Authors: Stephan Pollmann, Adrián González Ortega-Villaizán, Eoghan King, Manish K. Patel, Marta-Marina Pérez-Alonso, Sandra Scholz, Hitoshi Sakakibara, Takatoshi KIba, Mikiko Kojima, Yumiko Takebayashi, Patricio Ramos, Luis Morales-Quintana, Sarah Breitenbach, Ana Smolko, Branka Salopek-Sondi, Nataša Bauer, Jutta Ludwig-Müller, Anne Krapp, Ralf Oelmüller and Jesús Vicente-Carbajosa.

Authorea (2024)

Abstract: "Plants share their habitats with a multitude of different microbes. This close vicinity promoted the evolution of inter-organismic interactions between plants and many different microorganisms that provide mutual growth benefits both to the plant and the microbial partner. The symbiosis of Arabidopsis thaliana with the beneficial root colonizing endophyte Serendipita indica represents a well-studied system. Co-colonization of Arabidopsis roots with S. indica significantly promotes plant growth. Due to the notable phenotypic alterations of fungus-infected root systems, the involvement of a reprogramming of plant hormone levels, especially that of indole-3-acetic acid, has been suggested earlier. However, until now, the molecular mechanism by which S. indica promotes plant growth remains largely unknown. This study used comprehensive transcriptomics, metabolomics, reverse genetics, and life cell imaging to reveal the intricacies of auxin-related processes that affect root growth in the symbiosis between A. thaliana and S. indica. Our experiments revealed the essential role of tightly controlled auxin conjugation in the plant–fungus interaction. It particularly highlighted the importance of two GRETCHEN HAGEN 3 ( GH3) genes, GH3.5 and GH3.17, for the fungus infection-triggered stimulation of biomass production, thus broadening our knowledge about the function of GH3s in plants. Furthermore, we provide evidence for the transcriptional alteration of the PIN2 auxin transporter gene in roots of Arabidopsis seedlings infected with S. indica and demonstrate that this transcriptional adjustment affects auxin signaling in roots, which results in increased plant growth.

Authors: Zilin Zhang, Huihuang Chen, Shuaiying Peng and Huibin Han.

Journal of Experimental Botany (2024)

Highlights: An update of current understanding of slow and rapid responses that triggered by plant phytohormone auxin.

Excerpts: "The plant phytohormone auxin coordinates cellular responses to various developmental and environmental signals, thus optimizing plant growth and development. Auxin controls not only the slow transcriptional responses, but also transcriptional-independent rapid responses occurring in seconds (Box 1 and Box 2; Das et al., 2021; Dubey et al., 2021; Fiedler and Friml, 2023)."

"The nuclear auxin signaling pathway can explain majority of auxin effects on plant growth. However, several developmental processes including rapid root growth inhibition (Fendrych et al. 2018; Li et al., 2021), Ca2+ influxes (Shih et al., 2015), cytoplasmic streaming (Friml et al., 2022), apoplast alkalinization (Li et al., 2021) and membrane depolarization (Serre et al., 2021) are too fast to be mediated by the TIR1/AFB2-5-dependent transcriptional regulation, suggesting a rapid auxin-responsive system exists."

Authors: N. Butel, Y. Qiu, W. Xu, J. Santos-González and C. Köhler.

Nature Plants (2024)

Editor's view: In most flowering plants, early divisions of endosperm nuclei are not succeeded by cellularization. This study uncovered a family of clustered auxin response factors as dosage-sensitive, maternally expressed regulators of endosperm cellularization.

Abstract: "The endosperm is a reproductive tissue supporting embryo development. In most flowering plants, the initial divisions of endosperm nuclei are not succeeded by cellularization; this process occurs only after a specific number of mitotic cycles have taken place. The timing of cellularization significantly influences seed viability and size. Previous research implicated auxin as a key factor in initiating nuclear divisions and determining the timing of cellularization. Here we uncover the involvement of a family of clustered auxin response factors (cARFs) as dosage-sensitive regulators of endosperm cellularization. cARFs, maternally expressed and paternally silenced, are shown to induce cellularization, thereby restricting seed growth. Our findings align with the predictions of the parental conflict theory, suggesting that cARFs represent major molecular targets in this conflict. We further demonstrate a recurring amplification of cARFs in the Brassicaceae, suggesting an evolutionary response to parental conflict by reinforcing maternal control over endosperm cellularization. Our study highlights that antagonistic parental control on endosperm cellularization converges on auxin biosynthesis and signalling."

Authors: Arnau Rovira, Nil Veciana, Aina Basté-Miquel, Martí Quevedo, Antonella Locascio, Lynne Yenush, Gabriela Toledo-Ortiz, Pablo Leivar and Elena Monte.

Nature Communications (2024)

Editor's view: Stomata function is essential for photosynthesis and the global carbon and oxygen cycles. Here, the authors report the regulatory framework that establishes rhythmic pore movements to prevent water loss at night and allow CO2 uptake during the day.

Abstract: "Stomata govern the gaseous exchange between the leaf and the external atmosphere, and their function is essential for photosynthesis and the global carbon and oxygen cycles. Rhythmic stomata movements in daily dark/light cycles prevent water loss at night and allow CO2 uptake during the day. How the actors involved are transcriptionally regulated and how this might contribute to rhythmicity is largely unknown. Here, we show that morning stomata opening depends on the previous night period. The transcription factors PHYTOCHROME-INTERACTING FACTORS (PIFs) accumulate at the end of the night and directly induce the guard cell-specific K+ channel KAT1. Remarkably, PIFs and KAT1 are required for blue light-induced stomata opening. Together, our data establish a molecular framework for daily rhythmic stomatal movements under well-watered conditions, whereby PIFs are required for accumulation of KAT1 at night, which upon activation by blue light in the morning leads to the K+ intake driving stomata opening." 

Authors: Pierre Gautrat, Sanne E. A. Matton, Lisa Oskam, Siddhant S. Shetty, Kyra J. van der Velde and Ronald Pierik.

Journal of Experimental Botany (2024)

Abstract: "Plants growing in dense vegetation stands need to flexibly position their photosynthetic organs to ensure optimal light capture in a competitive environment. They do so through a suite of developmental responses referred to as the shade avoidance syndrome. Belowground, root development is also adjusted in response to aboveground neighbour proximity. Canopies are dynamic and complex environments with heterogeneous light cues in the far-red, red, blue and UV spectrum, which can be perceived with photoreceptors by spatially separated plant tissues. Molecular regulation of plant architecture adjustment via PHYTOCHROME-INTERACTING FACTOR (PIF) transcription factors and growth-related hormones such as auxin, gibberellic acid, brassinosteroids and abscisic acid were historically studied without much attention to spatial or tissue-specific context. Recent developments and technologies have, however, sparked strong interest in spatially explicit understanding of shade avoidance regulation. Other environmental factors such as temperature and nutrient availability interact with the molecular shade avoidance regulation network, often depending on the spatial location of the signals, and the responding organs. Here, we aim to review recent advances in how plants respond to heterogenous light cues and integrate these with other environmental signals."