Two Mitogen-Activated Protein Kinases, MPK3 and MPK6, Are Required for Funicular Guidance of Pollen Tubes in Arabidopsis    

Abstract

Double fertilization in flowering plants requires the delivery of two immotile sperm cells to the female gametes by a pollen tube, which perceives guidance cues, modifies its tip growth direction, and eventually enters the micropyle of the ovule. In spite of the recent progress, so far, little is known about the signaling events in pollen tubes in response to the guidance cues. Here, we show that MPK3 and MPK6, two Arabidopsis (Arabidopsis thaliana) mitogen-activated protein kinases, mediate the guidance response in pollen tubes. Genetic analysis revealed that mpk3 mpk6 double mutant pollen has reduced transmission. However, direct observation of mpk3 mpk6 mutant pollen phenotype was hampered by the embryo lethality of double homozygous mpk3 -/-mpk6 -/-plants. Utilizing a fluorescent reporter-tagged complementation method, we showed that the mpk3 mpk6 mutant pollen had normal pollen tube growth but impaired pollen tube guidance. In vivo pollination assays revealed that the mpk3 mpk6 mutant pollen tubes were defective in the funicular guidance phase. By contrast, semi-in vitro guidance assay showed that the micropylar guidance of the double mutant pollen tube was normal. Our results provide direct evidence to support that the funicular guidance phase of the pollen tube requires an in vivo signaling mechanism distinct from the micropyle guidance. Moreover, our finding opened up the possibility that the MPK3/MPK6 signaling pathway may link common signaling networks in plant stress response and pollen-pistil interaction.

Mitogen-activated protein kinase cascades in signaling plant growth and development

“…In addition to embryogenesis, MAPKs also play vital roles in other stages of plant reproduction including anther development [40,41], ovule development [42], pollen development [37,43,44], pollen tube guidance [45], and seeds development [27,46]. Abnormal subcellular structure or cell division account for some of the mutant phenotypes, for example, a disrupted plasma membrane and unusual After the sensing of tissue/cell-specific input signaling molecules (e.g., peptide ligands), receptor-like protein kinase (RLK) receptors activate MAPK cascades in a spatiotemporal-specific manner, either directly or through additional mediator(s).…”

AtVPS41-mediated endocytic pathway is essential for pollen tube–stigma interaction inArabidopsis

“…In vitro pollen germination assays were conducted according to previously reported protocols (37,38). The measurement of pollen tube length and pollen tube germination percentages were made with Image J software (rsbweb.nih.gov/ij/).…”

Gametophytic Pollen Tube Guidance: Attractant Peptides, Gametic Controls, and Receptors

“…The relationships between CNGC18 and MDIS1-MIK/PRK6 are of interest. Other pollen tube molecules for ovular guidance include glycosylphosphatidylinositol anchor and its biosynthesis proteins Dai et al, 2014), mitogen-activated protein kinases (Guan et al, 2014), AGC kinases for eukaryotic polarized cell growth (Zhang et al, 2009), endoplasmic reticulum-localized K + channel (Lu et al, 2011) and luminal proteins (Li et al, 2011), small secreted peptide phytosulfokine (Stührwohldt et al, 2015), and inositol polyphosphate kinases (Zhan et al, 2015). It remains to be elucidated whether these molecules are involved in sensing and competency control for LURE or other gametophytic attractant pathways .…”