Auxin Signaling and Polar Auxin Transport Play an Important Role in Regulating Root Development during Plant-Beneficial Microbes Interactions

Abstract

Plant roots are colonized by an immense number of microbes, including epiphytic and endophytic microbes, which can promote plant growth and alter host root development. Previous studies showed that auxin plays key roles in regulating many different aspects of plant growth and development. However, the precise role of auxin signaling and polar auxin transport in modulating root development during plant-beneficial microbe interactions is unknown. 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.

Keywords

Auxin signaling, Polar auxin transport, Plant-beneficial microbe interactions, Root development

Introduction

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. 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. LZR216 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]. Bacterial secretion of phytohormones can overproduce root hairs and lateral roots and subsequently increase nutrient and water uptake, thus contributing to growth promotion [8]. To elucidate auxin signaling mechanisms by which beneficial microbes modify plant root system architecture, we used Arabidopsis as a model plant to clarify the roles of auxin signaling pathway during microbes-regulated root development.

Roles of Auxin Signaling in Plant Root System Architecture Alteration by Beneficial Microbes

The auxin-responsive DR5::GUS transgenic lines can indirectly provide insights into changes of auxin levels and signaling during plant-beneficial microbe interactions. For example, rhizosphere Phyllobacterium brassicacearum STM196 enhances the DR5::GUS expression in primary and lateral root tips, but it does not enhance lateral root numbers in aux1 and axr1 mutants,indicating that auxin signaling plays an important role during beneficial microbe-regulated root development. Moreover, Pseudomonas fluorescens WCS417 can increase the shoot fresh weight and alter the root system architecture. Furthermore,WCS417 enhances the expression of DR5::vYFP and pAUX1: AUX1-YFP in Arabidopsis primary root tips, but it does not increase lateral root numbers and root hair numbers in the tir1afb2afb3 mutant [6]. Another study shows that Trichoderma virens, a plant beneficial fungus, can increase the DR5: GUS expression in shoots and primary root tips, but fails in increasing lateral root numbers in the axr1-3 mutant [9,10]. Auxin gene expression profiling studies have shed light on components of auxin signaling involved in microbe-induced root architectural changes. For example, The TIR1 gene expression level is down-regulated in the roots but slightly up-regulated in the shoots by Phyllobacterium brassicacearum STM196 [9]. In Populus, auxin-responsive transcription factors of the Aux/IAA family are transiently induced during co-culture with Laccaria bicolor [11]. Examples mentioned above show that auxin signaling is important during plant-microbes interactions.

Roles of Polar Auxin Transport in Plant Root System Architecture Alteration by Beneficial Microbes

Auxin transport inhibitor can block auxin accumulation at sites of biosynthesis in shoots of Arabidopsis inoculated with Bacillus subtilis , suppress the level of auxin in roots and reduce the growth-promoting effects[7]. This example indicates that polarauxin transport is also essential for plant-beneficial microbe interaction. Inoculation of Arabidopsis auxin transport mutants pin2 with Trichoderma virens reduces the growth compared with wild-type plants [10], indicating that normal auxin transport is important for the promoting effects of Trichoderma virens on root development. Recently, a study shows that Bacillus sp. LZR216 does not enhance lateral root number per plant in the mutant aux1-7 [5]. In Arabidopsis, after Laccaria bicolor treatment, aux1 and pin3 exhibit similar induction of lateral root numbers while less stimulation is observed in single mutant pin2 and quadruple mutant pin2,3,4,7 [11]. Inoculation with Phyllobacterium brassicacearum STM196 induces a weak increase in transcript levels of PIN1 and PIN2 in shoots, but not in roots[9]. Furthermore, the study shows that Bacillus sp. LZR216 significantly reduces the expression of PIN1, PIN2, PIN3, and AUX1 in root tips [5].

Conclusion

These aforementioned results provide evidence that auxin transport machinery is involved in plant-microbe interactions.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (31671595; 31670244), Foundation of Science and Technology Program of Gansu Province (1506RJZA209), The Agricultural Biotechnology Research and Application Development Program of Gansu Province (GNSW-2016-23), The Fundamental Research Funds for the Central Universities (lzujbky-2016-80), The Foundation of Science and Technology Program of Lanzhou City (2015-3-53), The Project of Qinghai Science & Technology Department (2016-ZJ-Y01), The Open Project of State Key Laboratory of Plateau Ecology and Agriculture, Qinghai University (201 -KF-05).

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