Anatomical and hormonal description of rootlet primordium development along white lupin cluster root

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.

Auxin triggers pectin modification during rootlet emergence in white lupin

Emergence of secondary roots through parental tissue is a highly controlled developmental process. Although the model plant Arabidopsis has been useful to uncover the predominant role of auxin in this process, its simple root structure is not representative of how emergence takes place in most plants, which display more complex root anatomy. White lupin is a legume crop producing structures called cluster roots, where closely spaced rootlets emerge synchronously. Rootlet primordia push their way through several cortical cell layers while maintaining the parent root integrity, reflecting more generally the lateral root emergence process in most multilayered species. In this study, we showed that lupin rootlet emergence is associated with an upregulation of cell wall pectin modifying and degrading genes under the active control of auxin. Among them, we identified LaPG3, a polygalacturonase gene typically expressed in cells surrounding the rootlet primordium and we showed that its downregulation delays emergence. Immunolabeling of pectin epitopes and their quantification uncovered a gradual pectin demethylesterification in the emergence zone, which was further enhanced by auxin treatment, revealing a direct hormonal control of cell wall properties. We also report rhamnogalacturonan-I modifications affecting cortical cells that undergo separation as a consequence of primordium outgrowth. In conclusion, we describe a model of how external tissues in front of rootlet primordia display cell wall modifications to allow for the passage of newly formed rootlets.

Flavonoids are involved in phosphorus-deficiency-induced cluster-root formation in white lupin

BACKGROUND AND AIMS

Initiation of cluster roots in white lupin (Lupinus albus) under phosphorus (P) deficiency requires auxin signalling, whereas flavonoids inhibit auxin transport. However, little information is available about the interactions between P deficiency and flavonoids in terms of cluster-root formation in white lupin.

METHODS

Hydroponic and aeroponic systems were used to investigate the role of flavonoids in cluster-root formation, with or without 75 μm P supply.

KEY RESULTS

Phosphorus-deficiency-induced flavonoid accumulation in cluster roots depended on developmental stage, based on in situ determination of fluorescence of flavonoids and flavonoid concentration. LaCHS8, which codes for a chalcone synthase isoform, was highly expressed in cluster roots, and silencing LaCHS8 reduced flavonoid production and rootlet density. Exogenous flavonoids suppressed cluster-root formation. Tissue-specific distribution of flavonoids in roots was altered by P deficiency, suggesting that P deficiency induced flavonoid accumulation, thus fine-tuning the effect of flavonoids on cluster-root formation. Furthermore, naringenin inhibited expression of an auxin-responsive DR5:GUS marker, suggesting an interaction of flavonoids and auxin in regulating cluster-root formation.

CONCLUSIONS

Phosphorus deficiency triggered cluster-root formation through the regulation of flavonoid distribution, which fine-tuned an auxin response in the early stages of cluster-root development. These findings provide valuable insights into the mechanisms of cluster-root formation under P deficiency.

Dynamic Development of White Lupin Rootlets Along a Cluster Root

White lupin produces cluster roots in response to phosphorus deficiency. Along the cluster root, numerous short rootlets successively appear, creating a spatial and temporal gradient of developmental stages that constitutes a powerful biological model to study the dynamics of the structural and functional evolution of these organs. The present study proposes a fine histochemical, transcriptomic and functional analysis of the rootlet development from its emergence to its final length. Between these two stages, the tissue structures of the rootlets were observed, the course of transcript expressions for the genes differentially expressed was monitored and some physiological events linked to Pi nutrition were followed. A switch between (i) a growing phase, in which a normal apical meristem is present and (ii) a specialized phase for nutrition, in which the rootlet is completely differentiated, was highlighted. In the final stage of its determinate growth, the rootlet is an organ with a very active metabolism, especially for the solubilization and absorption of several nutrients. This work discusses how the transition between a growing to a determinate state in response to nutritional stresses is found in other species and underlines the fundamental dilemma of roots between soil exploration and soil exploitation.

