Auxin and Target of Rapamycin Spatiotemporally Regulate Root Organogenesis

The programs associated with embryonic roots (ERs), primary roots (PRs), lateral roots (LRs), and adventitious roots (ARs) play crucial roles in the growth and development of roots in plants. The root functions are involved in diverse processes such as water and nutrient absorption and their utilization, the storage of photosynthetic products, and stress tolerance. Hormones and signaling pathways play regulatory roles during root development. Among these, auxin is the most important hormone regulating root development. The target of rapamycin (TOR) signaling pathway has also been shown to play a key role in root developmental programs. In this article, the milestones and influential progress of studying crosstalk between auxin and TOR during the development of ERs, PRs, LRs and ARs, as well as their functional implications in root morphogenesis, development, and architecture, are systematically summarized and discussed.

Plant long non-coding RNAs in the regulation of transcription

Eukaryotic genomes are pervasively transcribed, producing large numbers of non-coding RNAs (ncRNAs), including tens of thousands of long ncRNAs (lncRNAs), defined as ncRNAs longer than 200 nucleotides. Recent studies have revealed the important roles lncRNAs play in the regulation of gene expression at various levels in all eukaryotes; moreover, emerging research in plants has identified roles for lncRNAs in key processes such as flowering time control, root organogenesis, reproduction, and adaptation to environmental changes. LncRNAs participate in regulating most steps of gene expression, including reshaping nuclear organization and chromatin structure; governing multiple steps of transcription, splicing, mRNA stability, and translation; and affecting post-translational protein modifications. In this review, I present the latest progress on the lncRNA-mediated regulatory mechanisms modulating transcription in Arabidopsis thaliana, focusing on their functions in regulation of gene expression via chromatin structure and interactions with the transcriptional machinery.

Cytokinin Modulates Cellular Trafficking and the Cytoskeleton, Enhancing Defense Responses

The plant hormone cytokinin (CK) plays central roles in plant development and throughout plant life. The perception of CKs initiating their signaling cascade is mediated by histidine kinase receptors (AHKs). Traditionally thought to be perceived mostly at the endoplasmic reticulum (ER) due to receptor localization, CK was recently reported to be perceived at the plasma membrane (PM), with CK and its AHK receptors being trafficked between the PM and the ER. Some of the downstream mechanisms CK employs to regulate developmental processes are unknown. A seminal report in this field demonstrated that CK regulates auxin-mediated lateral root organogenesis by regulating the endocytic recycling of the auxin carrier PIN1, but since then, few works have addressed this issue. Modulation of the cellular cytoskeleton and trafficking could potentially be a mechanism executing responses downstream of CK signaling. We recently reported that CK affects the trafficking of the pattern recognition receptor LeEIX2, influencing the resultant defense output. We have also recently found that CK affects cellular trafficking and the actin cytoskeleton in fungi. In this work, we take an in-depth look at the effects of CK on cellular trafficking and on the actin cytoskeleton in plant cells. We find that CK influences the actin cytoskeleton and endomembrane compartments, both in the context of defense signaling—where CK acts to amplify the signal—as well as in steady state. We show that CK affects the distribution of FLS2, increasing its presence in the plasma membrane. Furthermore, CK enhances the cellular response to flg22, and flg22 sensing activates the CK response. Our results are in agreement with what we previously reported for fungi, suggesting a fundamental role for CK in regulating cellular integrity and trafficking as a mechanism for controlling and executing CK-mediated processes.

Effects associated with insertion of rol genes on morphogenic potential in explants derived from transgenic Bacopa monnieri (L.) Wettst

The present study deals with the establishment of rolA-transgenic and rolB-transgenic plants for the first time through Agrobacterium tumefaciens mediated transformation, exploiting the inherent morphogenic potential of an important medicinal plant, Bacopa monnieri (L.) Wettst. The rolA-transgenic and rolB-transgenic plants showed integration and expression of rolA and rolB genes respectively, whereas Ri-transformed plants showed integration and expression of rolA, rolB, rolC and rolD genes. Morphogenic potential of different types of explants derived from rolA-transgenic, rolB-transgenic and Ri-transformed plants on basal medium was evaluated. Shoot organogenesis was enhanced significantly in leaf (1.6-fold) and internode (1.4-fold) explants derived from rolA-transgenic plants, rolB-transgenic leaf (2.4-fold) and internode (1.6-fold) explants as well as leaf (5.2-fold) and internode (3.3-fold) explants derived from Ri-transformed plants compared to explants from non-transformed plants. Substantial increase in root organogenesis was also noticed in rolA-transgenic leaf (1.7-fold) explants, rolB-transgenic leaf (3.6-fold) and internode (1.4-fold) explants as well as leaf (4.1-fold) and internode (1.9-fold) explants derived from Ri-transformed plants compared to non-transformed ones. In addition to this, growth of root tip and shoot regeneration was also noticed from Ri-transformed root explants, but not in rolA-transgenic, rolB-transgenic and non-transformed roots. Clones of all three transgenic plants differed morphologically from non-transformed plants; however, rolB gene alone has a pronounced effect in alteration of plant phenotype in B. monnieri. Clones of rolB-transgenic plants were similar in shoot length, internode length, number of nodes/plant and number of leaves/plant when compared with Ri-transformed plant clones

