Strigolactones activate different hormonal pathways for regulation of root development in response to phosphate growth conditions

Background

Strigolactones (SLs) – a group of plant hormones and their derivatives – have been found to play a role in the regulation of root development, in addition to their role in suppression of lateral shoot branching: they alter root architecture and affect root-hair elongation, and SL signalling is necessary for the root response to low phosphate (Pi) conditions. These effects of SLs have been shown to be associated with differential activation of the auxin and ethylene signalling pathways.

Scope

The present review highlights recent findings on the activity of SLs as regulators of root development, in particular in response to low Pi stress, and discusses the different hormonal networks putatively acting with SLs in the root''s Pi response.

Conclusions

SLs are suggested to be key regulators of the adaptive responses to low Pi in the root by modulating the balance between auxin and ethylene signalling. Consequently, they impact different developmental programmes responsible for the changes in root system architecture under differential Pi supply.     

Strigolactones interact with ethylene and auxin in regulating root-hair elongation in Arabidopsis

Strigolactones (SLs) or derivatives thereof have been identified as phytohormones, and shown to act as long-distance shoot-branching inhibitors. In Arabidopsis roots, SLs have been suggested to have a positive effect on root-hair (RH) elongation, mediated via the MAX2 F-box. Two other phytohormones, auxin and ethylene, have been shown to have positive effects on RH elongation. Hence, in the present work, Arabidopsis RH elongation was used as a bioassay to determine epistatic relations between SLs, auxin, and ethylene. Analysis of the effect of hormonal treatments on RH elongation in the wild type and hormone-signalling mutants suggested that SLs and ethylene regulate RH elongation via a common regulatory pathway, in which ethylene is epistatic to SLs, whereas the effect of SLs on RH elongation requires ethylene synthesis. SL signalling was not needed for the auxin response, whereas auxin signalling was not necessary, but enhanced RH response to SLs, suggesting that the SL and auxin hormonal pathways converge for regulation of RH elongation. The ethylene pathway requirement for the RH response to SLs suggests that ethylene forms a cross-talk junction between the SL and auxin pathways.     

Strigolactones affect lateral root formation and root-hair elongation in Arabidopsis

Strigolactones (SLs) have been proposed as a new group of plant hormones, inhibiting shoot branching, and as signaling molecules for plant interactions. Here, we present evidence for effects of SLs on root development. The analysis of mutants flawed in SLs synthesis or signaling suggested that the absence of SLs enhances lateral root formation. In accordance, roots grown in the presence of GR24, a synthetic bioactive SL, showed reduced number of lateral roots in WT and in max3-11 and max4-1 mutants, deficient in SL synthesis. The GR24-induced reduction in lateral roots was not apparent in the SL signaling mutant max2-1. Moreover, GR24 led to increased root-hair length in WT and in max3-11 and max4-1 mutants, but not in max2-1. SLs effect on lateral root formation and root-hair elongation may suggest a role for SLs in the regulation of root development; perhaps, as a response to growth conditions.     

Role of vascular adrenergic mechanisms in the haemodynamic and PGI2 stimulating effects of angiotensin in diabetic dogs

In aware of the well-known altered vascular responsiveness in the diabetic vasculature, this study aimed to compare the haemodynamic and PGI₂ releasing effects of angiotensin in metabolically healthy (12) and alloxan-(560 umol/kg) diabetic (12) dogs as well as to analyze the role of vascular adrenoceptors in this. In vivo the effect of intracoronarially administered angiotensin (63-125-250-500-1000 pmol/kg/min) on coronary blood flow, mean arterial blood pressure, myocardial contractile force and heart rate was investigated without and with pretreatment of 2 umol/kg phentolamine. In vitro PGI₂ release by isolated coronary rings was induced by 50 nmol/1 angiotensin before and after pretreatment with 5 umol/1 phentolamine and measured by radioimmunoassay. Angiotensin enhances dose-dependently both the mean arterial blood pressure and coronary blood flow, while it provokes a considerable (p < 0.05) increase of PGI₂ formation by isolated coronary arterial rings. These alterations could be prevented by phentolamine administration both in vivo and in vitro, while this drug did not affect the angiotensin-induced enhancement of diabetic coronary blood flow. On the other hand the increase of blood pressure by angiotensin was found to be more (p < 0.05) expressed in diabetes and it could be further potentiated by phentolamine. PGI₂ synthesis by isolated diabetic coronary rings could not be modified either by angiotensin alone or in combination with phentolamine. On the basis of above data, the lack of stimulated vascular PGI₂ formation mediated by alpha-adrenergic mechanisms is supposed to causatively contribute to the diminished sensitivity of diabetic coronary arteries to vasodilation.     

Left ventricular diastolic function in diabetics

Left ventricular diastolic function was studied in 29 young diabetic patients (aged from 14 to 44 years) without any clinical sign of heart disease. The metabolic state, the presence and the degree of microvascular and neuropathic complications have been established. Age and sex matched 32 healthy subjects served as controls. The parameters of left ventricular diastolic function were determined by means of phonomechanocardiography. By this method in diabetic patients impaired diastolic function of the left ventricle was found. This alteration could be best characterized by the values of normalized relaxation index referring to the isovolumetric relaxation of the left ventricle. A close correlation was found between the microvascular and neuropathic complications and the left ventricular diastolic dysfunction, while no correlation could be demonstrated between the metabolic state and the diastolic cardiac disorder.     

Expression of endo-1,4-β-glucanase (cel1) in Arabidopsis thaliana is associated with plant growth, xylem development and cell wall thickening

Arabidopsis thaliana CEL1 protein was detected in young expanding tissues. Immunostaining revealed that CEL1 accumulated mostly in xylem cells. The primary, as well as the secondary xylem showed considerable CEL1 staining. CEL1 was also observed in young epidermal cells, in which the thicker lateral and tangential walls stained more intensely than the inner walls. In newly formed cell walls, the lateral tangential walls were labeled more intensively than the inner walls. Cellulase activity was found to be significantly higher in growing tissue compared to mature parts of the plant. Cel1 expression concurrently with cellulase activity could be restored in detached matured leaves by sucrose treatment after 48 h in the culture medium.