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Pin1At regulates PIN1 polar localization and root gravitropism
Description The importance of the plant growth regulator auxin for plant growth has long been recognized, even before the discovery of its chemical structures in the early 20th century. The present short communication summarizes the scattered evidence in support of known host root alterations by beneficial microbes , implying the key role of auxin signaling and polar auxin transport in modulating beneficial effects of microbes in plants.
Auxin signaling, Polar auxin transport, Plant-beneficial microbe interactions, Root development. Like animals, plants have their own microbiome that protects them from various adverse environmental conditions [ 1 ]. Plant roots live in close association with a large set of bacteria that thrive in the rhizosphere.
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Some of these microbes have a significant impact on root morphogenesis [ 2 , 3 ]. Plant growth-promoting bacteria PGPB refer to rhizobacteria and endophytes that enhance the growth of their hosts. Among these rhizobacteria, some can promote plant growth and provide a better environment for plant growth through indirect or direct means. For example, Bacillus megaterium can promote Arabidopsis shoot and root fresh weight and Arabidopsis endophytic microbe Bacillus sp. LZR can promote Arabidopsis shoot weight and alter the root system architecture [ 4 , 5 ]. The contribution of beneficial microbes to plant root development can be exerted by mechanisms including secretion of plant growth-regulating substance such as auxin and bacterial volatiles [ 6 , 7 ].
In wild-type root apices, auxin is enriched only at those cross-walls which are active in auxin export, being abundant not only at walls but also in adjacent endosomes. Similar evidence is provided by forcing roots to grow against the gravity vector due to placing them into thin glass capillaries. Such roots get progressively thinner and their root apices get depleted of dividing cells due to inhibited supply of auxin which can not be transported effectively against the gravity vector.
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- Pin1At regulates PIN1 polar localization and root gravitropism | Nature Communications.
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One of the most characteristic feature of the polar transport of auxin, which still awaits for its biological explanation, is that the auxin transport is tightly linked to the gravity being all the time perpendicular to the gravity vector. In the root cap, PIN3-mediated auxin asymmetry starts to be visible within few minutes. In other words, internal genetic and epigenetic programmes, which are extremely robust and resist single or even double-gene mutations, are prone to be easily overridden by epigenetic environmental information.
One can explain this gravity-dependent secretion of auxin via differential stretch-stress of the plasma membrane 6 , 19 which is expected, due to protoplast settling, to have the highest values at the physical bottom and the lowest values at the physical top of cells. This unique feature of gravisensing plant synapses implies that they are inherently asymmetric not only molecularly but also mechanically.
Polar Auxin Transport and Asymmetric Auxin Distribution
Importantly, epigenetic physical information, which underlies inherent mechanical asymmetry of plant synapse, is part of the plant synapse secreting auxin according to the gravity vector and shaping flexibly the plant body. Growing roots exhibit several animal-like features. Having two growth zones which can curve independently, 21 , 22 but in highly coordinated manner, 6 growing roots show animal-like curiosity in their exploratory growth.
Charles and Francis Darwins noted pages —54 in ref. Darwins proposed that root apex, covered with the root cap, resembles in many respects brains of lower animals. Interestingly, Julius Sachs was not able to repeat these experiments as he was apparently removing also parts of the apical root meristem pages —54 in ref. Besides gravity, root apices continuously monitor numerous other parameters from their environment to obtain sensory information which is integrated to allow adaptive behavior of exploratory root apices.
Recent advances identified sensor molecules for hydrosensing 26 and sensing of low phosphate27 in gravisensing root cap statocytes. Blue light sensor Phot1 is not expressed in root cap cells but localizes to plant synapses in the transition zone. Physiological and adaptive relevances of electrotropism and magnetotropis are still obscure and should be studied intensively in future in order to understand roots in their whole complexity.
In accordance with the complex behavior, root apices show much more complex auxin transport when up to 5 PINs and 4 ABC channels are involved. In contrast, shoot apices initiate primordia of leaves and lateral shoot branches closely to the shoot apex 30 which is morphologically much more complex. The shoot apex represent complex organogenic surface which is devoted to morphogenesis and initiates new organs in a highly controlled manner. In contrast, the lateral root primordia are formed far away of the root tip, only after root cells are ceasing their rapid elongation.
Obviously, the root apex represents some kind of sensory surface specialized for sensing of many parameters of environment. This feature allows roots to grow with the animal-like curiosity, to explore soil in searching for water and satisfying the mineral nutrition for the whole plant. Vesicular secretion of auxin can be expected to be under tight control allowing quantal release of auxin into the plant synaptic cleft after specific stimuli are received.
It can be expected that this feature would serve not only for synchronization of cells in one cell file but also for propagation of plant action potentials. Besides eliciting electrical responses in adjacent cells reviewed in ref. Among other aspects, vesicle recycling and secretion-based auxin efflux can be envisioned to act as the elusive flux sensor which is necessary for the canalization theory. This would then make redundant new hypotheses such as travelling-wave hypothesis proposed recently.
As root apices are well protected from predatory and other environmental insults, it would be logical to expect location of the putative master clock of the circadian system, which is in brains of animals and, perhaps, in the root system. In accordance with this scenario, recent study revealed that auxin signaling is tightly linked with the plant circadian clock 37 and that the root growth, in contrast to the shoot growth, is not showing circadian rhythms but shows rather steady growth behavior.
Vesicular secretion of auxin at root apices has several implications and consequences. First of all, the auxin status changes from plant hormone to multipurpose mobile signaling molecule which, depending on developmental or environmental context, acts as plant hormone, morphogen, or plant neurotransmitter-like molecule.
Recent Activity. The plant hormone auxin is secreted in root apices via phospholipase Dzeta2 PLDzeta2 activity which produces specific population of phosphatidic acid that stimulates secretion of vesicles enriched with auxin. The snippet could not be located in the article text. This may be because the snippet appears in a figure legend, contains special characters or spans different sections of the article.
Polar Auxin Transport and Asymmetric Auxin Distribution
Plant Signal Behav. PMID: Corresponding author. Received Oct 12; Accepted Oct This article has been cited by other articles in PMC. Auxin as Multipurpose and Mobile Signaling Molecule In recent years, the classical plant hormone auxin emerges to act rather as mobile multipurpose signaling and communicator molecule orchestrating plant development and integrating it with the environmental abiotic factors, especially light and gravity. Implications of Auxin Secretion: Gravisensing Plant Synapses One of the most characteristic feature of the polar transport of auxin, which still awaits for its biological explanation, is that the auxin transport is tightly linked to the gravity being all the time perpendicular to the gravity vector.
Outlook Vesicular secretion of auxin at root apices has several implications and consequences. References 1. The case for morphogens in plants.