Targeting and Regulation of Cell Wall Synthesis During Tip Growth in Plants

Root hairs and pollen tubes are formed through tip growth, a process requiring synthesis of new cell wall material and the precise targeting and integration of these components to a selected apical plasma membrane domain in the growing tips of these cells. Presence of a tip-focused calcium gradient, control of actin cytoskeleton dynamics, and formation and targeting of secretory vesicles are essential to tip growth. Similar to cells undergoing diffuse growth, cellulose, hemicelluloses, and pectins are also deposited in the growing apices of tip-growing cells. However, differences in the manner in which these cell wall components are targeted and inserted in the expanding portion of tip-growing cells is reflected by the identification of elements of the plant cell wall synthesis machinery which have been shown to play unique roles in tip-growing cells. In this review, we summarize our current understanding of the tip growth process, with a particular focus on the subcellular targeting of newly synthesized cell wall components, and their roles in this form of plant cell expansion.     

The Conserved Yeast Protein Knr4 Involved in Cell Wall Integrity Is a Multi-domain Intrinsically Disordered Protein

Knr4/Smi1 proteins are specific to the fungal kingdom and their deletion in the model yeast Saccharomyces cerevisiae and the human pathogen Candida albicans results in hypersensitivity to specific antifungal agents and a wide range of parietal stresses. In S. cerevisiae, Knr4 is located at the crossroads of several signalling pathways, including the conserved cell wall integrity and calcineurin pathways. Knr4 interacts genetically and physically with several protein members of those pathways. Its sequence suggests that it contains large intrinsically disordered regions. Here, a combination of small-angle X-ray scattering (SAXS) and crystallographic analysis led to a comprehensive structural view of Knr4. This experimental work unambiguously showed that Knr4 comprises two large intrinsically disordered regions flanking a central globular domain whose structure has been established. The structured domain is itself interrupted by a disordered loop. Using the CRISPR/Cas9 genome editing technique, strains expressing KNR4 genes deleted from different domains were constructed. The N-terminal domain and the loop are essential for optimal resistance to cell wall-binding stressors. The C-terminal disordered domain, on the other hand, acts as a negative regulator of this function of Knr4. The identification of molecular recognition features, the possible presence of secondary structure in these disordered domains and the functional importance of the disordered domains revealed here designate these domains as putative interacting spots with partners in either pathway. Targeting these interacting regions is a promising route to the discovery of inhibitory molecules that could increase the susceptibility of pathogens to the antifungals currently in clinical use.     

KINKY POLLEN encodes a SABRE-like protein required for tip growth in Arabidopsis and conserved among eukaryotes

In higher plants, pollen tubes and root hairs share an ancient growth process named tip growth. We have isolated three allelic Arabidopsis mutant lines showing kinky-shaped pollen tubes and, when homozygous, showing shorter and thicker root hairs. The ultrastructure of pollen tubes in these kinky pollen (kip) mutants is similar to that of the wild type; however, time-lapse studies suggest that aberrant pollen tube shape is caused by periodic growth arrests alternated with phases of tube axis reorientation. The KIP gene encodes a protein of 2587 amino acids that is predicted to be targeted to the secretory pathway. KIP mRNA was detected in all organs investigated but was most abundant in pollen and roots. KIP has putative homologues in many eukaryotes, including mammals and yeast, and is similar to the Arabidopsis SABRE gene, whose mutation causes a dwarf phenotype. The phenotype of the kip/sab double mutant suggests related functions for both genes, however, the KIP protein is mostly required for tip-growth.     

Arabidopsis pollen-specific glycerophosphodiester phosphodiesterase-like genes are essential for pollen tube tip growth

In angiosperms, pollen tube growth is critical for double fertilization and seed formation. Many of the factors involved in pollen tube tip growth are unknown. Here, we report the roles of pollen-specific GLYCEROPHOSPHODIESTER PHOSPHODIESTERASE-LIKE (GDPD-LIKE) genes in pollen tube tip growth. Arabidopsis thaliana GDPD-LIKE6 (AtGDPDL6) and AtGDPDL7 were specifically expressed in mature pollen grains and pollen tubes and green fluorescent protein (GFP)-AtGDPDL6 and GFP-AtGDPDL7 fusion proteins were enriched at the plasma membrane at the apex of forming pollen tubes. Atgdpdl6 Atgdpdl7 double mutants displayed severe sterility that was rescued by genetic complementation with AtGDPDL6 or AtGDPDL7. This sterility was associated with defective male gametophytic transmission. Atgdpdl6 Atgdpdl7 pollen tubes burst immediately after initiation of pollen germination in vitro and in vivo, consistent with the thin and fragile walls in their tips. Cellulose deposition was greatly reduced along the mutant pollen tube tip walls, and the localization of pollen-specific CELLULOSE SYNTHASE-LIKE D1 (CSLD1) and CSLD4 was impaired to the apex of mutant pollen tubes. A rice pollen-specific GDPD-LIKE protein also contributed to pollen tube tip growth, suggesting that members of this family have conserved functions in angiosperms. Thus, pollen-specific GDPD-LIKEs mediate pollen tube tip growth, possibly by modulating cellulose deposition in pollen tube walls.