A re-assessment of sucrose signaling involved in cluster-root formation and function in phosphate-deficient white lupin (Lupinus albus)

Transcriptomics Provide Insights into Early Responses to Sucrose Signaling in Lupinus albus, a Model Plant for Adaptations to Phosphorus and Iron Deficiency

Phosphorus (P) and iron (Fe) deficiency are major limiting factors for plant productivity worldwide. White lupin (Lupinus albus L.) has become a model plant for understanding plant adaptations to P and Fe deficiency, because of its ability to form cluster roots, bottle-brush-like root structures play an important role in the uptake of P and Fe from soil. However, little is known about the signaling pathways involved in sensing and responding to P and Fe deficiency. Sucrose, sent in increased concentrations from the shoot to the root, has been identified as a long-distance signal of both P and Fe deficiency. To unravel the responses to sucrose as a signal, we performed Oxford Nanopore cDNA sequencing of white lupin roots treated with sucrose for 10, 15, or 20 min compared to untreated controls. We identified a set of 17 genes, including 2 bHLH transcription factors, that were up-regulated at all three time points of sucrose treatment. GO (gene ontology) analysis revealed enrichment of auxin and gibberellin responses as early as 10 min after sucrose addition, as well as the emerging of ethylene responses at 20 min of sucrose treatment, indicating a sequential involvement of these hormones in plant responses to sucrose.

Soybean Root Transcriptomics: Insights into Sucrose Signaling at the Crossroads of Nutrient Deficiency and Biotic Stress Responses

Soybean (Glycine max) is an important agricultural crop, but nutrient deficiencies frequently limit soybean production. While research has advanced our understanding of plant responses to long-term nutrient deficiencies, less is known about the signaling pathways and immediate responses to certain nutrient deficiencies, such as P_i and Fe deficiencies. Recent studies have shown that sucrose acts as a long-distance signal that is sent in increased concentrations from the shoot to the root in response to various nutrient deficiencies. Here, we mimicked nutrient deficiency-induced sucrose signaling by adding sucrose directly to the roots. To unravel transcriptomic responses to sucrose acting as a signal, we performed Illumina RNA-sequencing of soybean roots treated with sucrose for 20 min and 40 min, compared to non-sucrose-treated controls. We obtained a total of 260 million paired-end reads, mapping to 61,675 soybean genes, some of which are novel (not yet annotated) transcripts. Of these, 358 genes were upregulated after 20 min, and 2416 were upregulated after 40 min of sucrose exposure. GO (gene ontology) analysis revealed a high proportion of sucrose-induced genes involved in signal transduction, particularly hormone, ROS (reactive oxygen species), and calcium signaling, in addition to regulation of transcription. In addition, GO enrichment analysis indicates that sucrose triggers crosstalk between biotic and abiotic stress responses.

Soilborne bacterium Klebsiella pneumoniae promotes cluster root formation in white lupin through ethylene mediation

Cluster roots of white lupin are induced by low phosphorus (LP) to efficiently access unavailable P, but how soilborne microbes are associated with cluster root formation (CRF) is unclear. We investigated the roles of soilborne bacteria in CRF response to LP by high-throughput sequencing and root-bacteria interactions. Cluster root number was significantly decreased in plants grown in sterilized soil compared with nonsterilized soil. Proteobacteria was enriched in CR, as shown by microbiome analysis of soil (bulk, rhizosphere, and rhizosheath) and roots (main, lateral, and CR). Large-scale gene expression level implicated ethylene mediation in CRF. Klebsiella pneumoniae (P7), a soilborne bacterium belonging to Proteobacteria, was isolated from CR. Among 11 isolated strains, P7 exhibited the highest 1-aminocyclopropane-1-carboxylate deaminase (ACCD) activity; this enzyme inhibits the biosynthesis of ethylene in plants by the cleavage of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid and promotes CRF under LP. We constructed an ACCD-deficit mutant accd in the P7 genetic background. The loss-of-function mutation failed to promote CRF under LP conditions. Also, auxin responses may be involved in K. pneumoniae-ethylene-mediated CRF. Overall, we propose that the soilborne bacterium K. pneumoniae promotes CRF of white lupin in response to LP by ethylene mediation.

