Medicago LINC Complexes Function in Nuclear Morphology, Nuclear Movement, and Root Nodule Symbiosis

Abstract

Nuclear movement is involved in cellular and developmental processes across eukaryotic life, often driven by Linker of Nucleoskeleton and Cytoskeleton (LINC) complexes, which bridge the nuclear envelope (NE) via the interaction of Klarsicht/ ANC-1/Syne-1 Homology (KASH) and Sad1/UNC-84 (SUN) proteins. Arabidopsis (Arabidopsis thaliana) LINC complexes are involved in nuclear movement and positioning in several cell types. Observations since the 1950s have described targeted nuclear movement and positioning during symbiosis initiation between legumes and rhizobia, but it has not been established whether these movements are functional or incidental. Here, we identify and characterize LINC complexes in the model legume Medicago truncatula. We show that LINC complex characteristics such as NE localization, dependence of KASH proteins on SUN protein binding for NE enrichment, and direct SUN-KASH binding are conserved between plant species. Using a SUN dominant-negative strategy, we demonstrate that LINC complexes are necessary for proper nuclear shaping and movement in Medicago root hairs, and are important for infection thread initiation and nodulation. 4

MLKS2 is an ARM domain and F-actin-associated KASH protein that functions in stomatal complex development and meiotic chromosome segregation

“…Nuclear morphology is commonly altered in LINC mutants, some of which present pathological conditions such as envelopathies [58,59]. In eudicot plants, disruption of LINC components cause alterations in root hair nuclear shape phenotypes [28,34,39,60]. In maize, we similarly observed that mlks2 nuclei change to more compact and spherical shapes than those from normal cells (Figure 5(a-c)).…”

“…Interestingly, GFP-AtSINE1, but not GFP-AtSINE2, colocalizes with F-actin when expressed in tobacco or Arabidopsis [45]. Medicago SINE-group KASH genes, MtSINE1 and MtSINE2, also encode proteins that colocalize with F-actin [34]. The ARM domains alone may not, therefore, be predictive of F-actin binding.…”

“…However, given the global challenges in agriculture and the need for plant-based biorenewable resources [33], it is important to better understand the plant NE and its fundamental components. Recently, progress towards understanding plant LINC biology has expanded beyond Arabidopsis to include Medicago and maize [34,35].…”

LINC-complex mediated positioning of the vegetative nucleus is involved in calcium and ROS signaling in Arabidopsis pollen tubes

“…Plant KASH proteins were first identified in Arabidopsis, through identification of a transmembrane domain, a shorter luminal tail domain (9-15 aa), and a similar characteristic C-terminal tail, terminating in a consensus 'XVPT' motif [3,4]. Homologs of Arabidopsis SUN and KASH proteins have subsequently been found in a variety of land plant species [5] and functionally investigated in Z. mays and M. truncatula [4,6,7,8]. The first identified plant KASH proteins, WPP-Interacting Proteins (WIPs) 1, 2 and 3, along with their ONM interaction partners the WPP-Interacting Tail Anchored Proteins 1 and 2 (WITs), form a LINC complex with Arabidopsis SUN1 and SUN2 [3].…”

What Drives Symbiotic Calcium Signalling In Legumes? Insights And Challenges Of Imaging

“…Ca 2+ signals have been linked to the infection thread growth and nodule formation by analysis of different mutants. The blue oval denotes the nucleus, which moves to guide the formation of the infection thread [3,4]. The nucleus is anchored by cytoplasmic strands which connect it to the cytoplasm (not represented, it presents different cytoarchitecture according to the root hair development stage).…”