The MKK7-MPK6 MAP Kinase Module Is a Regulator of Meristem Quiescence or Active Growth in Arabidopsis

Plant growth flexibly adapts to environmental conditions. Growth initiation itself may be conditional to a suitable environment, while the most common response of plants to adverse conditions is growth inhibition. Most of our understanding about environmental growth inhibition comes from studies on various plant hormones, while less is known about the signaling mechanisms involved. The mitogen-activated protein kinase (MAPK) cascades are central signal transduction pathways in all eukaryotes and their roles in plant stress responses is well-established, while increasing evidence points to their involvement in hormonal and developmental processes. Here we show that the MKK7-MPK6 module is a suppressor of meristem activity using genetic approaches. Shoot apical meristem activation during light-induced de-etiolation is accelerated in mpk6 and mkk7 seedlings, whereas constitutive or induced overexpression of MKK7 results in meristem defects or collapse, both in the shoot and the root apical meristems. These results underscore the role of stress-activated MAPK signaling in regulating growth responses at the whole plant level, which may be an important regulatory mechanism underlying the environmental plasticity of plant development

Mise en évidence de l'antagonisme hypaphorine / AIA lors du développement de l'ectomycorhize Pisolithus tinctorius-Eucalyptus globulus

Very little is known about the molecules regulating the interaction between plants and ectomycorrhizal fungi during root colonisatio'n. Therole of fungal auxin in ectomycorrhiza has repeatedly been suggested and questioned, suggesting that if fungal auxin controls sorne steps of colonised root development, its activity might be tightly controlled in time and in space by plant and/or fungal regulatory mechanisms. Hypaphorine, the major indolic compound isolated from theectomycorrhizal nmgus Pisolithus tinetorius, controls root hair elongation rate. At inhibitory concentrations, hypaphorine induces a transitory root hair tip swelling associated with root hairs cytoskeleton reorganisation. While IAA had no activity on root hair elongation, IAA was able to restore root hair polar growth following inhibition by hypaphorine. Hypaphorine activity could imply calcium flux modifications at the root hair tip. Furthennore, two experimental results confirm hypaphorine / IAA antagonism. We have demonstrated that hypaphorine counteracts the inhibiting activity of IAA on Eucalyptus taprootelongation. Likewise, while seedling treatment with ACC, the precursor of ethylene, results in formation of an hypocotyle apical hook, hypaphorine application as weil as root colonisation by Pisolithus tinetorius, stimulated hook opening. Hypaphorine counteraction with ACC is likely a consequence of hypaphorine interaction with IAA. In most plant microbe interactions studied, the interactions result in increased auxin synthesis or auxin accumulation in plant tissues. The P. tine/orius / Eucalyptus interaction is intriguing becausehere, the microbe down regulates the auxin activity in, the host plant. Hypaphorine might be the first specific IAA antagonist identified.La mise en place de la symbiose mycorhizienne implique l'établissement d'un dialogue entre deux partenaires, une plante et un champignon. Cette communication se fait par l'intermédiaire de molécules signal. La synthèse et l'accumulation d'hypaphorine (un alcaloïde indolique) dans les hyphes de Pisolithus tinctorius colonisant les racines d'Eucalyptus globulus a été précédemment rapportée. Bien que de structure moléculaire très proche de l'auxine naturelle (l'acide 3-indole acétique ou AIA), l'hypaphorine ne possède aucune activité auxinique sur le développement racinaire ou l'élongation des poils absorbants. Au contraire, l'hypaphorine se comporte comme un antagoniste de l'AIA. En effet, alors que les auxines (AIA et 2,4 d) stimulent faiblement l'allongement des poils absorbants, l'hypaphorine l'inhibe. La croissance apicale des poils inhibés par l'hypaphorine peut être restaurée en présence d'AIA. Cette inhibition s'accompagne de déformations à l'apex des poils, et d'une réorganisation du cytosquelette. L'activité de l'hypaphorine pourrait impliquer une modification des flux calciques à l'apex des poils. Par ailleurs, deux résultats expérimentaux confirment l'antagonisme hypaphorine / AIA. Nous avons montré que l'hypaphorine restaure l'allongement de pivots d'eucalyptus inhibés par de fortes concentrations d'AIA. De même l'hypaphorine synthétisée et accumulée par le champignon Pisolithus tinctorius au cours du développement de l'ectomycorhize, possède une activité anti-auxinique ; elle induit le redressement des hypocotyles d'eucalyptus cultivés en présence d'ACC (un précurseur de la synthèse d'éthylène). Il s'agit là de la première démonstration faisant état d'une réduction de l'activité de l'AIA dans les tissus de la plante hôte, par un microorganisme. Ainsi, certaines étapes critiques au cours de l'ontogenèse mycorhizienne, nécessiterait un subtil équilibre entre des molécules actives comme l'AIA et des molécules régulatrices comme l'hypaphorine

Lateral root stimulation in the early interaction between Arabidopsis thaliana and the ectomycorrhizal fungus Laccaria bicolor: Is fungal auxin the trigger?

Lateral root (LR) stimulation during early signal exchange between plant roots and ectomycorrhizal (ECM) fungi has recently been shown to be achieved by modulation of auxin gradients. We suggested that this modulation could occur through altered polar auxin transport (PAT) and through activation of auxin signalling pathways in the root. However, it remains unclear, which fungal molecules alter auxin pathways inside the plant partner. It has been suggested in previous studies that auxin released by the fungus could trigger observed plant responses during early signal exchange and later on during root colonization. Here we focus on the early interaction and we provide evidence for an alternative mechanism. Indeed, LR stimulation by the fungus in Arabidopsis thaliana followed a totally different timing than with exogenously applied auxin. Furthermore, experimental conditions that excluded the exchange of soluble molecules while allowing exchange of volatile(s) between the plant and the fungus were sufficient for LR induction, therefore questioning the role of secreted fungal auxin. These data suggest that volatiles released by the fungus and sensed by the plant may act upstream of altered auxin signaling in the plant

Settling for less: do statoliths modulate gravity perception?

Plants orientate their growth either towards (in roots) or away from (in shoots) the Earth’s gravitational field. While we are now starting to understand the molecular architecture of these gravity response pathways, the gravity receptor remains elusive. This perspective looks at the biology of statoliths and suggests it is conceivable that their immediate environment may be tuned to modulate the strength of the gravity response. It then suggests how mutant screens could use this hypothesis to identify the gravity receptor

Settling for Less: Do Statoliths Modulate Gravity Perception?

Plants orientate their growth either towards (in roots) or away from (in shoots) the Earth’s gravitational field. While we are now starting to understand the molecular architecture of these gravity response pathways, the gravity receptor remains elusive. This perspective looks at the biology of statoliths and suggests it is conceivable that their immediate environment may be tuned to modulate the strength of the gravity response. It then suggests how mutant screens could use this hypothesis to identify the gravity receptor.</jats:p