Indole-3-Butyric Acid Induces Ectopic Formation of Metaxylem in the Hypocotyl of Arabidopsis thaliana without Conversion into Indole-3-Acetic Acid and with a Positive Interaction with Ethylene

Genome-wide expression analysis of LACS gene family implies GhLACS25 functional responding to salt stress in cotton

BACKGROUND

Long-chain acyl-coenzyme A synthetase (LACS) is a type of acylating enzyme with AMP-binding, playing an important role in the growth, development, and stress response processes of plants.

RESULTS

The research team identified different numbers of LACS in four cotton species (Gossypium hirsutum, Gossypium barbadense, Gossypium raimondii, and Gossypium arboreum). By analyzing the structure and evolutionary characteristics of the LACS, the GhLACS were divided into six subgroups, and a chromosome distribution map of the family members was drawn, providing a basis for further research classification and positioning. Promoter cis-acting element analysis showed that most GhLACS contain plant hormones (GA, MeJA) or non-biological stress-related cis-elements. The expression patterns of GhLACS under salt stress treatment were analyzed, and the results showed that GhLACS may significantly participate in salt stress response through different mechanisms. The research team selected 12 GhLACSs responsive to salt stress for tissue expression analysis and found that these genes are expressed in different tissues.

CONCLUSIONS

There is a certain diversity of LACS among different cotton species. Analysis of promoter cis-acting elements suggests that GhLACS may be involved in regulating plant growth, development and stress response processes. GhLACS25 was selected for in-depth study, which confirmed its significant role in salt stress response through virus-induced gene silencing (VIGS) and induced expression in yeast cells.

Transcriptomics Analysis Reveals a Putative Role for Hormone Signaling and MADS-Box Genes in Mature Chestnut Shoots Rooting Recalcitrance

Maturation imposes several changes in plants, which are particularly drastic in the case of trees. In recalcitrant woody species, such as chestnut (Castanea sativa Mill.), one of the major maturation-related shifts is the loss of the ability to form adventitious roots in response to auxin treatment as the plant ages. To analyze the molecular mechanisms underlying this phenomenon, an in vitro model system of two different lines of microshoots derived from the same field-grown tree was established. While juvenile-like shoots root readily when treated with exogenous auxin, microshoots established from the crown of the tree rarely form roots. In the present study, a transcriptomic analysis was developed to compare the gene expression patterns in both types of shoots 24 h after hormone and wounding treatment, matching the induction phase of the process. Our results support the hypothesis that the inability of adult chestnut tissues to respond to the inductive treatment relies in a deep change of gene expression imposed by maturation that results in a significant transcriptome modification. Differences in phytohormone signaling seem to be the main cause for the recalcitrant behavior of mature shoots, with abscisic acid and ethylene negatively influencing the rooting ability of the chestnut plants. We have identified a set of related MADS-box genes whose expression is modified but not suppressed by the inductive treatment in mature shoots, suggesting a putative link of their activity with the rooting-recalcitrant behavior of this material. Overall, distinct maturation-derived auxin sensibility and homeostasis, and the related modifications in the balance with other phytohormones, seem to govern the outcome of the process in each type of shoots.

Convergence between Development and Stress: Ectopic Xylem Formation in Arabidopsis Hypocotyl in Response to 24-Epibrassinolide and Cadmium

Ectopic xylary element (EXE) formation in planta is a poorly investigated process, and it is unknown if it occurs as a response to the soil pollutant Cadmium (Cd). The pericycle cells of Arabidopsis thaliana hypocotyl give rise to EXEs under specific hormonal inputs. Cadmium triggers pericycle responses, but its role in EXE formation is unknown. Brassinosteroids (BRs) affect numerous developmental events, including xylogenesis in vitro, and their exogenous application by 24-epibrassinolide (eBL) helps to alleviate Cd-stress by increasing lateral/adventitious rooting. Epibrassinolide's effects on EXEs in planta are unknown, as well as its relationship with Cd in the control of the process. The research aims to establish an eBL role in pericycle EXE formation, a Cd role in the same process, and the possible interaction between the two. Results show that 1 nM eBL causes an identity reversal between the metaxylem and protoxylem within the stele, and its combination with Cd reduces the event. All eBL concentrations increase EXEs, also affecting xylary identity by changing from protoxylem to metaxylem in a concentration-dependent manner. Cadmium does not affect EXE identity but increases EXEs when combined with eBL. The results suggest that eBL produces EXEs to form a mechanical barrier against the pollutant.

