A light-sensitive mutation in Arabidopsis LEW3 reveals the important role of N-glycosylation in root growth and development.

Plant roots have the potential capacity to grow almost indefinitely if meristematic and lateral branching is sustained. In a genetic screen we identified an Arabidopsis mutant showing limited root growth (lrg1) due to defects in cell division and elongation in the root meristem. Positional cloning determined that lrg1 affects an alpha-1,2-mannosyltransferase gene, LEW3, involved in protein N-glycosylation. The lrg1 mutation causes a synonymous substitution that alters the correct splicing of the fourth intron in LEW3, causing a mix of wild-type and truncated protein. LRG1 RNA missplicing in roots and short root phenotypes in lrg1 are light-intensity dependent. This mutation disrupts a GC-base pair in a three-base-pair stem with a four-nucleotide loop, which seems to be necessary for correct LEW3 RNA splicing. We found that the lrg1 short root phenotype correlates with high levels of reactive oxygen species and low pH in the apoplast. Proteomic analyses of N-glycosylated proteins identified GLU23/PYK10 and PRX34 as N-glycosylation targets of LRG1 activity. The lrg1 mutation reduces the positive interaction between Arabidopsis and Serendipita indica. A prx34 mutant showed a significant reduction in root growth, which is additive to lrg1. Taken together our work highlights the important role of N-glycosylation in root growth and development.

The Emerging Role of Reactive Oxygen Species Signaling during Lateral Root Development.

Overall root architecture is the combined result of primary and lateral root growth and is influenced by both intrinsic genetic programs and external signals. One of the main questions for root biologists is how plants control the number of lateral root primordia and their emergence through the main root. We recently identified S-phase kinase-associated protein2 (SKP2B) as a new early marker for lateral root development. Here, we took advantage of its specific expression pattern in Arabidopsis (Arabidopsis thaliana) in a cell-sorting and transcriptomic approach to generate a lateral root-specific cell sorting SKP2B data set that represents the endogenous genetic developmental program. We first validated this data set by showing that many of the identified genes have a function during root growth or lateral root development. Importantly, genes encoding peroxidases were highly represented in our data set. Thus, we next focused on this class of enzymes and showed, using genetic and chemical inhibitor studies, that peroxidase activity and reactive oxygen species signaling are specifically required during lateral root emergence but, intriguingly, not for primordium specification itself.

Auxin and epigenetic regulation of SKP2B, an F-box that represses lateral root formation.

In plants, lateral roots originate from pericycle founder cells that are specified at regular intervals along the main root. Here, we show that Arabidopsis (Arabidopsis thaliana) SKP2B (for S-Phase Kinase-Associated Protein2B), an F-box protein, negatively regulates cell cycle and lateral root formation as it represses meristematic and founder cell divisions. According to its function, SKP2B is expressed in founder cells, lateral root primordia and the root apical meristem. We identified a novel motif in the SKP2B promoter that is required for its specific root expression and auxin-dependent induction in the pericycle cells. Next to a transcriptional control by auxin, SKP2B expression is regulated by histone H3.1/H3.3 deposition in a CAF-dependent manner. The SKP2B promoter and the 5' end of the transcribed region are enriched in H3.3, which is associated with active chromatin states, over H3.1. Furthermore, the SKP2B promoter is also regulated by H3 acetylation in an auxin- and IAA14-dependent manner, reinforcing the idea that epigenetics represents an important regulatory mechanism during lateral root formation.

Dissecting Arabidopsis lateral root development.

Recent studies in the model plant Arabidopsis provide new insight into the regulation of root architecture, a key determinant of nutrient- and water-use efficiency in crops. Lateral root (LR) primordia originate from a subset of pericycle founder cells. Sophisticated mass-spectroscopy-based techniques have been used to map the sites of biosynthesis of auxin and its distribution in Arabidopsis seedlings, highlighting the importance of the phytohormone during LR initiation and emergence. Key components of the cell cycle and signal-transduction pathway(s) that promote and attenuate auxin-dependent LR initiation have recently been identified. Additional signals, such as abscisic acid and nitrate, also regulate LR emergence, raising intriguing questions about the cross-talk between their transduction pathways.

Defence response of tomato seedlings to oxidative stress induced by phenolic compounds from dry olive mill residue.