A High-Throughput Phenotyping Tool to Identify Field-Relevant Anthracnose Resistance in White Lupin

The seed- and air-borne pathogen Colletotrichum lupini, the causal agent of lupin anthracnose, is the most important disease in white lupin (Lupinus albus) worldwide and can cause total yield loss. The aims of this study were to establish a reliable high-throughput phenotyping tool to identify anthracnose resistance in white lupin germplasm and to evaluate a genomic prediction model, accounting for previously reported resistance quantitative trait loci, on a set of independent lupin genotypes. Phenotyping under controlled conditions, performing stem inoculation on seedlings, showed to be applicable for high throughput, and its disease score strongly correlated with field plot disease assessments (r = 0.95, P < 0.0001) and yield (r = -0.64, P = 0.035). Traditional one-row field disease phenotyping showed no significant correlation with field plot disease assessments (r = 0.31, P = 0.34) and yield (r = -0.45, P = 0.17). Genomically predicted resistance values showed no correlation with values observed under controlled or field conditions, and the parental lines of the recombinant inbred line population used for constructing the prediction model exhibited a resistance pattern opposite to that displayed in the original (Australian) environment used for model construction. Differing environmental conditions, inoculation procedures, or population structure may account for this result. Phenotyping a diverse set of 40 white lupin accessions under controlled conditions revealed eight accessions with improved resistance to anthracnose. The standardized area under the disease progress curves (sAUDPC) ranged from 2.1 to 2.8, compared with the susceptible reference accession with a sAUDPC of 3.85. These accessions can be incorporated into white lupin breeding programs. In conclusion, our data support stem inoculation-based disease phenotyping under controlled conditions as a time-effective approach to identify field-relevant resistance, which can now be applied to further identify sources of resistance and their underlying genetics.

Cluster roots of Embothrium coccineum modify their metabolism and show differential gene expression in response to phosphorus supply

Embothrium coccineum produces cluster roots (CR) to acquire sparingly soluble phosphorus (P) from the soil through the exudation of organic compounds. However, the physiological mechanisms involved in carbon drainage through its roots, as well as the gene expression involved in the biosynthesis of carboxylates and P uptake, have not been explored. In this work, we evaluated the relationship between carboxylate exudation rate and phosphoenolpyruvate carboxylase (PEPC) activity in roots of E. coccineum seedlings grown in a nutrient-poor volcanic substrate. Second, we evaluated CR formation and the expression of genes involved in the production of carboxylates (PEPC) and P uptake (PHT1) in E. coccineum seedlings grown under three different P supplies in hydroponic conditions. Our results showed that the carboxylate exudation rate was higher in CR than in non-CR, which was consistent with the higher PEPC activity in CR. We found higher CR formation in seedlings grown at 5 μM of P supply, concomitant with a higher expression of EcPEPC and EcPHT1 in CR than in non-CR. Overall, mature CR of E. coccineum seedlings growing on volcanic substrates poor in nutrients modify their metabolism compared to non-CR, enhancing carboxylate biosynthesis and subsequent carboxylate exudation. Additionally, transcriptional responses of EcPEPC and EcPHT1 were induced simultaneously when E. coccineum seedlings were grown in P-limited conditions that favored CR formation. Our results showed, for the first time, changes at the molecular level in CR of a species of the Proteaceae family, demonstrating that these root structures are highly specialized in P mobilization and uptake.

Nitrogen and Phosphorus Interplay in Lupin Root Nodules and Cluster Roots

Nitrogen (N) and phosphorus (P) are two major plant nutrients, and their deficiencies often limit plant growth and crop yield. The uptakes of N or P affect each other, and consequently, understanding N-P interactions is fundamental. Their signaling mechanisms have been studied mostly separately, and integrating N-P interactive regulation is becoming the aim of some recent works. Lupins are singular plants, as, under N and P deficiencies, they are capable to develop new organs, the N2-fixing symbiotic nodules, and some species can also transform their root architecture to form cluster roots, hundreds of short rootlets that alter their metabolism to induce a high-affinity P transport system and enhance synthesis and secretion of organic acids, flavonoids, proteases, acid phosphatases, and proton efflux. These modifications lead to mobilization in the soil of, otherwise unavailable, P. White lupin (Lupinus albus) represents a model plant to study cluster roots and for understanding plant acclimation to nutrient deficiency. It tolerates simultaneous P and N deficiencies and also enhances uptake of additional nutrients. Here, we present the structural and functional modifications that occur in conditions of P and N deficiencies and lead to the organogenesis and altered metabolism of nodules and cluster roots. Some known N and P signaling mechanisms include different factors, including phytohormones and miRNAs. The combination of the individual N and P mechanisms uncovers interactive regulation pathways that concur in nodules and cluster roots. L. albus interlinks N and P recycling processes both in the plant itself and in nature.

