In this study, the role of the recently identified class of phytohormones, strigolactones, in shaping root architecture was addressed. Primary root lengths of strigolactone-deficient and -insensitive Arabidopsis (Arabidopsis thaliana) plants were shorter than those of wild-type plants. This was accompanied by a reduction in meristem cell number, which could be rescued by application of the synthetic strigolactone analog GR24 in all genotypes except in the strigolactone-insensitive mutant. Upon GR24 treatment, cells in the transition zone showed a gradual increase in cell length, resulting in a vague transition point and an increase in transition zone size. PIN1/3/7-green fluorescent protein intensities in provascular tissue of the primary root tip were decreased, whereas PIN3-green fluorescent protein intensity in the columella was not affected. During phosphate-sufficient conditions, GR24 application to the roots suppressed lateral root primordial development and lateral root forming potential, leading to a reduction in lateral root density. Moreover, auxin levels in leaf tissue were reduced. When auxin levels were increased by exogenous application of naphthylacetic acid, GR24 application had a stimulatory effect on lateral root development instead. Similarly, under phosphate-limiting conditions, endogenous strigolactones present in wild-type plants stimulated a more rapid outgrowth of lateral root primordia when compared with strigolactone-deficient mutants. These results suggest that strigolactones are able to modulate local auxin levels and that the net result of strigolactone action is dependent on the auxin status of the plant. We postulate that the tightly balanced auxin-strigolactone interaction is the basis for the mechanism of the regulation of the plants’ root-to-shoot ratio.

Nuclei of arbuscular endomycorrhizal fungi have been described as highly diverse due to their asexual nature and absence of a single cell stage with only one nucleus. This has raised fundamental questions concerning speciation, selection and transmission of the genetic make-up to next generations. Although this concept has become textbook knowledge, it is only based on studying a few loci, including 45S rDNA. To provide a more comprehensive insight into the genetic makeup of arbuscular endomycorrhizal fungi, we applied de novo genome sequencing of individual nuclei of Rhizophagus irregularis. This revealed a surprisingly low level of polymorphism between nuclei. In contrast, within a nucleus, the 45S rDNA repeat unit turned out to be highly diverged. This finding demystifies a long-lasting hypothesis on the complex genetic makeup of arbuscular endomycorrhizal fungi. Subsequent genome assembly resulted in the first draft reference genome sequence of an arbuscular endomycorrhizal fungus. Its length is 141 Mbps, representing over 27,000 protein-coding gene models. We used the genomic sequence to reinvestigate the phylogenetic relationships of Rhizophagus irregularis with other fungal phyla. This unambiguously demonstrated that Glomeromycota are more closely related to Mucoromycotina than to its postulated sister Dikarya. Endomycorrhizal fungi are known for their symbiosis with the vast majority of land plants. The fungus penetrates the root and facilitates uptake of nutrients for the plant. For a long time it was hypothesized that endomycorrhizal fungi have a complex genetic makeup, as they are asexual organisms. Their hyphae do not consist of individual cells, but rather form a continuous compartment in which numerous nuclei migrate. Several studies indicated that these nuclei are genetically highly diverse, suggesting that endomycorrhizal fungi evolved a unique genome structure. By sequencing individual nuclei of a single individual of the reference fungus Rhizophagus, we demystify this hypothesis and show that the nuclei are highly similar. Furthermore, we created the first genome sequence of these ancient fungi that will serve as a valuable resource to further understand and exploit this agriculturally and ecologically vital symbiosis.

Summary: Root hairs develop from bulges on root epidermal cells and elongate by tip growth, in which Golgi vesicles are targeted, released and inserted into the plasma membrane on one side of the cell. We studied the role of actin in vesicle delivery and retention by comparing the actin filament configuration during bulge formation, root hair initiation, sustained tip growth, growth termination, and in full‐grown hairs. Lipochito‐oligosaccharides (LCOs) were used to interfere with growth ( De Ruijter et al . 1998 , Plant J. 13, 341–350), and cytochalasin D (CD) was used to interfere with actin function. Actin filament bundles lie net‐axially in cytoplasmic strands in the root hair tube. In the subapex of growing hairs, these bundles flare out into fine bundles. The apex is devoid of actin filament bundles. This subapical actin filament configuration is not present in full‐grown hairs; instead, actin filament bundles loop through the tip. After LCO application, the tips of hairs that are terminating growth swell, and a new outgrowth appears from a site in the swelling. At the start of this outgrowth, net‐axial fine bundles of actin filaments reappear, and the tip region of the outgrowth is devoid of actin filament bundles. CD at 1.0 μ m , which does not affect cytoplasmic streaming, does not inhibit bulge formation and LCO‐induced swelling, but inhibits initiation of polar growth from bulges, elongation of root hairs and LCO‐induced outgrowth from swellings. We conclude that elongating net‐axial fine bundles of actin filaments, which we call FB‐actin, function in polar growth by targeting and releasing Golgi vesicles to the vesicle‐rich region, while actin filament bundles looping through the tip impede vesicle retention.

In growing Arabidopsis root hairs, the nucleus locates at a fixed distance from the apex, migrates to a random position during growth arrest, and moves from branch to branch in a mutant with branched hairs. Consistently, an artificial increase of the distance between the nucleus and the apex, achieved by entrapment of the nucleus in a laser beam, stops cell growth. Drug studies show that microtubules are not involved in the positioning of the nucleus but that subapical fine F-actin between the nucleus and the hair apex is required to maintain the nuclear position with respect to the growing apex. Injection of an antibody against plant villin, an actin filament-bundling protein, leads to actin filament unbundling and movement of the nucleus closer to the apex. Thus, the bundled actin at the tip side of the nucleus prevents the nucleus from approaching the apex. In addition, we show that the basipetal movement of the nucleus at root hair growth arrest requires protein synthesis and a functional actin cytoskeleton in the root hair tube.

Plant cells expand by exocytosis of wall material contained in Golgi-derived vesicles. We examined the role of local instability of the actin cytoskeleton in specifying the exocytosis site in Arabidopsis root hairs. During root hair growth, a specific actin cytoskeleton configuration is present in the cell's subapex, which consists of fine bundles of actin filaments that become more and more fine toward the apex, where they may be absent. Pulse application of low concentrations of the actin-depolymerizing drugs cytochalasin D and latrunculin A broadened growing root hair tips (i.e., they increased the area of cell expansion). Interestingly, recovery from cytochalasin D led to new growth in the original growth direction, whereas in the presence of oryzalin, a microtubule-depolymerizing drug, this direction was altered. Oryzalin alone, at the same concentration, had no influence on root hair elongation. These results represent an important step toward understanding the spatial and directional regulation of root hair growth.