Analysis of plant growth-promoting properties of Bacillus amyloliquefaciens UCMB5113 using Arabidopsis thaliana as host plant

This study showed that Bacillus amyloliquefaciens UCMB5113 colonizing Arabidopsis roots changed root structure and promoted growth implying the usability of this strain as a novel tool to support sustainable crop production.Root architecture plays a crucial role for plants to ensure uptake of water, minerals and nutrients and to provide anchorage in the soil. The root is a dynamic structure with plastic growth and branching depending on the continuous integration of internal and environmental factors. The rhizosphere contains a complex microbiota, where some microbes can colonize plant roots and support growth and stress tolerance. Here, we report that the rhizobacterium Bacillus amyloliquefaciens subsp. plantarum UCMB5113 stimulated the growth of Arabidopsis thaliana Col-0 by increased lateral root outgrowth and elongation and root-hair formation, although primary root elongation was inhibited. In addition, the growth of the above ground tissues was stimulated by UCMB5113. Specific hormone reporter gene lines were tested which suggested a role for at least auxin and cytokinin signaling during rhizobacterial modulation of Arabidopsis root architecture. UCMB5113 produced cytokinins and indole-3-acetic acid, and the formation of the latter was stimulated by root exudates and tryptophan. The plant growth promotion effect by UCMB5113 did not appear to depend on jasmonic acid in contrast to the disease suppression effect in plants. UCMB5113 exudates inhibited primary root growth, while a semi-purified lipopeptide fraction did not and resulted in the overall growth promotion indicating an interplay of many different bacterial compounds that affect the root growth of the host plant. This study illustrates that beneficial microbes interact with plants in root development via classic and novel signals

WOX5–IAA17 feedback circuit-mediated cellular auxin response is crucial for the patterning of root stem cell niches in Arabidopsis

In plants, the patterning of stem cell-enriched meristems requires a graded auxin response maximum that emerges from the concerted action of polar auxin transport, auxin biosynthesis, auxin metabolism, and cellular auxin response machinery. However, mechanisms underlying this auxin response maximum-mediated root stem cell maintenance are not fully understood. Here, we present unexpected evidence that WUSCHEL-RELATED HOMEOBOX 5 (WOX5) transcription factor modulates expression of auxin biosynthetic genes in the quiescent center (QC) of the root and thus provides a robust mechanism for the maintenance of auxin response maximum in the root tip. This WOX5 action is balanced through the activity of indole-3-acetic acid 17 (IAA17) auxin response repressor. Our combined genetic, cell biology, and computational modeling studies revealed a previously uncharacterized feedback loop linking WOX5-mediated auxin production to IAA17-dependent repression of auxin responses. This WOX5–IAA17 feedback circuit further assures the maintenance of auxin response maximum in the root tip and thereby contributes to the maintenance of distal stem cell (DSC) populations. Our experimental studies and in silico computer simulations both demonstrate that the WOX5–IAA17 feedback circuit is essential for the maintenance of auxin gradient in the root tip and the auxin-mediated root DSC differentiation

Inter-regulation of the unfolded protein response and auxin signaling

The unfolded protein response (UPR) is a signaling network triggered by overload of protein-folding demand in the endoplasmic reticulum (ER), a condition termed ER stress. The UPR is critical for growth and development; nonetheless, connections between the UPR and other cellular regulatory processes remain largely unknown. Here, we identify a link between the UPR and the phytohormone auxin, a master regulator of plant physiology. We show that ER stress triggers down-regulation of auxin receptors and transporters in Arabidopsis thaliana. We also demonstrate that an Arabidopsis mutant of a conserved ER stress sensor IRE1 exhibits defects in the auxin response and levels. These data not only support that the plant IRE1 is required for auxin homeostasis, they also reveal a species-specific feature of IRE1 in multicellular eukaryotes. Furthermore, by establishing that UPR activation is reduced in mutants of ER-localized auxin transporters, including PIN5, we define a long-neglected biological significance of ER-based auxin regulation. We further examine the functional relationship of IRE1 and PIN5 by showing that an ire1 pin5 triple mutant enhances defects of UPR activation and auxin homeostasis in ire1 or pin5. Our results imply that the plant UPR has evolved a hormone-dependent strategy for coordinating ER function with physiological processes

The effects of ABA on endoreduplication.

<p>The influence of ABA on nuclear ploidy was investigated using hypocotyl segments of <i>sit</i> and WT seedlings grown in darkness on untreated media and media containing 100 nM ABA (A). Triangles represent all measured values (n = 40) for each genotype and treatment. The bars within the boxes indicate the median values in each case, while the boxes’ upper and lower boundaries indicate the boundaries of the first and third quartiles. Three independent experiments were performed. The Kruskal-Wallis ANOVA test with multiple comparisons revealed that only the <i>sit</i> control sample differed significantly from the WT control (p < 0.01). No other significant differences were found. (B) Analysis of the expression of the <i>SlKRP1</i> and <i>SlKRP3</i> genes based on the mean of three independent experiments ± SE. <i>Tip41like</i> and <i>PP2ACs</i> were used as housekeeping genes. All expression values are quoted relative to those for the “WT control” sample.</p

Effect of ABA deficiency on the expansion of hypocotyl cells.

<p>The influence of ABA on the elongation of epidermal cells of etiolated hypocotyls in <i>sit</i> and WT (cv. Rheinlands Ruhm) was investigated using SEM imaging analysis. Triangles represent all measured values (n = 471) for each genotype. The bars within the boxes indicate the median values in each case, while the boxes’ upper and lower boundaries indicate the boundaries of the first and third quartiles. The Mann-Whitney test was used to prove statistical significance (p < 0.01).</p

Endogenous Abscisic Acid Promotes Hypocotyl Growth and Affects Endoreduplication during Dark-Induced Growth in Tomato (<i>Solanum lycopersicum</i> L.)

<div><p>Dark-induced growth (skotomorphogenesis) is primarily characterized by rapid elongation of the hypocotyl. We have studied the role of abscisic acid (ABA) during the development of young tomato (<i>Solanum lycopersicum</i> L.) seedlings. We observed that ABA deficiency caused a reduction in hypocotyl growth at the level of cell elongation and that the growth in ABA-deficient plants could be improved by treatment with exogenous ABA, through which the plants show a concentration dependent response. In addition, ABA accumulated in dark-grown tomato seedlings that grew rapidly, whereas seedlings grown under blue light exhibited low growth rates and accumulated less ABA. We demonstrated that ABA promotes DNA endoreduplication by enhancing the expression of the genes encoding inhibitors of cyclin-dependent kinases <i>SlKRP1</i> and <i>SlKRP3</i> and by reducing cytokinin levels. These data were supported by the expression analysis of the genes which encode enzymes involved in ABA and CK metabolism. Our results show that ABA is essential for the process of hypocotyl elongation and that appropriate control of the endogenous level of ABA is required in order to drive the growth of etiolated seedlings.</p></div

Effect of <i>sit</i> and <i>not</i> mutations on the endogenous ABA content in etiolated hypocotyls.

<p>The dark-grown hypocotyls of <i>sit</i> and <i>not</i> mutants and corresponding WTs were harvested five days after germination and ABA content was measured. Free ABA levels are reported as relative means [a.u.] ± SE based on two biological repeats. The mutant values are expressed relative to those for corresponding WT, which were assigned a value of 100 a.u.</p