Abstract
In this paper, we describe the role of the receptor-like kinase ERULUS (ERU) in PT growth of Arabidopsis thaliana. In silico analysis and transcriptional reporter lines revealed that ERU is only expressed in pollen and root hairs (RHs), making it a tip growth-specific kinase. Deviations from Mendelian inheritance were observed in the offspring of self-pollinated heterozygous eru plants. We found that in vivo eru PT targeting was disturbed, providing a possible explanation for the observed decrease in eru fertilization competitiveness. Extracellular calcium perception and intracellular calcium dynamics lie at the basis of in vivo pollen tube (PT) tip growth and guidance. In vitro, ERU loss-of-function lines displayed no obvious PT phenotype, unless grown on low extracellular calcium ([Ca2+]ext) medium. When grown at 12 the normal [Ca2+]ext, eru PTs grew 37% slower relative to WT PTs. Visualization of cytoplasmic [Ca2+]cyt oscillations using the Yellow Cameleon 3.6 (YC3.6) calcium sensor showed that, unlike in WT PTs, eru apical [Ca2+]cyt oscillations occur at a lower frequency when grown at lower [Ca2+]ext, consistent with the observed reduced growth velocity. Our results show that the tip growth-specific kinase ERULUS is involved in regulating Ca2+-dependent PT growth, and most importantly, fertilization efficiency through successful PT targeting to the ovules.
Highlights
Pollen tube (PT) tip growth is a highly polarized form of cell elongation leading to the delivery of sperm cells at the embryo sac and subsequent double fertilization (Obermeyer and Feijó, 2017)
Recent data showed that ERULUS, a receptor-like kinase (RLKs) from the Arabidopsis thaliana subfamily of Catharanthus roseus RECEPTOR-LIKE KINASE 1-LIKE proteins (CrRLK1Ls), is expressed in trichoblast cells and that it controls tip growth of root hairs (RHs) in Arabidopsis (Bai et al, 2014a; Haruta et al, 2014)
Tip growth is restricted to RHs and PTs
Summary
Pollen tube (PT) tip growth is a highly polarized form of cell elongation leading to the delivery of sperm cells at the embryo sac and subsequent double fertilization (Obermeyer and Feijó, 2017). A number of highly dynamic cellular processes occur during PT growth (Obermeyer and Feijó, 2017), including the formation and maintenance of intra- and extracellular ion dynamics, apical endo-/exocytosis and the modification of the cytoskeleton, all of which depend on tight spatial and temporal control of cytosolic [Ca2+] oscillations at the tip (Schoenaers et al, 2017). Ca2+ ions have a pivotal role in controling cell wall flexibility, a sine qua non condition for (oscillatory) PT elongation. Despite these findings, it remains poorly understood how small changes in the [Ca2+]cyt oscillatory regime are regulated, and how they can lead to alterations of PT growth. Dependent PT growth and the control of in vivo PT guidance and fertilization
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