Effects of Elevated Temperature on Root System Development of Two Lupine Species

The aim of this study was to assess the effect of elevated temperature on the growth, morphology and spatial orientation of lupine roots at the initial stages of development and on the formation of lupine root architecture at later stages. Two lupine species were studied—the invasive Lupinus polyphyllus Lindl. and the non-invasive L. luteus L. The plants were grown in climate chambers under 25 °C and simulated warming at 30 °C conditions. The angle of root curvature towards the vector of gravity was measured at the 48th hour of growth, and during a 4-h period after 90° reorientation. Root biometrical, histological measurements were carried out on 7-day-old and 30-day-old plants. The elevation of 5 °C affected root formation of the two lupine species differently. The initial roots of L. polyphyllus were characterized by worse spatial orientation, reduced growth and reduced mitotic index of root apical meristem at 30 °C compared with 25 °C. The length of primary roots of 30-day-old lupines and the number of lateral roots decreased by 14% and 16%, respectively. More intense root development and formation were observed in non-invasive L. luteus at 30 °C. Our results provide important information on the effect of elevated temperature on the formation of root architecture in two lupine species and suggest that global warming may impact the invasiveness of these species.

RETINOBLASTOMA RELATED (RBR) interaction with key factors of the RNA-directed DNA methylation (RdDM) pathway

● Transposable elements and other repetitive elements are silenced by the RNA-directed DNA methylation pathway (RdDM). In RdDM, POLIV-derived transcripts are converted into double stranded RNA (dsRNA) by the activity of RDR2 and subsequently processed into 24 nucleotide short interfering RNAs (24-nt siRNAs) by DCL3. 24-nt siRNAs are recruited by AGO4 and serve as guides to direct AGO4-siRNA complexes to chromatin bound POLV-derived transcripts generated from the template/target DNA. The interaction between POLV, AGO4, DMS3, DRD1, RDM1 and DRM2 promotes DRM2-mediated de novo DNA methylation. In silico exploration of Arabidopsis RBR protein partners revealed that several members of the RdDM pathway contain a motif that confers high affinity binding to RBR, including the largest subunits of POLIV and POLV (NRPD1 and NRPE1), the shared second largest subunit of POLIV and POLV (NRPD/E2), RDR1, RDR2, DCL3, DRM2 and SUVR2. We demonstrate that RBR binds to DRM2, DRD1 and SUVR2. We also report that seedlings from loss-of-function mutants in RdDM and in RBR show similar phenotypes in the root apical meristem. Furthermore, we show that RdDM and SUVR2 targets are up-regulated in the 35S::AmiGO-RBR background. Our results suggest a novel mechanism for RBR function in transcriptional gene silencing based on the interaction with key players of the RdDM pathway and opens several new hypotheses, including the convergence of RBR-DRM2 on the transcriptional control of TEs and several cell/tissue and stage-specific target genes.

Whole-Tissue Three-Dimensional Imaging of Rice at Single-Cell Resolution

The three-dimensional (3D) arrangement of cells in tissues provides an anatomical basis for analyzing physiological and biochemical aspects of plant and animal cellular development and function. In this study, we established a protocol for tissue clearing and 3D imaging in rice. Our protocol is based on three improvements: clearing with iTOMEI (clearing solution suitable for plants), developing microscopic conditions in which the Z step is optimized for 3D reconstruction, and optimizing cell-wall staining. Our protocol successfully 3D imaged rice shoot apical meristems, florets, and root apical meristems at cellular resolution throughout whole tissues. Using fluorescent reporters of auxin signaling in rice root tips, we also revealed the 3D distribution of auxin signaling events that are activated in the columella, quiescent center, and multiple rows of cells in the stele of the root apical meristem. Examination of cells with higher levels of auxin signaling revealed that only the central row of cells was connected to the quiescent center. Our method provides opportunities to observe the 3D arrangement of cells in rice tissues.

Boron Supply Restores Aluminum-Blocked Auxin Transport by Modulation PIN2 Trafficking in the Arabidopsis Roots

This study tested a hypothesis that boron (B) supply alleviates aluminum (Al) toxicity by modifying auxin distribution in functionally different root zones. Auxin distribution and transport at various Al and B ratios were analyzed using the range of molecular and imaging techniques. Al stress resulted in increased auxin accumulation in root apical meristem (MZ) and transition zones (TZ) while reducing its content in elongation zone (EZ). This phenomenon was explained by reduction in basipetal auxin transport caused by Al blockage of PIN2 endocytosis, regulated at posttranscriptional level. This inhibition of PIN2 endocytosis was dependent on actin filaments and microtubules. B supply facilitated the endocytosis and exocytosis of PIN2 carriers via recycling endosomes conjugated with IAA to modify Al-induced auxin depletion in the EZ. However, disruption of auxin signaling with auxinole did not alleviate Al-induced inhibition of root growth. B supply alleviates Al-induced inhibition of root growth via restoring the endocytic recycling of PIN2 proteins involved in the basipetal (shootward) auxin transport, restoring Al-induced auxin depletion in the elongation zone.Short summaryAluminum-intensified PIN2 abundance, nontranscriptional, via repressing PIN2 endocytosis to block polar auxin transport, and this adverse effect could be alleviated by boron supply.

