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1. Regulation of AUXIN RESPONSE FACTOR condensation and nucleo-cytoplasmic partitioning

   https://doi.org/10.1038/s41467-022-31628-2  

   Jing, Hongwei; Korasick, David A.; Emenecker, Ryan J.; Morffy, Nicholas; Wilkinson, Edward G.; Powers, Samantha K.; Strader, Lucia C. (December 2022, Nature Communications)

   Abstract Auxin critically regulates plant growth and development. Auxin-driven transcriptional responses are mediated through the AUXIN RESPONSE FACTOR (ARF) family of transcription factors. ARF protein condensation attenuates ARF activity, resulting in dramatic shifts in the auxin transcriptional landscape. Here, we perform a forward genetics screen for ARF hypercondensation, identifying an F-box protein, which we named AUXIN RESPONSE FACTOR F-BOX1 (AFF1). Functional characterization of SCF^AFF1revealed that this E3 ubiquitin ligase directly interacts with ARF19 and ARF7 to regulate their accumulation, condensation, and nucleo-cytoplasmic partitioning. Mutants defective inAFF1display attenuated auxin responsiveness, and developmental defects, suggesting that SCF^AFF1-mediated regulation of ARF protein drives aspects of auxin response and plant development. 

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2. Rapid biosensor development using plant hormone receptors as reprogrammable scaffolds

   https://doi.org/10.1038/s41587-022-01364-5  

   Beltrán, Jesús; Steiner, Paul J.; Bedewitz, Matthew; Wei, Shuang; Peterson, Francis C.; Li, Zongbo; Hughes, Brigid E.; Hartley, Zachary; Robertson, Nicholas R.; Medina-Cucurella, Angélica V.; et al (June 2022, Nature Biotechnology)

   Abstract A general method to generate biosensors for user-defined molecules could provide detection tools for a wide range of biological applications. Here, we describe an approach for the rapid engineering of biosensors using PYR1 (Pyrabactin Resistance 1), a plant abscisic acid (ABA) receptor with a malleable ligand-binding pocket and a requirement for ligand-induced heterodimerization, which facilitates the construction of sense–response functions. We applied this platform to evolve 21 sensors with nanomolar to micromolar sensitivities for a range of small molecules, including structurally diverse natural and synthetic cannabinoids and several organophosphates. X-ray crystallography analysis revealed the mechanistic basis for new ligand recognition by an evolved cannabinoid receptor. We demonstrate that PYR1-derived receptors are readily ported to various ligand-responsive outputs, including enzyme-linked immunosorbent assay (ELISA)-like assays, luminescence by protein-fragment complementation and transcriptional circuits, all with picomolar to nanomolar sensitivity. PYR1 provides a scaffold for rapidly evolving new biosensors for diverse sense–response applications. 

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3. The leaf polarity factors SGS3 and YABBYs regulate style elongation through auxin signaling in Mimulus lewisii

   https://doi.org/10.1111/nph.17702  

   Ding, Baoqing; Li, Jingjian; Gurung, Vandana; Lin, Qiaoshan; Sun, Xuemei; Yuan, Yao‐Wu (September 2021, New Phytologist)

   Summary Style length is a major determinant of breeding strategies in flowering plants and can vary dramatically between and within species. However, little is known about the genetic and developmental control of style elongation.We characterized the role of two classes of leaf adaxial–abaxial polarity factors, SUPPRESSOR OF GENE SILENCING3 (SGS3) and the YABBY family transcription factors, in the regulation of style elongation inMimulus lewisii. We also examined the spatiotemporal patterns of auxin response during style development.Loss ofSGS3function led to reduced style length via limiting cell division, and downregulation ofYABBYgenes by RNA interference resulted in shorter styles by decreasing both cell division and cell elongation. We discovered an auxin response minimum between the stigma and ovary during the early stages of pistil development that marks style differentiation. Subsequent redistribution of auxin response to this region was correlated with style elongation. Auxin response was substantially altered when bothSGS3andYABBYfunctions were disrupted.We suggest that auxin signaling plays a central role in style elongation and that the way in which auxin signaling controls the different cell division and elongation patterns underpinning natural style length variation is a major question for future research. 

