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Plant roots are colonized by an immense number of microbes, referred to as the root microbiome. Selected strains of beneficial soil-borne bacteria can protect against abiotic stress and prime the plant immune system against a broad range of pathogens. Pseudomonas spp. rhizobacteria represent one of the most abundant genera of the root microbiome. Here, by employing a germ-free experimental system, we demonstrate the ability of selected Pseudomonas spp. strains to promote plant growth and drive developmental plasticity in the roots of Arabidopsis (Arabidopsis thaliana) by inhibiting primary root elongation and promoting lateral root and root hair formation. By studying cell type-specific developmental markers and employing genetic and pharmacological approaches, we demonstrate the crucial role of auxin signaling and transport in rhizobacteria-stimulated changes in the root system architecture of Arabidopsis. We further show that Pseudomonas spp.-elicited alterations in root morphology and rhizobacteria-mediated systemic immunity are mediated by distinct signaling pathways. This study sheds new light on the ability of soil-borne beneficial bacteria to interfere with postembryonic root developmental programs. [ABSTRACT FROM AUTHOR]

Factors with a graded distribution can program fields of cells in a dose-dependent manner, but no evidence has hitherto surfaced for such mechanisms in plants. In the Arabidopsis thaliana root, two PLETHORA (PLT) genes encoding AP2-domain transcription factors have been shown to maintain the activity of stem cells. Here we show that a clade of four PLT homologues is necessary for root formation. Promoter activity and protein fusions of PLT homologues display gradient distributions with maxima in the stem cell area. PLT activities are largely additive and dosage dependent. High levels of PLT activity promote stem cell identity and maintenance; lower levels promote mitotic activity of stem cell daughters; and further reduction in levels is required for cell differentiation. Our findings indicate that PLT protein dosage is translated into distinct cellular responses. [ABSTRACT FROM AUTHOR]

The article reports that the phytohormone auxin plays a major role in plant growth. To determine the direction of auxin flow researchers studied the directional auxin translocation during root gravitropism in Arabidopsis thaliana. They designed hemagglutinin epitope-tagged PIN1 and PIN2 genes under the transcriptional control of the PIN2 promoter. It was observed that in cortex cells, both proteins were localized at the lower basal side. Researchers tested the relationship between polarity and the direction of auxin flow by examining auxin translocation during gravity response. It was found that the localization of PIN proteins at the upper side of epidermal cells was related to their ability to facilitate upward auxin movement for root gravitropic response. Finally it was concluded that PIN polarity is a primary direction determining factor in auxin transport in meristematic tissues. It also provides information on how auxin flows can be redirected through rapid changes in PIN polarity in response to developmental and environmental signals.