Subtoxic levels of some heavy metals cause differential root-shoot structure, morphology and auxins levels in Arabidopsis thaliana

Contamination of soil by heavy metals severely affects plant growth and causes soil pollution. While effects on plant growth have been investigated for metals taken individually or in groups, less is known about their comparative effects. In this study Arabidopsis thaliana seedlings were grown for 14 days in Petri dishes containing medium contaminated by six common heavy metals (Hg, Cd, Pb, Cu, Ni and Zn), at the minimum concentrations defined as toxic by the most recent EU legislation on contamination of agricultural soils. (a) Root structure and morphology, (b) metal composition and translocation, and (c) the levels of indole-3-acetic acid (IAA) and indole-3-butyric acid (IBA) were analyzed. Metals accumulated more in roots than in shoots, with concentrations that differed by several orders of magnitude depending on the metal: Cd (ca. 700 × and ca. 450 × in roots and shoots, respectively), Hg (150 × , 80 × ), Ni (50 × , 20 × ), Cu (48 × , 20 × ), Zn (23 × , 6 × ), and Pb (9 × , 4 × ). Responses were significant for at least nine of the ten root parameters (with the exception of Hg), and five of the six shoot parameters (with the exception of Zn). Cu and Zn induced respectively the strongest responses in root hormonal (up to ca. 240% the control values for IBA, 190% for IAA) and structural parameters (up to 210% for main root length, 330% for total lateral root length, 220% for number of root tips, 600% for total root surface, and from 2.5° to 26.0° of root growth angle). Regarding the shoots, the largest changes occurred for shoot height (down to 60% for Ni), rosette diameter (down to 45% for Hg), leaf number (up to 230% for Zn) and IBA (up to 240% for Pb and Cu). A microscope analysis revealed that shape and conformation of root hairs were strongly inhibited after Cd exposure, and enhanced under Hg and Pb. The results could have positive applications such as for defining toxicity thresholds (in phytoremediation) and acceptable concentration levels (for policies) for some of the most common heavy metals in agricultural soils.

Jasmonates, Ethylene and Brassinosteroids Control Adventitious and Lateral Rooting as Stress Avoidance Responses to Heavy Metals and Metalloids

Developmental and environmental signaling networks often converge during plant growth in response to changing conditions. Stress-induced hormones, such as jasmonates (JAs), can influence growth by crosstalk with other signals like brassinosteroids (BRs) and ethylene (ET). Nevertheless, it is unclear how avoidance of an abiotic stress triggers local changes in development as a response. It is known that stress hormones like JAs/ET and BRs can regulate the division rate of cells from the first asymmetric cell divisions (ACDs) in meristems, suggesting that stem cell activation may take part in developmental changes as a stress-avoidance-induced response. The root system is a prime responder to stress conditions in soil. Together with the primary root and lateral roots (LRs), adventitious roots (ARs) are necessary for survival in numerous plant species. AR and LR formation is affected by soil pollution, causing substantial root architecture changes by either depressing or enhancing rooting as a stress avoidance/survival response. Here, a detailed overview of the crosstalk between JAs, ET, BRs, and the stress mediator nitric oxide (NO) in auxin-induced AR and LR formation, with/without cadmium and arsenic, is presented. Interactions essential in achieving a balance between growth and adaptation to Cd and As soil pollution to ensure survival are reviewed here in the model species Arabidopsis and rice.

Recent Advances in Adventitious Root Formation in Chestnut

The genus Castanea includes several tree species that are relevant because of their geographical extension and their multipurpose character, that includes nut and timber production. However, commercial exploitation of the trees is hindered by several factors, particularly by their limited regeneration ability. Regardless of recent advances, there exists a serious limitation for the propagation of elite genotypes of chestnut due to decline of rooting ability as the tree ages. In the present review, we summarize the research developed in this genus during the last three decades concerning the formation of adventitious roots (ARs). Focusing on cuttings and in vitro microshoots, we gather the information available on several species, particularly C. sativa, C. dentata and the hybrid C.sativa × C. crenata, and analyze the influence of several factors on the achievements of the applied protocols, including genotype, auxin treatment, light regime and rooting media. We also pay attention to the acclimation phase, as well as compile the information available about biochemical and molecular related aspects. Furthermore, we considerate promising biotechnological approaches that might enable the improvement of the current protocols.

Auxin and Its Interaction With Ethylene Control Adventitious Root Formation and Development in Apple Rootstock

Adventitious root (AR) formation is indispensable for vegetative asexual propagation. Indole-3-butyric acid (IBA) functioned indirectly as precursor of IAA in regulating AR formation. Ethylene affects auxin synthesis, transport, and/or signaling processes. However, the interactions between auxin and ethylene that control AR formation in apple have not been elucidated. In this study, we investigated the effects of IBA and its interaction with ethylene on AR development in apple. The results revealed that IBA stimulated the formation of root primordia, increased the number of ARs, and upregulated expression of genes (MdWOX11, MdLBD16, and MdLBD29) involved in AR formation. Comparison of different periods of IBA application indicated that IBA was necessary for root primordium formation, while long time IBA treatment obviously inhibited root elongation. RNA-seq analysis revealed that many plant hormone metabolism and signal transduction related genes were differentially expressed. IBA stimulated the production of ethylene during AR formation. Auxin inhibiting ARs elongation depended on ethylene. Together, our results suggest that the inhibitory role of auxin on AR elongation in apples is partially mediated by stimulated ethylene production.

