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Roots provide physical and nutritional support to plant organs that are above ground and play critical roles for adaptation via intricate movements and growth patterns. Through screening the effects of bacterial isolates from roots of halophyte Mesquite (Prosopis sp.) on Arabidopsis thaliana, we identified Achromobacter sp. 5B1 as a probiotic bacterium that influences plant functional traits. Detailed genetic and architectural analyses in Arabidopsis grown in vitro and in soil, cell division measurements, auxin transport and response gene expression and brefeldin A treatments demonstrated that root colonization with Achromobacter sp. 5B1 changes the growth and branching patterns of roots, which were related to auxin perception and redistribution. Expression analysis of auxin transport and signaling revealed a redistribution of auxin within the primary root tip of wild-type seedlings by Achromobacter sp. 5B1 that is disrupted by brefeldin A and correlates with repression of auxin transporters PIN1 and PIN7 in root provasculature, and PIN2 in the epidermis and cortex of the root tip, whereas expression of PIN3 was enhanced in the columella. In seedlings harboring AUX1, EIR1, AXR1, ARF7ARF19, TIR1AFB2AFB3 single, double or triple loss-of-function mutations, or in a dominant (gain-of-function) mutant of SLR1, the bacterium caused primary roots to form supercoils that are devoid of lateral roots. The changes in growth and root architecture elicited by the bacterium helped Arabidopsis seedlings to resist salt stress better. Thus, Achromobacter sp. 5B1 fine tunes both root movements and the auxin response, which may be important for plant growth and environmental adaptation.

Melatonin (N-acetyl-5-methoxytryptamine) is a tryptophan-derived signal with important physiological roles in mammals. Although the presence of melatonin in plants may be universal, its endogenous function in plant tissues is unknown. On the basis of its structural similarity to indole-3-acetic acid, recent studies mainly focusing on root growth in several plant species have suggested a potential auxin-like activity of melatonin. However, direct evidence about the mechanisms of action of this regulator is lacking. In this work, we used Arabidopsis thaliana seedlings as a model system to evaluate the effects of melatonin on plant growth and development. Melatonin modulated root system architecture by stimulating lateral and adventitious root formation but minimally affected primary root growth or root hair development. The auxin activity of melatonin in roots was investigated using the auxin-responsive marker constructs DR5:uidA, BA3:uidA, and HS::AXR3NT-GUS. Our results show that melatonin neither activates auxin-inducible gene expression nor induces the degradation of HS::AXR3NT-GUS, indicating that root developmental changes elicited by melatonin were independent of auxin signaling. Taken together, our results suggest that melatonin is beneficial to plants by increasing root branching and that root development processes elicited by this novel plant signal are likely independent of auxin responses.

N-acyl-homoserine lactones (AHLs) belong to a class of bacterial quorum-sensing signals important for bacterial cell-to-cell communication. We evaluated Arabidopsis thaliana growth responses to a variety of AHLs ranging from 4 to 14 carbons in length, focusing on alterations in post-embryonic root development as a way to determine the biological activity of these signals. The compounds affected primary root growth, lateral root formation and root hair development, and in particular, N-decanoyl-HL (C10-HL) was found to be the most active AHL in altering root system architecture. Developmental changes elicited by C10-HL were related to altered expression of cell division and differentiation marker lines pPRZ1:uidA, CycB1:uidA and pAtEXP7:uidA in Arabidopsis roots. Although the effects of C10-HL were similar to those produced by auxins in modulating root system architecture, the primary and lateral root response to this compound was found to be independent of auxin signalling. Furthermore, we show that mutant and overexpressor lines for an Arabidopsis fatty acid amide hydrolase gene (AtFAAH) sustained altered growth response to C10-HL. All together, our results suggest that AHLs alter root development in Arabidopsis and that plants posses the enzymatic machinery to metabolize these compounds.