Salt hypersensitivity is associated with excessive xylem development in a thermospermine-deficient mutant of Arabidopsis thaliana

A Solanum lycopersicum polyamine oxidase contributes to the control of plant growth, xylem differentiation, and drought stress tolerance

Polyamines are involved in several plant physiological processes. In Arabidopsis thaliana, five FAD-dependent polyamine oxidases (AtPAO1 to AtPAO5) contribute to polyamine homeostasis. AtPAO5 catalyzes the back-conversion of thermospermine (T-Spm) to spermidine and plays a role in plant development, xylem differentiation, and abiotic stress tolerance. In the present study, to verify whether T-Spm metabolism can be exploited as a new route to improve stress tolerance in crops and to investigate the underlying mechanisms, tomato (Solanum lycopersicum) AtPAO5 homologs were identified (SlPAO2, SlPAO3, and SlPAO4) and CRISPR/Cas9-mediated loss-of-function slpao3 mutants were obtained. Morphological, molecular, and physiological analyses showed that slpao3 mutants display increased T-Spm levels and exhibit changes in growth parameters, number and size of xylem elements, and expression levels of auxin- and gibberellin-related genes compared to wild-type plants. The slpao3 mutants are also characterized by improved tolerance to drought stress, which can be attributed to a diminished xylem hydraulic conductivity that limits water loss, as well as to a reduced vulnerability to embolism. Altogether, this study evidences conservation, though with some significant variations, of the T-Spm-mediated regulatory mechanisms controlling plant growth and differentiation across different plant species and highlights the T-Spm role in improving stress tolerance while not constraining growth.

Thermospermine Is an Evolutionarily Ancestral Phytohormone Required for Organ Development and Stress Responses in Marchantia Polymorpha

Thermospermine suppresses auxin-inducible xylem differentiation, whereas its structural isomer, spermine, is involved in stress responses in angiosperms. The thermospermine synthase, ACAULIS5 (ACL5), is conserved from algae to land plants, but its physiological functions remain elusive in non-vascular plants. Here, we focused on MpACL5, a gene in the liverwort Marchantia polymorpha, that rescued the dwarf phenotype of the acl5 mutant in Arabidopsis. In the Mpacl5 mutants generated by genome editing, severe growth retardation was observed in the vegetative organ, thallus, and the sexual reproductive organ, gametangiophore. The mutant gametangiophores exhibited remarkable morphological defects such as short stalks, fasciation and indeterminate growth. Two gametangiophores fused together, and new gametangiophores were often initiated from the old ones. Furthermore, Mpacl5 showed altered responses to heat and salt stresses. Given the absence of spermine in bryophytes, these results suggest that thermospermine has a dual primordial function in organ development and stress responses in M. polymorpha. The stress response function may have eventually been assigned to spermine during land plant evolution.

Overexpression of OsACL5 triggers environmentally-dependent leaf rolling and reduces grain size in rice

Major polyamines include putrescine, spermidine, spermine and thermospermine, which play vital roles in growth and adaptation against environmental changes in plants. Thermospermine (T-Spm) is synthetised by ACL5. The function of ACL5 in rice is still unknown. In this study, we used a reverse genetic strategy to investigate the biological function of OsACL5. We generated several knockout mutants by pYLCRISPR/Cas9 system and overexpressing (OE) lines of OsACL5. Interestingly, the OE plants exhibited environmentally-dependent leaf rolling, smaller grains, lighter 1000-grain weight and reduction in yield per plot. The area of metaxylem vessels of roots and leaves of OE plants were significantly smaller than those of WT, which possibly caused reduction in leaf water potential, resulting in leaf rolling with rise in the environmental temperature and light intensity and decrease in humidity. Additionally, the T-Spm contents were markedly increased by over ninefold whereas the ethylene evolution was reduced in OE plants, suggesting that T-Spm signalling pathway interacts with ethylene pathway to regulate multiple agronomic characters. Moreover, the osacl5 exhibited an increase in grain length, 1000-grain weight, and yield per plot. OsACL5 may affect grain size via mediating the expression of OsDEP1, OsGS3 and OsGW2. Furthermore, haplotypes analysis indicated that OsACL5 plays a conserved function on regulating T-Spm levels during the domestication of rice. Our data demonstrated that identification of OsACL5 provides a theoretical basis for understanding the physiological mechanism of T-Spm which may play roles in triggering environmentally dependent leaf rolling; OsACL5 will be an important gene resource for molecular breeding for higher yield.

