Sofie Goormachtig

Strigolactones Suppress Adventitious Rooting in Arabidopsis and Pea

Adventitious root formation is essential for the propagation of many commercially important plant species and involves the formation of roots from nonroot tissues such as stems or leaves. Here, we demonstrate that the plant hormone strigolactone suppresses adventitious root formation in Arabidopsis (Arabidopsis thaliana) and pea (Pisum sativum). Strigolactone-deficient and response mutants of both species have enhanced adventitious rooting. CYCLIN B1 expression, an early marker for the initiation of adventitious root primordia in Arabidopsis, is enhanced in more axillary growth2 (max2), a strigolactone response mutant, suggesting that strigolactones restrain the number of adventitious roots by inhibiting the very first formative divisions of the founder cells. Strigolactones and cytokinins appear to act independently to suppress adventitious rooting, as cytokinin mutants are strigolactone responsive and strigolactone mutants are cytokinin responsive. In contrast, the interaction between the strigolactone and auxin signaling pathways in regulating adventitious rooting appears to be more complex. Strigolactone can at least partially revert the stimulatory effect of auxin on adventitious rooting, and auxin can further increase the number of adventitious roots in max mutants. We present a model depicting the interaction of strigolactones, cytokinins, and auxin in regulating adventitious root formation.

CLE Peptides Control Medicago truncatula Nodulation Locally and Systemically

The CLAVATA3/embryo-surrounding region (CLE) peptides control the fine balance between proliferation and differentiation in plant development. We studied the role of CLE peptides during indeterminate nodule development and identified 25 MtCLE peptide genes in the Medicago truncatula genome, of which two genes, MtCLE12 and MtCLE13, had nodulation-related expression patterns that were linked to proliferation and differentiation. MtCLE13 expression was up-regulated early in nodule development. A high-to-low expression gradient radiated from the inner toward the outer cortical cell layers in a region defining the incipient nodule. At later stages, MtCLE12 and MtCLE13 were expressed in differentiating nodules and in the apical part of mature, elongated nodules. Functional analysis revealed a putative role for MtCLE12 and MtCLE13 in autoregulation of nodulation, a mechanism that controls the number of nodules and involves systemic signals mediated by a leucine-rich repeat receptor-like kinase, SUNN, which is active in the shoot. When MtCLE12 and MtCLE13 were ectopically expressed in transgenic roots, nodulation was abolished at the level of the nodulation factor signal transduction, and this inhibition involved long-distance signaling. In addition, composite plants with roots ectopically expressing MtCLE12 or MtCLE13 had elongated petioles. This systemic effect was not observed in transgenic roots ectopically expressing MtCLE12 and MtCLE13 in a sunn-1 mutant background, although nodulation was still strongly reduced. These results suggest multiple roles for CLE signaling in nodulation.

Reactive oxygen species and ethylene play a positive role in lateral root base nodulation of a semiaquatic legume

Lateral root base nodulation on the tropical, semiaquatic legume Sesbania rostrata results from two coordinated, Nod factor-dependent processes: formation of intercellular infection pockets and induction of cell division. Infection pocket formation is associated with cell death and production of hydrogen peroxide. Pharmacological experiments showed that ethylene and reactive oxygen species mediate Nod factor responses and are required for nodule initiation, whereby induction of division and infection could not be uncoupled. Application of purified Nod factors triggered cell division, and both Nod factors and ethylene induced cavities and cell death features in the root cortex. Thus, in S. rostrata, ethylene and reactive oxygen species act downstream from the Nod factors in pathways that lead to formation of infection pockets and initiation of nodule primordia.

Aging in Legume Symbiosis. A Molecular View on Nodule Senescence in Medicago truncatula

Rhizobia reside as symbiosomes in the infected cells of legume nodules to fix atmospheric nitrogen. The symbiotic relation is strictly controlled, lasts for some time, but eventually leads to nodule senescence. We present a comprehensive transcriptomics study to understand the onset of nodule senescence in the legume Medicago truncatula. Distinct developmental stages with characteristic gene expression were delineated during which the two symbiotic partners were degraded consecutively, marking the switch in nodule tissue status from carbon sink to general nutrient source. Cluster analysis discriminated an early expression group that harbored regulatory genes that might be primary tools to interfere with pod filling-related or stress-induced nodule senescence, ultimately causing prolonged nitrogen fixation. Interestingly, the transcriptomes of nodule and leaf senescence had a high degree of overlap, arguing for the recruitment of similar pathways.

Strigolactones are involved in root response to low phosphate conditions in Arabidopsis.

Strigolactones (SLs) are plant hormones that suppress lateral shoot branching, and act to regulate root hair elongation and lateral root formation. Here, we show that SLs are regulators of plant perception of or response to low inorganic phosphate (Pi) conditions. This regulation is mediated by MORE AXILLARY GROWTH2 (MAX2) and correlated with transcriptional induction of the auxin receptor TRANSPORT INHIBITOR RESPONSE1 (TIR1). Mutants of SL signaling (max2-1) or biosynthesis (max4-1) showed reduced response to low Pi conditions relative to the wild type. In max4-1, but not max2-1, the reduction in response to low Pi was compensated by the application of a synthetic strigolactone GR24. Moreover, AbamineSG, which decreases SL levels in plants, reduced the response to low Pi in the wild type, but not in SL-signaling or biosynthesis mutants. In accordance with the reduced response of max2-1 to low Pi relative to the wild type, several phosphate-starvation response and phosphate-transporter genes displayed reduced induction in max2-1, even though Pi content in max2-1 and the wild type were similar. Auxin, but not ethylene, was sufficient to compensate for the reduced max2-1 response to low Pi conditions. Moreover, the expression level of TIR1 was induced under low Pi conditions in the wild type, but not in max2-1. Accordingly, the tir1-1 mutant showed a transient reduction in root hair density in comparison with the wild type under low Pi conditions. Therefore, we suggest that the response of plants to low Pi is regulated by SLs; this regulation is transmitted via the MAX2 component of SL signaling and is correlated with transcriptional induction of the TIR1 auxin receptor.