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4. Auxin resistant 2 and short hypocotyl 2 regulate cotton fiber initiation and elongation

   https://doi.org/10.1093/plphys/kiae183  

   Jin, Fei; Zhu, Liping; Hou, Liyong; Li, Hongbin; Li, Ling; Xiao, Guanghui (March 2024, Plant Physiology)

   Abstract Auxin, a pivotal regulator of diverse plant growth processes, remains central to development. The auxin-responsive genes auxin/indole-3-acetic acids (AUX/IAAs) are indispensable for auxin signal transduction, which is achieved through intricate interactions with auxin response factors (ARFs). Despite this, the potential of AUX/IAAs to govern the development of the most fundamental biological unit, the single cell, remains unclear. In this study, we harnessed cotton (Gossypium hirsutum) fiber, a classic model for plant single-cell investigation, to determine the complexities of AUX/IAAs. Our research identified 2 pivotal AUX/IAAs, auxin resistant 2 (GhAXR2) and short hypocotyl 2 (GhSHY2), which exhibit opposite control over fiber development. Notably, suppressing GhAXR2 reduced fiber elongation, while silencing GhSHY2 fostered enhanced fiber elongation. Investigating the mechanistic intricacies, we identified specific interactions between GhAXR2 and GhSHY2 with distinct ARFs. GhAXR2's interaction with GhARF6-1 and GhARF23-2 promoted fiber cell development through direct binding to the AuxRE cis-element in the constitutive triple response 1 promoter, resulting in transcriptional inhibition. In contrast, the interaction of GhSHY2 with GhARF7-1 and GhARF19-1 exerted a negative regulatory effect, inhibiting fiber cell growth by activating the transcription of xyloglucan endotransglucosylase/hydrolase 9 and cinnamate-4-hydroxylase. Thus, our study reveals the intricate regulatory networks surrounding GhAXR2 and GhSHY2, elucidating the complex interplay of multiple ARFs in AUX/IAA-mediated fiber cell growth. This work enhances our understanding of single-cell development and has potential implications for advancing plant growth strategies and agricultural enhancements. 

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5. Initiation of aboveground organ primordia depends on combined action of auxin, ERECTA family genes, and PINOID

   https://doi.org/10.1093/plphys/kiac288  

   DeGennaro, Daniel; Urquidi_Camacho, Ricardo_Andres; Zhang, Liang; Shpak, Elena_D (June 2022, Plant Physiology)

   Abstract Leaves and flowers are produced by the shoot apical meristem (SAM) at a certain distance from its center, a process that requires the hormone auxin. The amount of auxin and the pattern of its distribution in the initiation zone determine the size and spatial arrangement of organ primordia. Auxin gradients in the SAM are formed by PIN-FORMED (PIN) auxin efflux carriers whose polar localization in the plasma membrane depends on the protein kinase PINOID (PID). Previous work determined that ERECTA (ER) family genes (ERfs) control initiation of leaves. ERfs are plasma membrane receptors that enable cell-to-cell communication by sensing extracellular small proteins from the EPIDERMAL PATTERNING FACTOR/EPF-LIKE (EPF/EPFL) family. Here, we investigated whether ERfs regulate initiation of organs by altering auxin distribution or signaling in Arabidopsis (Arabidopsis thaliana). Genetic and pharmacological data suggested that ERfs do not regulate organogenesis through PINs while transcriptomics data showed that ERfs do not alter primary transcriptional responses to auxin. Our results indicated that in the absence of ERf signaling the peripheral zone cells inefficiently initiate leaves in response to auxin signals and that increased accumulation of auxin in the er erecta-like1 (erl1) erl2 SAM can partially rescue organ initiation defects. We propose that both auxin and ERfs are essential for leaf initiation and that they have common downstream targets. Genetic data also indicated that the role of PID in initiation of cotyledons and leaves cannot be attributed solely to regulation of PIN polarity and PID is likely to have other functions in addition to regulation of auxin distribution. 

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