24-Epibrassinolide Facilitates Adventitious Root Formation by Coordinating Cell-Wall Polyamine Oxidase- and Plasma Membrane Respiratory Burst Oxidase Homologue-Derived Reactive Oxygen Species in Capsicum annuum L

Adventitious root (AR) formation is a critical process in cutting propagation of horticultural plants. Brassinosteroids (BRs) have been shown to regulate AR formation in several plant species; however, little is known about their exact effects on pepper AR formation, and the downstream signaling of BRs also remains elusive. In this study, we showed that treatment of 24-Epibrassinolide (EBL, an active BR) at the concentrations of 20-100 nM promoted AR formation in pepper (Capsicum annuum). Furthermore, we investigated the roles of apoplastic reactive oxygen species (ROS), including hydrogen peroxide (H₂O₂) and superoxide radical (O₂^•-), in EBL-promoted AR formation, by using physiological, histochemical, bioinformatic, and biochemical approaches. EBL promoted AR formation by modulating cell-wall-located polyamine oxidase (PAO)-dependent H₂O₂ production and respiratory burst oxidase homologue (RBOH)-dependent O₂^•- production, respectively. Screening of CaPAO and CaRBOH gene families combined with gene expression analysis suggested that EBL-promoted AR formation correlated with the upregulation of CaPAO1, CaRBOH2, CaRBOH5, and CaRBOH6 in the AR zone. Transient expression analysis confirmed that CaPAO1 was able to produce H₂O₂, and CaRBOH2, CaRBOH5, and CaRBOH6 were capable of producing O₂^•-. The silencing of CaPAO1, CaRBOH2, CaRBOH5, and CaRBOH6 in pepper decreased the ROS accumulation and abolished the EBL-induced AR formation. Overall, these results uncover one of the regulatory pathways for BR-regulated AR formation, and extend our knowledge of the functions of BRs and of the BRs-ROS crosstalk in plant development.

Impairment of sugar transport in the vascular system acts on nitrogen remobilization and nitrogen use efficiency in Arabidopsis

Carbon (C) and nitrogen (N) metabolisms have long been known to be coupled, and this is required for adjusting nitrogen use efficiency (NUE). Despite this intricate relationship, it is still unclear how deregulation of sugar transport impacts N allocation. Here, we investigated in Arabidopsis the consequences of the simultaneous downregulation of the genes coding for the sugar transporters SWEET11, SWEET12, SWEET16, and SWEET17 on various anatomical and physiological traits ranging from the stem's vascular system development to plant biomass production, seed yield, and N remobilization and use efficiency. Our results show that intracellular sugar exchanges mediated by SWEET16 and SWEET17 proteins specifically impact vascular development but do not play a significant role in the distribution of N. Most importantly, we showed that the double mutant swt11 swt12, which has an impacted vascular development, displays an improved NUE and nitrogen remobilization to the seeds. In addition, a significant negative correlation between sugar and amino acids contents and the inflorescence stem radial growth exists, highlighting the complex interaction between the maintenance of C/N homeostasis and the inflorescence stem development. Our results thus deepen the link between sugar transport, C/N allocation, and vascular system development.

Salinity induces discontinuous protoxylem via a DELLA-dependent mechanism promoting salt tolerance in Arabidopsis seedlings

Salinity is detrimental to plants and developmental adjustments limiting salt uptake and transport is therefore important for acclimation to high salt. These parameters may be influenced by xylem morphology, however how plant root xylem development is affected by salt stress remains unclear. Using molecular and genetic techniques and detailed phenotypic analyses, we demonstrate that salt causes distinct effects on Arabidopsis seedling root xylem and reveal underlying molecular mechanisms. Salinity causes intermittent inhibition of protoxylem cell differentiation, generating protoxylem gaps, in Arabidopsis and several other eudicot seedlings. The extent of protoxylem gaps in seedlings positively correlates with salt tolerance. Reduced gibberellin signalling is required for protoxylem gap formation. Mutant analyses reveal that the xylem differentiation regulator VASCULAR RELATED NAC DOMAIN 6 (VND6), along with secondary cell wall producing and cell wall modifying enzymes, including EXPANSIN A1 (EXP1), are involved in protoxylem gap formation, in a DELLA-dependent manner. Salt stress is likely to reduce levels of bioactive gibberellins, stabilising DELLAs, which in turn activates multiple factors modifying protoxylem differentiation. Salt stress impacts seedling survival and formation of protoxylem gaps may be a measure to enhance salt tolerance.