Use of differential display to identify novel Sesbania rostrata genes enhanced by Azorhizobium caulinodans infection.

Upon infection of the tropical legume Sesbania rostrata with Azorhizobium caulinodans ORS571, nodules are formed on the roots as well as on the stems. Stem nodules appear at multiple predetermined sites consisting of dormant root primordia, which are positioned in vertical rows along the stem of the plant. We used the differential display method to isolate and characterize three cDNA clones (differential display; didi-2, didi-13, and didi-20), corresponding to genes whose expression is enhanced in the dormant root primordia after inoculation. Database searches revealed that the deduced (partial) didi-2 gene product shares significant similarity with hydroxyproline-rich cell wall proteins. The (partial) didi-13 and didi-20 products are similar to chitinases and chalcone reductases, respectively. Transcripts corresponding to the cDNA clones didi-2 and didi-13 were first detectable 1 day after inoculation. In contrast, didi-20 transcripts were found at low levels in uninfected root primordia and were enhanced significantly around 3 days after inoculation. In addition, a cDNA was isolated (didi-42) that corresponds to the previously identified leghemoglobin gene Srlb6. These studies show that differential display is a useful method for the isolation of infection-related genes.

Srchi13, a Novel Early Nodulin from Sesbania rostrata, Is Related to Acidic Class III Chitinases

On the tropical legume Sesbania rostrata, stem-borne nodules develop after inoculation of adventitious root primordia with the microsymbiont Azorhizobium caulinodans. A cDNA clone, Srchi13, with homology to acidic class III chitinase genes, corresponds to an early nodulin gene with transiently induced expression during nodule ontogeny. Srchi13 transcripts accumulated strongly 2 days after inoculation, decreased from day 7 onward, and disappeared in mature nodules. Induction was dependent on Nod factor–producing bacteria. Srchi13 was expressed around infection pockets, in infection centra, around the developing nodule and its vascular bundles, and in uninfected cells of the central tissue. The specific and transient transcript accumulation together with the lipochitooligosaccharide degradation activity of the recombinant protein hint at a role of Srchi13 in normal nodule ontogeny by limiting the action of Nod factors.

Never too many? How legumes control nodule numbers

Restricted availability of nitrogen compounds in soils is often a major limiting factor for plant growth and productivity. Legumes circumvent this problem by establishing a symbiosis with soil-borne bacteria, called rhizobia that fix nitrogen for the plant. Nitrogen fixation and nutrient exchange take place in specialized root organs, the nodules, which are formed by a coordinated and controlled process that combines bacterial infection and organ formation. Because nodule formation and nitrogen fixation are energy-consuming processes, legumes develop the minimal number of nodules required to ensure optimal growth. To this end, several mechanisms have evolved that adapt nodule formation and nitrogen fixation to the plants needs and environmental conditions, such as nitrate availability in the soil. In this review, we give an updated view on the mechanisms that control nodulation.

Nodule numbers are governed by interaction between CLE peptides and cytokinin signaling

CLE peptides are involved in the balance between cell division and differentiation throughout plant development, including nodulation. Previously, two CLE genes of Medicago truncatula, MtCLE12 and MtCLE13, had been identified whose expression correlated with nodule primordium formation and meristem establishment. Gain-of-function analysis indicated that both MtCLE12 and MtCLE13 interact with the SUPER NUMERIC NODULES (SUNN)-dependent auto-regulation of nodulation to control nodule numbers. Here we demonstrate that cytokinin, which is essential for nodule organ formation, regulates MtCLE13 expression. In addition, simultaneous knockdown of MtCLE12 and MtCLE13 resulted in an increase in nodule number, implying that both genes play a role in controlling nodule number. Additionally, a weak link may exist with the ethylene-dependent mechanism that locally controls nodule number.

The protein quality control system manages plant defence compound synthesis

Jasmonates are ubiquitous oxylipin-derived phytohormones that are essential in the regulation of many development, growth and defence processes. Across the plant kingdom, jasmonates act as elicitors of the production of bioactive secondary metabolites that serve in defence against attackers. Knowledge of the conserved jasmonate perception and early signalling machineries is increasing, but the downstream mechanisms that regulate defence metabolism remain largely unknown. Here we show that, in the legume Medicago truncatula, jasmonate recruits the endoplasmic-reticulum-associated degradation (ERAD) quality control system to manage the production of triterpene saponins, widespread bioactive compounds that share a biogenic origin with sterols. An ERAD-type RING membrane-anchor E3 ubiquitin ligase is co-expressed with saponin synthesis enzymes to control the activity of 3-hydroxy-3-methylglutaryl-CoA reductase (HMGR), the rate-limiting enzyme in the supply of the ubiquitous terpene precursor isopentenyl diphosphate. Thus, unrestrained bioactive saponin accumulation is prevented and plant development and integrity secured. This control apparatus is equivalent to the ERAD system that regulates sterol synthesis in yeasts and mammals but that uses distinct E3 ubiquitin ligases, of the HMGR degradation 1 (HRD1) type, to direct destruction of HMGR. Hence, the general principles for the management of sterol and triterpene saponin biosynthesis are conserved across eukaryotes but can be controlled by divergent regulatory cues.