ADOR is an aqueous extract obtained from the dry olive mill residue (DOR) which contains the majority of its soluble phenolic compounds, which are responsible for its phytotoxic properties. Some studies have shown that ADOR negatively affects seed germination. However, to date, few studies have been carried out on the effect of ADOR on the oxidative stress of the plant. It is well known that saprobe fungi can detoxify these phenolic compounds and reduce the potential negative effects of ADOR on plants. To gain a better understanding of the phytotoxic effects and oxidative stress caused by this residue, tomato seeds were germinated in the presence of ADOR, treated and untreated with Coriolopsis rigida, Trametes versicolor, Pycnoporus cinnabarinus and Penicillium chrysogenum-10 saprobe fungi. ADOR sharply reduced tomato seed germination and also generated high levels of malondialdehyde (MDA), O(2)(-) and H(2)O(2). However, bioremediated ADOR did not negatively affect germination and reduced MDA, O(2)(-) and H(2)O(2) content in different ways depending on the fungus used. In addition, the induced defense response was studied by analyzing the activity of both antioxidant enzymes (superoxide dismutase (SOD), catalase, ascorbate peroxidasa, glutathione reductase (GR), peroxidases and coniferil alcohol peroxidasa) and detoxification enzymes (glutathione-S-transferase (GST)). Our findings suggest that, because ADOR is capable of inducing oxidative stress, tomato seedlings trigger a defense response through SOD, GR, and GST activity and through antioxidant and lignification processes. On the other hand, the bioremediation of ADOR plays an important role in counteracting the oxidative stress induced by the untreated residue.

Oxidative stress induced in sunflower seedling roots by aqueous dry olive-mill residues.

The contamination of soils with dry olive-mill residue can represent a serious problem as being an environmental stressor in plants. It has been demonstrated that inoculation of aqueous extract of olive oil-mill residue (ADOR) with saprobe fungi removes some phenolic compounds. In this paper we studied the effect of ADOR uninoculated or inoculated with saprobe fungi in sunflower seedling roots. The germination and root growth, O(2)·(-) generation, superoxide dismutase (SOD) and extracellular peroxidases (EC-POXs) activities, and the content of some metabolites involved in the tolerance of stress were tested. The roots germinated in ADOR uninoculated show a decrease in meristem size, resulting in a reduction of the root length and fresh weight, and in the number of layers forming the cortex, but did not alter the dry weight, protein and soluble amino acid content. ADOR caused the decreases in O(2)·(-) generation and EC-POX's activities and protein oxidation, but enhanced SOD activity, lipid peroxidation and proline content. Fluorescence imaging showed that ADOR induced O(2)·(-) and H(2)O(2) accumulation in the roots. The increase in SOD and the decrease in EC-POX's activities might be involved in the enhancement of H(2)O(2) content and lipid peroxidation. Control roots treated with ADOR for 10 min show an oxidative burst. Roots germinated in ADOR inoculated with saprobe fungi partially recovered normal levels of ROS, morphological characteristics and antioxidant activities. These results suggested that treatment with ADOR caused a phytotoxic effect during germination inducing an oxidative stress. The inoculation of ADOR with saprobe fungi limited the stress.

AUX1 promotes lateral root formation by facilitating indole-3-acetic acid distribution between sink and source tissues in the Arabidopsis seedling.

Arabidopsis root architecture is regulated by shoot-derived signals such as nitrate and auxin. We report that mutations in the putative auxin influx carrier AUX1 modify root architecture as a result of the disruption in hormone transport between indole-3-acetic acid (IAA) source and sink tissues. Gas chromatography-selected reaction monitoring-mass spectrometry measurements revealed that the aux1 mutant exhibited altered IAA distribution in young leaf and root tissues, the major IAA source and sink organs, respectively, in the developing seedling. Expression studies using the auxin-inducible reporter IAA2::uidA revealed that AUX1 facilitates IAA loading into the leaf vascular transport system. AUX1 also facilitates IAA unloading in the primary root apex and developing lateral root primordium. Exogenous application of the synthetic auxin 1-naphthylacetic acid is able to rescue the aux1 lateral root phenotype, implying that root auxin levels are suboptimal for lateral root primordium initiation in the mutant.