Understanding the root xylem plasticity for designing resilient crops

Xylem is the main route for transporting water, minerals and a myriad of signalling molecules within the plant. With its onset during early embryogenesis, the development of the xylem relies on hormone gradients, the activity of unique transcription factors, the distribution of mobile microRNAs, and receptor-ligand pathways. These regulatory mechanisms are often interconnected and together contribute to the plasticity of this water-conducting tissue. Environmental stresses, such as drought and salinity, have a great impact on xylem patterning. A better understanding of how the structural properties of the xylem are regulated in normal and stress conditions will be instrumental in developing crops of the future. In addition, vascular wilt pathogens that attack the xylem are becoming increasingly problematic. Further knowledge of xylem development in response to these pathogens will bring new solutions against these diseases. In this review, we summarize recent findings on the molecular mechanisms of xylem formation that largely come from Arabidopsis research with additional insights from tomato and monocot species. We emphasize the impact of abiotic factors and pathogens on xylem plasticity and the urgent need to uncover the underlying mechanisms. Finally, we discuss the multidisciplinary approach to model xylem capacities in crops.

Root plasticity under abiotic stress

Abiotic stresses increasingly threaten existing ecological and agricultural systems across the globe. Plant roots perceive these stresses in the soil and adapt their architecture accordingly. This review provides insights into recent discoveries showing the importance of root system architecture (RSA) and plasticity for the survival and development of plants under heat, cold, drought, salt, and flooding stress. In addition, we review the molecular regulation and hormonal pathways involved in controlling RSA plasticity, main root growth, branching and lateral root growth, root hair development, and formation of adventitious roots. Several stresses affect root anatomy by causing aerenchyma formation, lignin and suberin deposition, and Casparian strip modulation. Roots can also actively grow toward favorable soil conditions and avoid environments detrimental to their development. Recent advances in understanding the cellular mechanisms behind these different root tropisms are discussed. Understanding root plasticity will be instrumental for the development of crops that are resilient in the face of abiotic stress.

How roots and shoots communicate through stressful times

When plants face an environmental stress such as water deficit, soil salinity, high temperature, or shade, good communication between above- and belowground organs is necessary to coordinate growth and development. Various signals including hormones, peptides, proteins, hydraulic signals, and metabolites are transported mostly through the vasculature to distant tissues. How shoots and roots synchronize their response to stress using mobile signals is an emerging field of research. We summarize recent advances on mobile signals regulating shoot stomatal movement and root development in response to highly localized environmental cues. In addition, we highlight how the vascular system is not only a conduit but is also flexible in its development in response to abiotic stress.

Recognizing the hidden half in wheat: root system attributes associated with drought tolerance

Improving drought tolerance in wheat is crucial for maintaining productivity and food security. Roots are responsible for the uptake of water from soil, and a number of root traits are associated with drought tolerance. Studies have revealed many quantitative trait loci and genes controlling root development in plants. However, the genetic dissection of root traits in response to drought in wheat is still unclear. Here, we review crop root traits associated with drought, key genes governing root development in plants, and quantitative trait loci and genes regulating root system architecture under water-limited conditions in wheat. Deep roots, optimal root length density and xylem diameter, and increased root surface area are traits contributing to drought tolerance. In view of the diverse environments in which wheat is grown, the balance among root and shoot traits, as well as individual and population performance, are discussed. The known functions of key genes provide information for the genetic dissection of root development of wheat in a wide range of conditions, and will be beneficial for molecular marker development, marker-assisted selection, and genetic improvement in breeding for drought tolerance.

