypocotyl growth induced by UV-B in Arabidopsis thaliana. However, GAs do usually not work independently but integrate in complex networks linking to other plant hormones and responses to external environmental signals. Until now, our understanding of the regulatory network underlying GA-involved UV-B photomorphogenesis had remained elusive. In the present research, we investigate the crosstalk between the GA and UV-B signaling pathways in UV-B-induced photomorphogenesis of Arabidopsis thaliana. Compared with wild type Landsberg erecta (Ler), the abundance of HY5, CHS, FLS, and UF3GT were found to be down-regulated in rga-24 and gai-t6 mutants under UV-B radiation, indicating that DELLA is a positive regulator in UV-B-induced photomorphogenesis. Our results indicate that BBX24 interacts with RGA (one of the functional DELLA family members). Furthermore, we also found that RGA interacts with HY5 (the master regulator in plant photomorphogenesis). Collectively, our findings suggest that the HY5−BBX24−DELLA module serves as an important signal regulating network, in which GA is involved in UV-B signaling to regulate hypocotyl inhibition.

The chromosome-level genome assembly of Astragalus sinicus and comparative genomic analyses provide new resources and insights for understanding legume-rhizobial interactions

The legume species Astragalus sinicus (Chinese milk vetch [CMV]) has been widely cultivated for centuries in southern China as one of the most important green manures/cover crops for improving rice productivity and preventing soil degeneration. In this study, we generated the first chromosome-scale reference genome of CMV by combining PacBio and Illumina sequencing with high-throughput chromatin conformation capture (Hi-C) technology. The CMV genome was 595.52 Mb in length, with a contig N50 size of 1.50 Mb. Long terminal repeats (LTRs) had been amplified and contributed to genome size expansion in CMV. CMV has undergone two whole-genome duplication (WGD) events, and the genes retained after the WGD shared by Papilionoideae species shaped the rhizobial symbiosis and the hormonal regulation of nodulation. The chalcone synthase (CHS) gene family was expanded and was expressed primarily in the roots of CMV. Intriguingly, we found that resistance genes were more highly expressed in roots than in nodules of legume species, suggesting that their expression may be increased to bolster plant immunity in roots to cope with pathogen infection in legumes. Our work sheds light on the genetic basis of nodulation and symbiosis in CMV and provides a benchmark for accelerating genetic research and molecular breeding in the future.

Interactions of Gibberellins with Phytohormones and Their Role in Stress Responses

Gibberellins are amongst the main plant growth regulators. Discovered over a century ago, the interest in gibberellins research is growing due to their current and potential applications in crop production and their role in the responses to environmental stresses. In the present review, the current knowledge on gibberellins’ homeostasis and modes of action is outlined. Besides this, the complex interrelations between gibberellins and other plant growth regulators are also described, providing an intricate network of interactions that ultimately drives towards precise and specific gene expression. Thus, genes and proteins identified as being involved in gibberellin responses in model and non-model species are highlighted. Furthermore, the molecular mechanisms governing the gibberellins’ relation to stress responses are also depicted. This review aims to provide a comprehensive picture of the state-of-the-art of the current perceptions of the interactions of gibberellins with other phytohormones, and their responses to plant stresses, thus allowing for the identification of the specific mechanisms involved. This knowledge will help us to improve our understanding of gibberellins’ biology, and might help increase the biotechnological toolbox needed to refine plant resilience, particularly under a climate change scenario.

Regulation of the Later Stages of Nodulation Stimulated by IPD3/CYCLOPS Transcription Factor and Cytokinin in Pea Pisum sativum L

The IPD3/CYCLOPS transcription factor was shown to be involved in the regulation of nodule primordia development and subsequent stages of nodule differentiation. In contrast to early stages, the stages related to nodule differentiation remain less studied. Recently, we have shown that the accumulation of cytokinin at later stages may significantly impact nodule development. This conclusion was based on a comparative analysis of cytokinin localization between pea wild type and ipd3/cyclops mutants. However, the role of cytokinin at these later stages of nodulation is still far from understood. To determine a set of genes involved in the regulation of later stages of nodule development connected with infection progress, intracellular accommodation, as well as plant tissue and bacteroid differentiation, the RNA-seq analysis of pea mutant SGEFix--2 (sym33) nodules impaired in these processes compared to wild type SGE nodules was performed. To verify cytokinin's influence on late nodule development stages, the comparative RNA-seq analysis of SGEFix--2 (sym33) mutant plants treated with cytokinin was also conducted. Findings suggest a significant role of cytokinin in the regulation of later stages of nodule development.