Formation and Development of Taproots in Deciduous Tree Species

Trees are generally long-lived and are therefore exposed to numerous episodes of external stimuli and adverse environmental conditions. In certain trees e.g., oaks, taproots evolved to increase the tree’s ability to acquire water from deeper soil layers. Despite the significant role of taproots, little is known about the growth regulation through internal factors (genes, phytohormones, and micro-RNAs), regulating taproot formation and growth, or the effect of external factors, e.g., drought. The interaction of internal and external stimuli, involving complex signaling pathways, regulates taproot growth during tip formation and the regulation of cell division in the root apical meristem (RAM). Assuming that the RAM is the primary regulatory center responsible for taproot growth, factors affecting the RAM function provide fundamental information on the mechanisms affecting taproot development.

The Pellicle–Another Strategy of the Root Apex Protection against Mechanical Stress?

In grasses, the apical part of the root is covered by a two-layered deposit of extracellular material, the pellicle, which together with the outer periclinal wall of protodermal cells forms the three-layered epidermal surface. In this study, the effect of mechanical stress on the pellicle was examined. An experiment was performed, in which maize roots were grown in narrow diameter plastic tubes with conical endings for 24 h. Two groups of experimental roots were included in the analysis: stressed (S) roots, whose tips did not grow out of the tubes, and recovering (R) roots, whose apices grew out of the tube. Control (C) roots grew freely between the layers of moist filter paper. Scanning electron microscopy and confocal microscopy analysis revealed microdamage in all the layers of the epidermal surface of S roots, however, protodermal cells in the meristematic zone remained viable. The outermost pellicle layer was twice as thick as in C roots. In R roots, large areas of dead cells were observed between the meristematic zone and the transition zone. The pellicle was defective with a discontinuous and irregular outermost layer. In the meristematic zone the pellicle was undamaged and the protodermal cells were intact. The results lead to the conclusion that the pellicle may prevent damage to protodermal cells, thus protecting the root apical meristem from the negative effects of mechano-stress.

Methods of In Situ Quantitative Root Biology

When dealing with plant roots, a multiscale description of the functional root structure is needed. Since the beginning of 21st century, new devices such as laser confocal microscopes have been accessible for coarse root structure measurements, including three-dimensional (3D) reconstruction. Most researchers are familiar with using simple 2D geometry visualization that does not allow quantitative determination of key morphological features from an organ-like perspective. We provide here a detailed description of the quantitative methods available for 3D analysis of root features at single-cell resolution, including root asymmetry, lateral root analysis, cell size and nuclear organization, cell-cycle kinetics, and chromatin structure analysis. Quantitative maps of the root apical meristem (RAM) are shown for different species, including Arabidopsis thaliana (L.), Heynh, Nicotiana tabacum L., Medicago sativa L., and Setaria italica (L.) P. Beauv. The 3D analysis of the RAM in these species showed divergence in chromatin organization and cell volume distribution that might be used to study root zonation for each root tissue. Detailed protocols and possible pitfalls in the usage of the marker lines are discussed. Therefore, researchers who need to improve their quantitative root biology portfolio can use them as a reference.

Morphological, Histological, and Protein Profiling of Tea Embryo Axis at Early Stage of Culture

Tissue culture is an alternative choice of plant propagation either through somatic embryogenesis or in vitro organogenesis techniques. TRI2025 tea clone has been cultured successfully, however, the scientific information related to morphology, histology, and protein profile at an early event of culturing time has not been reported yet. This study aimed to determine the differences between those pathways, in the context of morphology, histology, and protein profile. The explants were the embryo axis of TRI2025 tea clone cultured on two different induction mediums; somatic embryogenesis and in vitro organogenesis induction medium. The results showed that most of the explants cultured on A medium developed to be a globular-like structure at 11-day after culture (DAC), while all explants cultured on B medium showed the initiation stage of in vitro organogenesis. Histological analysis showed meristem reconstruction at shoot apical meristem (SAM) and root apical meristem (RAM) at 11-DAC at explants cultured on B medium, while explants cultured on A medium showed callusing at 21-DAC. Protein profile analysis using SDS-PAGE showed protein bands of 54 and 81 KDa that only appeared at explants cultured on A medium start from 14-DAC, and those two protein bands thought to be a differentiator at the early stages of the two tissue culture techniques. Thus, these parameters can be used as early detection for plant tissue culture, especially in tea.

Salt Stress Promotes Abscisic Acid Accumulation to Affect Cell Proliferation and Expansion of Primary Roots in Rice

The primary root is the basic component of the root system and plays a key role in early seedling growth in rice. Its growth is easily affected by environmental cues, such as salt stress. Abscisic acid (ABA) plays an essential role in root development, but the molecular mechanism underlying ABA-regulated root growth in response to salt stress remains poorly understood. In this study, we report that salt stress inhibits primary root elongation and promotes primary root swelling. Moreover, salt stress induces the expression of ABA-responsive genes and ABA accumulation in the primary root, revealing that ABA plays an essential role in salt-modulated root growth. Transgenic lines of OsSAPK10-OE and OsABIL2-OE, which constitutively express OsSAPK10 or OsABIL2, with enhanced or attenuated ABA signaling, show increased and decreased sensitivity to salt, correspondingly. Microscopic analysis indicates that salt and ABA inhibits cell proliferation and promotes cell expansion in the root apical meristem. Transcriptome analysis showed that ABA induces the expression of EXPANSIN genes. Further investigations indicate that ABA exerts these effects largely through ABA signaling. Thus, our findings deepen our understanding of the role of ABA in controlling primary root growth in response to salt stress, and this knowledge can be used by breeders to cultivate rice varieties suitable for saline–alkali land.