Leaf Phosphorus Concentration Regulates the Development of Cluster Roots and Exudation of Carboxylates in Macadamia integrifolia

Phosphorus (P) deficiency induces cluster-root formation and carboxylate exudation in most Proteaceae. However, how external P supply regulates these root traits in Macadamia integrifolia remains unclear. Macadamia plants were grown hydroponically with seven P levels to characterize biomass allocation, cluster-root development, and exudation of carboxylates and acid phosphatases. Plant biomass increased with increasing P supply, peaking at 5 μM P, was the same at 5-25 μM P, and declined at 50-100 μM P. Leaf P concentration increased with increasing P supply, but shoot biomass was positively correlated with leaf P concentration up to 0.7-0.8 mg P g^-1 dry weight (DW), and declined with further increasing leaf P concentration. The number of cluster roots declined with increasing P supply, with a critical value of leaf P concentration at 0.7-0.8 mg P g^-1 DW. We found a similar trend for carboxylate release, with a critical value of leaf P concentration at 0.5 mg g^-1 DW, but the activity of acid phosphatases showed a gradually-decreasing trend with increasing P supply. Our results suggest that leaf P concentration regulates the development and functioning of cluster roots, with a critical P concentration of 0.5-0.8 mg g^-1, above which macadamia growth is inhibited.

Stress tolerance in flax plants inoculated with Bacillus and Azotobacter species under deficit irrigation

Drought stress affects not only crop growth but also its morpho-physiological and biochemical traits to reduce crop productivity. The study reported in this article was designed and implemented to determine the effects of deficit irrigation and bacterial inoculation on flax plants. For this purpose, seeds were inoculated with Bacillus amyloliquefaciens (B₁ ), Bacillus sp. Strain1 (B₂ ), and Azotobacter chroococcum (A) as plant growth promoting rhizobacteria (PGPR). The individual inoculated plants were then grown under field conditions in 2015, while individually and in combination in pots in 2016. The irrigation regimes in either experiments included 50, 75 and 100% crop water requirement. Bacterial cultures were observed to produce ammonia (except B₂ ), indole acetic acid and siderophores. Results showed that the PGPRs significantly mitigated the effects of water deficit. Compared with the control plants, the bacterially-inoculated plants had an enhanced relative water content, plant height, water-soluble carbohydrate and proline contents and antioxidant enzyme activities, but a decreased malondialdehyde content. B₁ exhibited greater effects on most of the traits investigated under the field conditions rather than those with moderate and severe drought stress, while application of the triple bacteria in pots had greater effects on relative water content, carbohydrate and proline contents as well as malondialdehyde. The significant differences in abiotic stress indicators in PGPR-treated plants suggest that these bacteria could be used as biofertilizers to assist plant growth and to reduce the adverse effects of deficit irrigation.

Lateral Roots: Random Diversity in Adversity

Lateral roots are essential for soil foraging and uptake of minerals and water. They feature a large morphological diversity that results from divergent primordia or root growth and development patterns. Besides a structured diversity, resulting from the hierarchical and developmental organization of root systems, there exists a random diversity, occurring between roots of similar age, of the same hierarchical order, and exposed to uniform conditions. The physiological bases and functional consequences of this random diversity are largely ignored. Here we review the evidence for such random diversity throughout the plant kingdom, present innovative approaches based on statistical modeling to account for such diversity, and set the list of its potential benefits in front of a variable and unpredictable soil environment.