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1. SHORT‐ROOT and SCARECROW homologs regulate patterning of diverse cell types within and between species

   https://doi.org/10.1111/nph.18654  

   Shaar‐Moshe, Lidor ; Brady, Siobhan M. ( December 2022 , New Phytologist)

   The roles ofSHORT‐ROOT(SHR) andSCARECROW(SCR) in ground tissue patterning and differentiation have been well established in the root ofArabidopsis thaliana. Recently, work in additional organs and species revealed the extensive functional diversification of these genes, including regulation of cortical divisions essential for nodule organogenesis in legume roots, bundle sheath specification in the Arabidopsis leaf, patterning of inner leaf cell layers in maize, and stomatal development in rice. The co‐option of distinct functions and cell types is attributed to different mechanisms, including paralog retention, spatiotemporal changes in gene expression, and novel protein functions. Elaborating our knowledge of theSHR–SCRmodule further unravels the developmental regulation that controls diverse forms and functions within and between species.

    

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2. Ground tissue circuitry regulates organ complexity in maize and Setaria

   https://doi.org/10.1126/science.abj2327  

   Ortiz-Ramírez, Carlos ; Guillotin, Bruno ; Xu, Xiaosa ; Rahni, Ramin ; Zhang, Sanqiang ; Yan, Zhe ; Coqueiro Dias Araujo, Poliana ; Demesa-Arevalo, Edgar ; Lee, Laura ; Van Eck, Joyce ; et al ( December 2021 , Science)

   Most plant roots have multiple cortex layers that make up the bulk of the organ and play key roles in physiology, such as flood tolerance and symbiosis. However, little is known about the formation of cortical layers outside of the highly reduced anatomy of Arabidopsis . Here, we used single-cell RNA sequencing to rapidly generate a cell-resolution map of the maize root, revealing an alternative configuration of the tissue formative transcription factor SHORT-ROOT (SHR) adjacent to an expanded cortex. We show that maize SHR protein is hypermobile, moving at least eight cell layers into the cortex. Higher-order SHR mutants in both maize and Setaria have reduced numbers of cortical layers, showing that the SHR pathway controls expansion of cortical tissue to elaborate anatomical complexity. 

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3. Distinct mechanisms orchestrate the contra-polarity of IRK and KOIN, two LRR-receptor-kinases controlling root cell division

   https://doi.org/10.1038/s41467-021-27913-1  

   Rodriguez-Furlan, Cecilia ; Campos, Roya ; Toth, Jessica N. ; Van Norman, Jaimie M. ( January 2022 , Nature Communications)

   In plants, cell polarity plays key roles in coordinating developmental processes. Despite the characterization of several polarly localized plasma membrane proteins, the mechanisms connecting protein dynamics with cellular functions often remain unclear. Here, we introduce a polarized receptor, KOIN, that restricts cell divisions in the Arabidopsis root meristem. In the endodermis, KOIN polarity is opposite to IRK, a receptor that represses endodermal cell divisions. Their contra-polar localization facilitates dissection of polarity mechanisms and the links between polarity and function. We find that IRK and KOIN are recognized, sorted, and secreted through distinct pathways. IRK extracellular domains determine its polarity and partially rescue the mutant phenotype, whereas KOIN’s extracellular domains are insufficient for polar sorting and function. Endodermal expression of an IRK/KOIN chimera generates non-cell-autonomous misregulation of root cell divisions that impacts patterning. Altogether, we reveal two contrasting mechanisms determining these receptors’ polarity and link their polarity to cell divisions in root tissue patterning.

    

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4. Polarly localized receptor-like kinases PXC2 and IRK act redundantly during Arabidopsis root development in the radial axis

   https://doi.org/10.1101/2021.02.11.429611  

   Goff, Jason B ( January 2021 , bioRxiv)

   In plants, coordination of cell division and differentiation is critical for tissue patterning and organ development. Directional cell signaling and cell polarity have been proposed to participate in coordination of these developmental processes. For instance, a leucine-rich repeat receptor-like kinase (LRR-RLK) named INFLORESCENCE AND ROOT APICES KINASE (IRK) functions to restrict stele area and inhibit longitudinal anticlinal divisions (LADs) in the endodermis where it is polarly localized. The LRR-RLK most closely related to IRK is PXY/TDR CORRELATED 2 (PXC2) and we find that PXC2 shows similar polarized accumulation as IRK in root cell types. To further understand how these proteins operate in directional cell-cell signaling and root development we explored PXC2 function. pxc2 roots have an increase in stele area, indicating that PXC2 also functions to restrict stele size. Additionally, compared to either single mutant, irk pxc2 roots have an enhanced phenotype with further increases in endodermal LADs and stele area indicating redundant activities of these receptors. The double mutant also exhibits abnormal root growth, suggesting broader functions of PXC2 and IRK in the root. However, PXC2 is not functionally equivalent to IRK, as endodermal misexpression of PXC2 did not fully rescue irk. We propose that PXC2 is at least partially redundant to IRK with a more predominant role for IRK in repression of endodermal LADs. Our results are consistent with the hypothesis that repression of specific endodermal cell divisions and stele area through a PXC2/IRK-mediated directional signaling pathway is required for coordinated root growth and development. 

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5. Mutation of BAM2 rescues the sunn hypernodulation phenotype in Medicago truncatula, suggesting that a signaling pathway like CLV1/BAM in Arabidopsis affects nodule number

   https://doi.org/10.3389/fpls.2023.1334190  

   Thomas, Jacklyn ; Frugoli, Julia ( January 2024 , Frontiers in Plant Science)

   The unique evolutionary adaptation of legumes for nitrogen-fixing symbiosis leading to nodulation is tightly regulated by the host plant. The autoregulation of nodulation (AON) pathway negatively regulates the number of nodules formed in response to the carbon/nitrogen metabolic status of the shoot and root by long-distance signaling to and from the shoot and root. Central to AON signaling in the shoots ofMedicago truncatulais SUNN, a leucine-rich repeat receptor-like kinase with high sequence similarity with CLAVATA1 (CLV1), part of a class of receptors inArabidopsisinvolved in regulating stem cell populations in the root and shoot. This class of receptors inArabidopsisincludes the BARELY ANY MERISTEM family, which, like CLV1, binds to CLE peptides and interacts with CLV1 to regulate meristem development.M. truncatulacontains five members of theBAMfamily, but onlyMtBAM1andMtBAM2are highly expressed in the nodules 48 hours after inoculation. Plants carry mutations in individualMtBAMs, and several doubleBAMmutant combinations all displayed wild-type nodule number phenotypes. However,Mtbam2suppressed thesunn-5hypernodulation phenotype and partially rescued the short root length phenotype ofsunn-5 when present in asunn-5background. Grafting determined thatbam2suppresses supernodulation from the roots, regardless of theSUNNstatus of the root. Overexpression ofMtBAM2in wild-type plants increases nodule numbers, while overexpression ofMtBAM2in somesunnmutants rescues the hypernodulation phenotype, but not the hypernodulation phenotypes of AON mutantrdn1-2orcrn. Relative expression measurements of the nodule transcription factor MtWOX5 downstream of the putativebam2 sunn-5complex revealed disruption of meristem signaling; while bothbam2andbam2 sunn-5influenceMtWOX5expression, the expression changes are in different directions. We propose a genetic model wherein the specific root interactions of BAM2/SUNN are critical for signaling in nodule meristem cell homeostasis inM. truncatula.

    

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