Dynamic cytokinin signalling landscapes during lateral root formation in Arabidopsis

By forming lateral roots, plants expand their root systems to improve anchorage and absorb more water and nutrients from the soil. Each phase of this developmental process in Arabidopsis is tightly regulated by dynamic and continuous signalling of the phytohormones cytokinin and auxin. While the roles of auxin in lateral root organogenesis and spatial accommodation by overlying cell layers have been well studied, insights on the importance of cytokinin is still somewhat limited. Cytokinin is a negative regulator of lateral root formation with versatile modes of action being activated at different root developmental zones. Here, we review the latest progress made towards our understanding of these spatially separated mechanisms of cytokinin-mediated signalling that shape lateral root initiation, outgrowth and emergence and highlight some of the enticing open questions.

A plant lipocalin is required for retinal-mediated de novo root organogenesis

Branching of root systems enables the exploration and colonization of the soil environment. In Arabidopsis roots, de novo organogenesis of lateral roots is patterned by an oscillatory mechanism called the root clock, which is dependent on metabolites derived from the β-carotene pathway1, 2. Retinoids are β-carotene-derived regulators of organogenesis in the animal kingdom. To determine if retinoids function in plant development, we conducted time-lapse imaging of a chemical reporter for retinoid binding proteins. We found that it oscillates with a comparable frequency to the root clock and accurately predicts sites of lateral root organogenesis. Exogenous application of retinal to wild-type plants is sufficient to induce root clock oscillations and lateral root organogenesis. A homology search yielded a potential Arabidopsis homolog, TEMPERATURE INDUCED LIPOCALIN (TIL) to vertebrate retinoid binding proteins. Genetic analysis indicates that TIL is necessary for normal lateral root development and a til mutant has decreased retinal sensitivity. TIL expression in a heterologous system conferred retinal binding activity, suggesting that it may directly interact with this molecule. Together, these results demonstrate an essential role for retinal and for plant retinal binding proteins in lateral root organogenesis.

EXPANSIN A1-mediated radial swelling of pericycle cells positions anticlinal cell divisions during lateral root initiation

In plants, postembryonic formation of new organs helps shape the adult organism. This requires the tight regulation of when and where a new organ is formed and a coordination of the underlying cell divisions. To build a root system, new lateral roots are continuously developing, and this process requires the tight coordination of asymmetric cell division in adjacent pericycle cells. We identified EXPANSIN A1 (EXPA1) as a cell wall modifying enzyme controlling the divisions marking lateral root initiation. Loss ofEXPA1leads to defects in the first asymmetric pericycle cell divisions and the radial swelling of the pericycle during auxin-driven lateral root formation. We conclude that a localized radial expansion of adjacent pericycle cells is required to position the asymmetric cell divisions and generate a core of small daughter cells, which is a prerequisite for lateral root organogenesis.

Systemic signalling through TCTP1 controls lateral root formation in Arabidopsis

As in animals, the plant body plan and primary organs are established during embryogenesis. However, plants have the ability to generate new organs and functional units throughout their whole life. These are produced through the specification, initiation and differentiation of secondary meristems, governed by the intrinsic genetic program and cues from the environment. They give plants an extraordinary developmental plasticity to modulate their size and architecture according to environmental constraints and opportunities. How this plasticity is regulated at the whole organism level is still largely elusive. In particular the mechanisms regulating the iterative formation of lateral roots along the primary root remain little known. A pivotal role of auxin is well established and recently the role of local mechanical signals and oscillations in transcriptional activity has emerged. Here we provide evidence for a role of Translationally Controlled Tumor Protein (TCTP), a vital ubiquitous protein in eukaryotes. We show that Arabidopsis AtTCTP1 controls root system architecture through a dual function: as a general constitutive growth promoter locally, and as a systemic signalling agent via mobility from the shoot. Our data indicate that this signalling function is specifically targeted to the pericycle and modulates the frequency of lateral root initiation and emergence sites along the primary root, and the compromise between branching and elongating, independent of shoot size. Plant TCTP genes show high similarity among species. TCTP messengers and proteins have been detected in the vasculature of diverse species. This suggests that the mobility and extracellular signalling function of AtTCTP1 to control root organogenesis might be widely conserved within the plant kingdom, and highly relevant to a better understanding of post-embryonic formation of lateral organs in plants, and the elusive coordination of shoot and root morphogenesis.