Cytokinins as central regulators during plant growth and stress response

Cytokinins are a class of phytohormone that participate in the regulation of the plant growth, development, and stress response. In this review, the potential regulating mechanism during plant growth and stress response are discussed. Cytokinins are a class of phytohormone that participate in the regulation of plant growth, physiological activities, and yield. Cytokinins also play a key role in response to abiotic stresses, such as drought, salt and high or low temperature. Through the signal transduction pathway, cytokinins interact with various transcription factors via a series of phosphorylation cascades to regulate cytokinin-target gene expression. In this review, we systematically summarize the biosynthesis and metabolism of cytokinins, cytokinin signaling, and associated gene regulation, and highlight the function of cytokinins during plant development and resistance to abiotic stress. We also focus on the importance of crosstalk between cytokinins and other classes of phytohormones, including auxin, ethylene, strigolactone, and gibberellin. Our aim is to provide a comprehensive overview of recent findings on the mechanisms by which cytokinins act as central regulators of plant development and stress reactions, and highlight topics for future research.

Identification of BELL Transcription Factors Involved in Nodule Initiation and Development in the Legumes Pisum sativum and Medicago truncatula

Single three-amino acid loop extension (TALE) homeodomain proteins, including the KNOTTED-like (KNOX) and BEL-like (BELL) families in plants, usually work as heterodimeric transcription factor complexes to regulate different developmental processes, often via effects on phytohormonal pathways. Nitrogen-fixing nodule formation in legumes is regulated by different families of homeodomain transcription factors. Whereas the role of KNOX transcription factors in the control of symbiosis was studied early, BELL transcription factors have received less attention. Here, we report the identification and expression analysis of BELL genes in the legume plants Medicago truncatula and Pisum sativum, which are involved in regulating symbiosis initiation and development. A more precise analysis was performed for the most significantly upregulated PsBELL1-2 gene in pea. We found that the PsBELL1-2 transcription factor could be a potential partner of PsKNOX9. In addition, we showed that PsBELL1-2 can interact with the PsDELLA1 (LA) protein-regulator of the gibberellin pathway, which has a previously demonstrated important role in symbiosis development.

DELLA family duplication events lead to different selective constraints in angiosperms

Gibberellic acid (GA) is a major plant hormone involved in several biological processes from the flowering to the symbiosis with microorganisms. Thus, the GA regulation is crucial for plant biology. This regulation occurs via the DELLA proteins that belong to the GRAS transcription factor family. DELLA proteins are characterised by a DELLA N-terminal and a GRAS C-terminal domains. It is well known that DELLA activity appears after the bryophytes divergence and then evolved in the vascular plant lineages. Here we present the phylogeny of DELLA across 75 species belonging to various lineages from algae, liverworts and angiosperms. Our study confirmed two main duplication events, the first occurring before the angiosperms divergence and the other specific to the eudicots lineage. Comparative analysis of DELLA subclades in angiosperms revealed the loss in Poaceae and strong alteration in other species of the DELLA functional domain in the DELLA2 clade. In addition, molecular evolution analysis suggests that each of the clades (named DELLA1.1, DELLA1.2 and DELLA2) evolved differently but copies of each subclade are under strong purifying selection. This also suggests that, although the DELLA functional domain is altered in DELLA2, DELLA2 orthologs are still functional and operate in a different way compared to DELLA1 copies. In angiosperms, additional duplication events occurred and led to duplicate copies in species, genus or family such as in the Fabaceae subfamily Papilionoideae. This duplication led to the formation of additional paralogs in the DELLA1.2 subclade (DELLA1.2.1 and DELLA1.2.2). Interestingly, both copies appeared to be under relaxing selection revealing different evolutionary fate of the DELLA duplicated copies.

Mutational analysis indicates that abnormalities in rhizobial infection and subsequent plant cell and bacteroid differentiation in pea (Pisum sativum) nodules coincide with abnormal cytokinin responses and localization

BACKGROUND AND AIMS

Recent findings indicate that Nod factor signalling is tightly interconnected with phytohormonal regulation that affects the development of nodules. Since the mechanisms of this interaction are still far from understood, here the distribution of cytokinin and auxin in pea (Pisum sativum) nodules was investigated. In addition, the effect of certain mutations blocking rhizobial infection and subsequent plant cell and bacteroid differentiation on cytokinin distribution in nodules was analysed.

METHODS

Patterns of cytokinin and auxin in pea nodules were profiled using both responsive genetic constructs and antibodies.

KEY RESULTS

In wild-type nodules, cytokinins were found in the meristem, infection zone and apical part of the nitrogen fixation zone, whereas auxin localization was restricted to the meristem and peripheral tissues. We found significantly altered cytokinin distribution in sym33 and sym40 pea mutants defective in IPD3/CYCLOPS and EFD transcription factors, respectively. In the sym33 mutants impaired in bacterial accommodation and subsequent nodule differentiation, cytokinin localization was mostly limited to the meristem. In addition, we found significantly decreased expression of LOG1 and A-type RR11 as well as KNOX3 and NIN genes in the sym33 mutants, which correlated with low cellular cytokinin levels. In the sym40 mutant, cytokinins were detected in the nodule infection zone but, in contrast to the wild type, they were absent in infection droplets.

CONCLUSIONS

In conclusion, our findings suggest that enhanced cytokinin accumulation during the late stages of symbiosis development may be associated with bacterial penetration into the plant cells and subsequent plant cell and bacteroid differentiation.