The vacuolar transporter LaMTP8.1 detoxifies manganese in leaves of Lupinus albus

Manganese (Mn) is an essential microelement, but overaccumulation is harmful to many plant species. Most plants have similar minimal Mn requirements, but the tolerance to elevated Mn varies considerably. Mobilization of phosphate (P) by plant roots leads to increased Mn uptake, and shoot Mn levels have been reported to serve as an indicator for P mobilization efficiency in the presence of P deficiency. White lupin (Lupinus albus L.) mobilizes P and Mn with outstanding efficiency due to the formation of determinate cluster roots that release carboxylates. The high Mn tolerance of L. albus goes along with shoot Mn accumulation, but the molecular basis of this detoxification mechanism has been unknown. In this study, we identify LaMTP8.1 as the transporter mediating vacuolar sequestration of Mn in the shoot of white lupin. The function of Mn transport was demonstrated by yeast complementation analysis, in which LaMTP8.1 detoxified Mn in pmr1∆ mutant cells upon elevated Mn supply. In addition, LaMTP8.1 also functioned as an iron (Fe) transporter in yeast assays. The expression of LaMTP8.1 was particularly high in old leaves under high Mn stress. However, low P availability per se did not result in transcriptional upregulation of LaMTP8.1. Moreover, LaMTP8.1 expression was strongly upregulated under Fe deficiency, where it was accompanied by Mn accumulation, indicating a role in the interaction of these micronutrients in L. albus. In conclusion, the tonoplast-localized Mn transporter LaMTP8.1 mediates Mn detoxification in leaf vacuoles, providing a mechanistic explanation for the high Mn accumulation and Mn tolerance in this species.

Serine Hydroxymethyltransferase 1 Is Essential for Primary-Root Growth at Low-Sucrose Conditions

Plant roots are essential organs for absorbing nutrients from the soil or medium. Sucrose functions as a vital carbon source in root development, and sucrose starvation interferes with the redox state of plant cells. However, the mechanism of root growth at sucrose starvation remains unclear. Here, we report that SHMT1 (serine hydroxymethyltransferase 1) plays a crucial role in primary-root growth. SHMT1 mutation caused decreased sugar levels, excessive H₂O₂ accumulation, and severe root-growth arrest at sucrose-free conditions, whereas plants with SHMT1 overexpression had increased sugar and decreased H₂O₂ levels, and longer primary roots. Sucrose supply fully restored root growth of shm1-2, but CO₂ alone could not, and SHMT1 is much more stable in roots than shoots at sucrose conditions, suggesting that SHMT1 accumulation in roots is critical for sucrose accumulation and root growth. Further ROS scavenging by GSH application or ROS synthesis inhibition by apocynin application or RBOHD mutation reduced H₂O₂ levels and partially restored the root-growth arrest phenotype of shm1-2 at low-sucrose conditions, suggesting that SHMT1 modulates root growth via sucrose-mediated ROS accumulation. Our findings demonstrated the role of SHMT1 in primary-root growth by regulating sucrose accumulation and ROS homeostasis in roots.

Root Handling Affects Carboxylates Exudation and Phosphate Uptake of White Lupin Roots

The reliable quantification of root exudation and nutrient uptake is a very challenging task, especially when considering single root segments. Most methods used necessitate root handling e.g. root dissecting/cutting. However, there is a knowledge gap on how much these techniques affect root physiology. Thus, this study aimed at assessing the effect of different root handling techniques on the phosphate (P_i) uptake and carboxylate exudation of white lupin roots. White lupin plants were grown hydroponically in a full and P_i-deficient nutrient solution for 60 days. Phosphate uptake and carboxylate exudation of cluster and non-cluster roots were measured using custom made cells 1, 4, and 8 h after the onset of light. Three different experimental set-ups were used: i) without cutting the root apparatus from the shoots, nor dissecting the root into smaller root sections - named intact plant (IP); ii) separating the roots from the shoots, without dissecting the root into smaller sections - named intact root (IR); iii) separating the roots form the shoots and dissecting the roots in different sections-named dissected roots (DR). The sampling at 8 h led to the most significant alterations of the root P_i uptake induced by the sampling method. Generally, roots were mainly affected by the DR sampling method, indicating that results of studies in which roots are cut/dissected should be interpreted carefully. Additionally, the study revealed that the root tip showed a very high P_i uptake rate, suggesting that the tip could act as a P_i sensor. Citrate, malate and lactate could be detected in juvenile, mature and senescent cluster root exudation. We observed a significant effect of the handling method on carboxylate exudation only at sampling hours 1 and 8, although no clear and distinctive trend could be observed. Results here presented reveal that the root handling as well as the sampling time point can greatly influence root physiology and therefore should not be neglected when interpreting rhizosphere dynamics.