Pseudomonas fluorescens DN16 Enhances Cucumber Defense Responses Against the Necrotrophic Pathogen Botrytis cinerea by Regulating Thermospermine Catabolism

Plants can naturally interact with beneficial rhizobacteria to mediate defense responses against foliar pathogen infection. However, the mechanisms of rhizobacteria-mediated defense enhancement remain rarely clear. In this study, beneficial rhizobacterial strain Pseudomonas fluorescens DN16 greatly increased the resistance of cucumber plants against Botrytis cinerea infection. RNA-sequencing analyses showed that several polyamine-associated genes including a thermospermine (TSpm) synthase gene (CsACL5) and polyamine catabolic genes (CsPAO1, CsPAO5, and CsCuAO1) were notably induced by DN16. The associations of TSpm metabolic pathways with the DN16-mediated cucumber defense responses were further investigated. The inoculated plants exhibited the increased leaf TSpm levels compared with the controls. Accordantly, overexpression of CsACL5 in cucumber plants markedly increased leaf TSpm levels and enhanced defense against B. cinerea infection. The functions of TSpm catabolism in the DN16-mediated defense responses of cucumber plants to B. cinerea were further investigated by pharmacological approaches. Upon exposure to pathogen infection, the changes of leaf TSpm levels were positively related to the enhanced activities of polyamine catabolic enzymes including polyamine oxidases (PAOs) and copper amine oxidases (CuAOs), which paralleled the transcription of several defense-related genes such as pathogenesis-related protein 1 (CsPR1) and defensin-like protein 1 (CsDLP1). However, the inhibited activities of polyamine catabolic enzymes abolished the DN16-induced cucumber defense against B. cinerea infection. This was in line with the impaired expression of defense-related genes in the inoculated plants challenged by B. cinerea. Collectively, our findings unraveled a pivotal role of TSpm catabolism in the regulation of the rhizobacteria-primed defense states by mediating the immune responses in cucumber plants after B. cinerea infection.

A Beginner's Guide to Osmoprotection by Biostimulants

Water is indispensable for the life of any organism on Earth. Consequently, osmotic stress due to salinity and drought is the greatest threat to crop productivity. Ongoing climate change includes rising temperatures and less precipitation over large areas of the planet. This is leading to increased vulnerability to the drought conditions that habitually threaten food security in many countries. Such a scenario poses a daunting challenge for scientists: the search for innovative solutions to save water and cultivate under water deficit. A search for formulations including biostimulants capable of improving tolerance to this stress is a promising specific approach. This review updates the most recent state of the art in the field.

Plant vascular development: mechanisms and environmental regulation

Plant vascular development is a complex process culminating in the generation of xylem and phloem, the plant transporting conduits. Xylem and phloem arise from specialized stem cells collectively termed (pro)cambium. Once developed, xylem transports mainly water and mineral nutrients and phloem transports photoassimilates and signaling molecules. In the past few years, major advances have been made to characterize the molecular, genetic and physiological aspects that govern vascular development. However, less is known about how the environment re-shapes the process, which molecular mechanisms link environmental inputs with developmental outputs, which gene regulatory networks facilitate the genetic adaptation of vascular development to environmental niches, or how the first vascular cells appeared as an evolutionary innovation. In this review, we (1) summarize the current knowledge of the mechanisms involved in vascular development, focusing on the model species Arabidopsis thaliana, (2) describe the anatomical effect of specific environmental factors on the process, (3) speculate about the main entry points through which the molecular mechanisms controlling of the process might be altered by specific environmental factors, and (4) discuss future research which could identify the genetic factors underlying phenotypic plasticity of vascular development.

Coping With Water Limitation: Hormones That Modify Plant Root Xylem Development

Periods of drought, that threaten crop production, are expected to become more prominent in large parts of the world, making it necessary to explore all aspects of plant growth and development, to breed, modify and select crops adapted to such conditions. One such aspect is the xylem, where influencing the size and number of the water-transporting xylem vessels, may impact on hydraulic conductance and drought tolerance. Here, we focus on how plants adjust their root xylem as a response to reduced water availability. While xylem response has been observed in a wide array of species, most of our knowledge on the molecular mechanisms underlying xylem plasticity comes from studies on the model plant Arabidopsis thaliana. When grown under water limiting conditions, Arabidopsis rapidly adjusts its development to produce more xylem strands with altered identity in an abscisic acid (ABA) dependent manner. Other hormones such as auxin and cytokinin are essential for vascular patterning and differentiation. Their balance can be perturbed by stress, as evidenced by the effects of enhanced jasmonic acid signaling, which results in similar xylem developmental alterations as enhanced ABA signaling. Furthermore, brassinosteroids and other signaling molecules involved in drought tolerance can also impact xylem development. Hence, a multitude of signals affect root xylem properties and, potentially, influence survival under water limiting conditions. Here, we review the likely entangled signals that govern root vascular development, and discuss the importance of taking root anatomical traits into account when breeding crops for enhanced resilience toward changes in water availability.