Much of plant development depends on cell-to-cell redistribution of the plant hormone auxin, which is facilitated by the plasma membrane (PM) localized PIN FORMED (PIN) proteins. Auxin export activity, developmental roles, subcellular trafficking, and polarity of PINs have been well studied, but their structure remains elusive besides a rough outline that they contain two groups of 5 alpha-helices connected by a large hydrophilic loop (HL). Here, we focus on the PIN1 HL as we could produce it in sufficient quantities for biochemical investigations to provide insights into its secondary structure. Circular dichroism (CD) studies revealed its nature as an intrinsically disordered protein (IDP), manifested by the increase of structure content upon thermal melting. Consistent with IDPs serving as interaction platforms, PIN1 loops homodimerize. PIN1 HL cytoplasmic overexpression in Arabidopsis disrupts early endocytic trafficking of PIN1 and PIN2 and causes defects in the cotyledon vasculature formation. In summary, we demonstrate that PIN1 HL has an intrinsically disordered nature, which must be considered to gain further structural insights. Some secondary structures may form transiently during pairing with known and yet-to-be-discovered interactors.

• Keywords
• PIN1, dimerization, hydrophilic hoop, intrinsic disorder, subcellular trafficking,
• MeSH
• Arabidopsis * metabolism   MeSH
• Biological Transport MeSH
• Plant Roots metabolism   MeSH
• Indoleacetic Acids metabolism   MeSH
• Membrane Transport Proteins genetics   metabolism   MeSH
• Arabidopsis Proteins * genetics   metabolism   MeSH
• Intrinsically Disordered Proteins * genetics   metabolism   MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Indoleacetic Acids MeSH
• Membrane Transport Proteins MeSH
• PIN1 protein, Arabidopsis MeSH Browser
• Arabidopsis Proteins * MeSH
• Intrinsically Disordered Proteins * MeSH

To overcome nitrogen deficiency, legume roots establish symbiotic interactions with nitrogen-fixing rhizobia that are fostered in specialized organs (nodules). Similar to other organs, nodule formation is determined by a local maximum of the phytohormone auxin at the primordium site. However, how auxin regulates nodule development remains poorly understood. Here, we found that in soybean, (Glycine max), dynamic auxin transport driven by PIN-FORMED (PIN) transporter GmPIN1 is involved in nodule primordium formation. GmPIN1 was specifically expressed in nodule primordium cells and GmPIN1 was polarly localized in these cells. Two nodulation regulators, (iso)flavonoids trigger expanded distribution of GmPIN1b to root cortical cells, and cytokinin rearranges GmPIN1b polarity. Gmpin1abc triple mutants generated with CRISPR-Cas9 showed the impaired establishment of auxin maxima in nodule meristems and aberrant divisions in the nodule primordium cells. Moreover, overexpression of GmPIN1 suppressed nodule primordium initiation. GmPIN9d, an ortholog of Arabidopsis thaliana PIN2, acts together with GmPIN1 later in nodule development to acropetally transport auxin in vascular bundles, fine-tuning the auxin supply for nodule enlargement. Our findings reveal how PIN-dependent auxin transport modulates different aspects of soybean nodule development and suggest that the establishment of auxin gradient is a prerequisite for the proper interaction between legumes and rhizobia.

• MeSH
• Biological Transport MeSH
• Glycine max genetics   growth & development   metabolism   MeSH
• Root Nodules, Plant growth & development   metabolism   MeSH
• Indoleacetic Acids metabolism   MeSH
• Plant Proteins genetics   metabolism   MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Indoleacetic Acids MeSH
• Plant Proteins MeSH

Plant growth and development are critically influenced by unpredictable abiotic factors. To survive fluctuating changes in their environments, plants have had to develop robust adaptive mechanisms. The dynamic and complementary actions of the auxin and cytokinin pathways regulate a plethora of developmental processes, and their ability to crosstalk makes them ideal candidates for mediating stress-adaptation responses. Other crucial signaling molecules responsible for the tremendous plasticity observed in plant morphology and in response to abiotic stress are reactive oxygen species (ROS). Proper temporal and spatial distribution of ROS and hormone gradients is crucial for plant survival in response to unfavorable environments. In this regard, the convergence of ROS with phytohormone pathways acts as an integrator of external and developmental signals into systemic responses organized to adapt plants to their environments. Auxin and cytokinin signaling pathways have been studied extensively. Nevertheless, we do not yet understand the impact on plant stress tolerance of the sophisticated crosstalk between the two hormones. Here, we review current knowledge on the function of auxin and cytokinin in redirecting growth induced by abiotic stress in order to deduce their potential points of crosstalk.

• Keywords
• ROS, abiotic stress, adaptation, auxin, crosstalk, cytokinin, growth,
• MeSH
• Biological Transport MeSH
• Cytokinins metabolism   MeSH
• Adaptation, Physiological MeSH
• Stress, Physiological * MeSH
• Plant Physiological Phenomena * MeSH
• Gene Regulatory Networks MeSH
• Indoleacetic Acids metabolism   MeSH
• Gene Expression Regulation, Plant MeSH
• Plant Growth Regulators metabolism   MeSH
• Plants genetics   metabolism   MeSH
• Signal Transduction MeSH
• Publication type
• Journal Article MeSH
• Review MeSH
• Names of Substances
• Cytokinins MeSH
• Indoleacetic Acids MeSH
• Plant Growth Regulators MeSH