Root cap at soil interface: a driving force towards plant adaptation and development

Land plants harbour robust roots to grow in diverse soil ecosystems. The distal end of the primary root tip has specialized tissue, called “root cap.” The evolution of root cap-like structures in early plants rudimentary roots and well-developed root caps in vascular plants hints towards developing an adaptive trait for a localized plant habitat. Root cap interacts with soil and assists roots in penetrating the below ground, avoid/adsorb metals, uptake water, minerals, and regulates rhizosphere microbiota that drives plant-soil feedback. Besides, the root cap governs lateral root patterning and directs root growth in varying conditions. This review article presents the retrospective and our perspective on root cap characters for root-soil interaction. We discussed the anatomy of root cap among the different taxa of land plants and their relevance in diverse habitats and elucidated the root cap functions under various growth conditions. We took advantage of recently published single-cell RNAseq data and shed light on biological relevance of root cap cell-type enriched genes from arabidopsis, rice, maize, and tomato. Additionally, analyzed the transcription factor binding site enrichment in root cap enriched genes and constructed gene-regulatory networks operating in root cap to contribute its multi-faceted role in plant growth and adaptation.

Genetic variation of BnaA3.NIP5;1 expressing in the lateral root cap contributes to boron deficiency tolerance in Brassica napus

Boron (B) is essential for vascular plants. Rapeseed (Brassica napus) is the second leading crop source for vegetable oil worldwide, but its production is critically dependent on B supplies. BnaA3.NIP5;1 was identified as a B-efficient candidate gene in B. napus in our previous QTL fine mapping. However, the molecular mechanism through which this gene improves low-B tolerance remains elusive. Here, we report genetic variation in BnaA3.NIP5;1 gene, which encodes a boric acid channel, is a key determinant of low-B tolerance in B. napus. Transgenic lines with increased BnaA3.NIP5;1 expression exhibited improved low-B tolerance in both the seedling and maturity stages. BnaA3.NIP5;1 is preferentially polar-localized in the distal plasma membrane of lateral root cap (LRC) cells and transports B into the root tips to promote root growth under B-deficiency conditions. Further analysis revealed that a CTTTC tandem repeat in the 5’UTR of BnaA3.NIP5;1 altered the expression level of the gene, which is tightly associated with plant growth and seed yield. Field tests with natural populations and near-isogenic lines (NILs) confirmed that the varieties carried BnaA3.NIP5;1Q allele significantly improved seed yield. Taken together, our results provide novel insights into the low-B tolerance of B. napus, and the elite allele of BnaA3.NIP5;1 could serve as a direct target for breeding low-B-tolerant cultivars.

Statoliths displacement in root statocytes in real and simulated microgravity

Despite the long-term employment of different types of clinostats in space and gravitational biology, the discussions about their reliability to mimic microgravity in space flight are still ongoing. In this paper, we present some data about the behaviour of amyloplasts-statoliths in root cap statocytes of higher plant seedlings growing during 3–5 days under slow and fast 2-D clinorotation and real microgravity in orbital flight. In addition, data on the displacement of amyloplasts in the statocytes of seedlings subjected to vibration and acceleration in the launch mode of a spacecraft are also given. A comparative analysis showed sharp differences in statolith responses to slow and fast clinorotation with a speed of 50 rpm. In the first case, the behaviour of amyloplasts was more or less similar to that in space flight, they did not touch the plasmalemma. In the second case, the contacts of statoliths with the plasmalemma or its invaginations (plasmalomasomes), like those under the action of vibration and acceleration, were clearly observed. Thus, slow 2-D clinostat is more suitable to study gravity sensing by root cap amyloplasts-statoliths and their responses to microgravity in the ground-based experiments.

A Root Tip-Specific Expressing Anthocyanin Marker for Direct Identification of Transgenic Tissues by the Naked Eye in Symbiotic Studies

The Agrobacterium rhizogenes hairy root transformation system is widely used in symbiotic studies of model legumes. It typically relies on fluorescent reporters, such as DsRed, for identification of transgenic roots. The MtLAP1 transcription factor has been utilized as a reporter system in Medicago truncatula based on production of anthocyanin pigment. Here, we describe a version of this reporter driven by a root-cap specific promoter for direct observation of anthocyanin accumulation in root tips, which allows the identification of transgenic hairy roots by the naked eye. Results from our analysis suggest that the reporter had no significant effects on nodulation of M. truncatula. This approach, by virtue of its strong and specific expression in root cap cells, greatly reduces false positives and false negatives, and its use of an easily scored visible pigment should allow greater versatility and efficiency in root biology studies.

Integrated analysis of plant gene expression and bacterial and protozoan community composition reveals changes related to root zonation, root cap and root hair formation

<p>This study was conducted within the framework of the DFG project SPP2089 “Rhizosphere Spatiotemporal Organization – a Key to Rhizosphere Functions” (project number 403641192).</p><p>As plant roots grow into the soil, the formation of biological gradients occurs at different spatial scales. It has been shown that plants recruit specific subsets of the soil bacterial community at their roots through excretion of mucilage at root tips and exudates at the sites of root hair formation. The promotion of or defense against certain bacterial taxa is also reflected in the composition of the protist communities that feed on bacteria.</p><p>Using high-throughput sequencing methods, we investigated emerging patterns in root gene expression in relation to bacterial and protozoan community structures. We found highly distinct root region specific patterns relating to differential root gene expression relating to growth, defense and transporter activity, as well as bacterial and protist (cercozoan) diversity. Root cap removal led to differently composed microbial communities, as well as a regulation of root genes relating to stress and defense. The lack of root hairs was only reflected in the amount of microbial carbon in soil and a small number of differentially expressed genes involved in cell wall processes.</p><p>We could show that the rhizosphere microbiome, is as dynamic as its environment. Root regions differentially affect microbial communities, which is also reflected in the expression of plant genes of categories relating to defense, immunity and stress. Our findings will further enhance our understanding of microbial root interactions at single root scale.</p>

AtPiezo Plays an Important Role in Root Cap Mechanotransduction

Plants encounter a variety of mechanical stimuli during their growth and development. It is currently believed that mechanosensitive ion channels play an essential role in the initial perception of mechanical force in plants. Over the past decade, the study of Piezo, a mechanosensitive ion channel in animals, has made significant progress. It has been proved that the perception of mechanical force in various physiological processes of animals is indispensable. However, little is still known about the function of its homologs in plants. In this study, by investigating the function of the AtPiezo gene in the model plant Arabidopsis thaliana, we found that AtPiezo plays a role in the perception of mechanical force in plant root cap and the flow of Ca2+ is involved in this process. These findings allow us to understand the function of AtPiezo from the perspective of plants and provide new insights into the mechanism of plant root cap in response to mechanical stimuli.