The LaCEP1 peptide modulates cluster root morphology in Lupinus albus

White lupin cluster roots are specialized brush-like root structures that are formed in some species under phosphorus (P)-deficient conditions. They intensely secrete protons and organic acid anions for solubilization and acquisition of sparingly soluble phosphates. Phytohormones and sucrose modulate cluster root number, but the molecular mechanisms of cluster root formation have been elusive. Here, a novel peptide phytohormone was identified that affects cluster root development. It belongs to the C-TERMINALLY-ENCODED PEPTIDE (CEP) family. Members of that family arrest root growth and modulate branching in model species. LaCEP1 was highly expressed in the pre-emergence zone of clusters. Over-expression of the gene encoding the LaCEP1 propeptide resulted in moderate inhibition of cluster root formation. The primary and lateral root lengths of lupin were little affected by the overexpression, but LaCEP1 reduced cluster rootlet and root hair elongation. Addition of a 15-mer core peptide derived from LaCEP1 similarly altered root morphology and modified cluster activity, suggesting that a core sequence of the propeptide is functionally sufficient. Stable overexpression in Arabidopsis confirmed the LaCEP1 function in root growth inhibition across species. Taken together, the root inhibitory effects of the LaCEP1 phytohormone suggest a role as of a regulatory module involved in cluster root development in white lupin.

Metabolic analysis of two contrasting wild barley genotypes grown hydroponically reveals adaptive strategies in response to low nitrogen stress

Nitrogen (N) is an essential macronutrient for plants. The increasingly severe environmental problems caused by N fertilizer application urge alleviation of N fertilizer dependence in crop production. In previous studies, we identified the Tibetan wild barley accessions with high tolerance to low nitrogen (LN). In this study, metabolic analysis was done on two wild genotypes (XZ149, tolerant and XZ56, sensitive) to understand the mechanism of LN tolerance, using a hydroponic experiment. Leaf and root samples were taken at seven time points within 18 d after LN treatment, respectively. XZ149 was much less affected by low N stress than XZ56 in plant biomass. A total of 51 differentially accumulated metabolites were identified between LN and normal N treated plants. LN stress induced tissue-specific changes in carbon and nitrogen partitioning, and XZ149 had a pattern of energy-saving amino acids accumulation and carbon distribution in favor of root growth that contribute to its higher LN tolerance. Moreover, XZ149 is highly capable of producing energy and maintaining the redox homeostasis under LN stress. The current results revealed the mechanisms underlying the wild barley in high LN tolerance and provided the valuable references for developing barley cultivars with LN tolerance.

Metabolic Adaptations of White Lupin Roots and Shoots under Phosphorus Deficiency

White lupin (Lupinus albus L.) is highly adapted to phosphorus-diminished soils. P-deficient white lupin plants modify their root architecture and physiology to acquire sparingly available soil phosphorus. We employed gas chromatography-mass spectrometry (GC-MS) for metabolic profiling of P-deficient white lupins, to investigate biochemical pathways involved in the P-acquiring strategy. After 14 days of P-deficiency, plants showed reduced levels of fructose, glucose, and sucrose in shoots. Phosphorylated metabolites such as glucose-6-phosphate, fructose-6-phosphate, myo-inositol-phosphate and glycerol-3-phosphate were reduced in both shoots and roots. After 22 days of P-deficiency, no effect on shoot or root sugar metabolite levels was found, but the levels of phosphorylated metabolites were further reduced. Organic acids, amino acids and several shikimate pathway products showed enhanced levels in 22-day-old P-deficient roots and shoots. These results indicate that P-deficient white lupins adapt their carbohydrate partitioning between shoot and root in order to supply their growing root system as an early response to P-deficiency. Organic acids are released into the rhizosphere to mobilize phosphorus from soil particles. A longer period of P-deficiency leads to scavenging of Pi from P-containing metabolites and reduced protein anabolism, but enhanced formation of secondary metabolites. The latter can serve as stress protection molecules or actively acquire phosphorus from the soil.

The Role of Ethylene in Plant Adaptations for Phosphate Acquisition in Soils - A Review

Although a role of ethylene in the regulation of senescence and plant stress responses in general has a long history, a possible involvement in the regulation of adaptive responses to nutrient deficiencies has been mainly investigated since the last two decades. In the case of plant responses to phosphate (Pi) starvation, ethylene was identified as a modulator of adaptive responses in root growth and morphology. The molecular base of these adaptations has been elucidated in supplementation studies with ethylene precursors and antagonists, as well as analysis of mutants and transgenic plants with modified ethylene biosynthesis and responsiveness, using mainly Arabidopsis thaliana as a model plant. However, increasing evidence suggests that apart from root growth responses, ethylene may be involved in various additional plant adaptations to Pi limitation including Pi mobilization in the rhizosphere, Pi uptake and internal Pi recycling. The ethylene-mediated responses are frequently characterized by high genotypic variability and may partially share common pathways in different nutrient limitations.