Nitric Oxide Cooperates With Auxin to Mitigate the Alterations in the Root System Caused by Cadmium and Arsenic

Oryza sativa L. is a worldwide food-crop frequently growing in cadmium (Cd)/arsenic (As) polluted soils, with its root-system as the first target of the pollutants. Root-system development involves the establishment of optimal indole-3-acetic acid (IAA) levels, also requiring the conversion of the IAA natural precursor indole-3-butyric acid (IBA) into IAA, causing nitric oxide (NO) formation. Nitric oxide is a stress-signaling molecule. In rice, a negative interaction of Cd or As with endogenous auxin has been demonstrated, as some NO protective effects. However, a synergism between the natural auxins (IAA and/or IBA) and NO was not yet determined and might be important for ameliorating rice metal(oid)-tolerance. With this aim, the stress caused by Cd/As toxicity in the root cells and the possible recovery by either NO or auxins (IAA/IBA) were evaluated after Cd or As (arsenate) exposure, combined or not with the NO-donor compound sodium-nitroprusside (SNP). Root fresh weight, membrane electrolyte leakage, and H2O2 production were also measured. Moreover, endogenous IAA/IBA contents, transcription-levels of OsYUCCA1 and OsASA2 IAA-biosynthetic-genes, and expression of the IAA-influx-carrier OsAUX1 and the IAA-responsive DR5::GUS construct were analyzed, and NO-epifluorescence levels were measured. Results showed that membrane injury by enhanced electrolyte leakage occurred under both pollutants and was reduced by the treatment with SNP only in Cd-presence. By contrast, no membrane injury was caused by either exogenous NO or IAA or IBA. Cd- and As-toxicity also resulted into a decreased root fresh weight, mitigated by the combination of each pollutant with either IAA or IBA. Cd and As decreased the endogenous NO-content, increased H2O2 formation, and altered auxin biosynthesis, levels and distribution in both adventitious (ARs) and mainly lateral roots (LRs). The SNP-formed NO counteracted the pollutants' effects on auxin distribution/levels, reduced H2O2 formation in Cd-presence, and enhanced AUX1-expression, mainly in As-presence. Each exogenous auxin, but mainly IBA, combined with Cd or As at 10 µM, mitigated the pollutants' effects by increasing LR-production and by increasing NO-content in the case of Cd. Altogether, results demonstrate that NO and auxin(s) work together in the rice root system to counteract the specific toxic-effects of each pollutant.

Auxin-Induced Adventitious Root Formation in Nodal Cuttings of Camellia sinensis

Adventitious root (AR) formation is essential for the successful propagation of Camellia sinensis and auxins play promotive effects on this process. Nowadays, the mechanism of auxin-induced AR formation in tea cuttings is widely studied. However, a lack of global view of the underlying mechanism has largely inhibited further studies. In this paper, recent advances including endogenous hormone changes, nitric oxide (NO) and hydrogen peroxide (H2O2) signals, secondary metabolism, cell wall reconstruction, and mechanisms involved in auxin signaling are reviewed. A further time course analysis of transcriptome changes in tea cuttings during AR formation is also suggested to deepen our understanding. The purpose of this paper is to offer an overview on the most recent developments especially on those key aspects affected by auxins and that play important roles in AR formation in tea plants.

Jasmonic Acid Methyl Ester Induces Xylogenesis and Modulates Auxin-Induced Xylary Cell Identity with NO Involvement

In Arabidopsis basal hypocotyls of dark-grown seedlings, xylary cells may form from the pericycle as an alternative to adventitious roots. Several hormones may induce xylogenesis, as Jasmonic acid (JA), as well as indole-3-acetic acid (IAA) and indole-3-butyric acid (IBA) auxins, which also affect xylary identity. Studies with the ethylene (ET)-perception mutant ein3eil1 and the ET-precursor 1-aminocyclopropane-1-carboxylic acid (ACC), also demonstrate ET involvement in IBA-induced ectopic metaxylem. Moreover, nitric oxide (NO), produced after IBA/IAA-treatments, may affect JA signalling and interact positively/negatively with ET. To date, NO-involvement in ET/JA-mediated xylogenesis has never been investigated. To study this, and unravel JA-effects on xylary identity, xylogenesis was investigated in hypocotyls of seedlings treated with JA methyl-ester (JAMe) with/without ACC, IBA, IAA. Wild-type (wt) and ein3eil1 responses to hormonal treatments were compared, and the NO signal was quantified and its role evaluated by using NO-donors/scavengers. Ectopic-protoxylem increased in the wt only after treatment with JAMe(10 μM), whereas in ein3eil1 with any JAMe concentration. NO was detected in cells leading to either xylogenesis or adventitious rooting, and increased after treatment with JAMe(10 μM) combined or not with IBA(10 μM). Xylary identity changed when JAMe was applied with each auxin. Altogether, the results show that xylogenesis is induced by JA and NO positively regulates this process. In addition, NO also negatively interacts with ET-signalling and modulates auxin-induced xylary identity.

What Makes Adventitious Roots?

The spermatophyte root system is composed of a primary root that develops from an embryonically formed root meristem, and of different post-embryonic root types: lateral and adventitious roots. Adventitious roots, arising from the stem of the plants, are the main component of the mature root system of many plants. Their development can also be induced in response to adverse environmental conditions or stresses. Here, in this review, we report on the morphological and functional diversity of adventitious roots and their origin. The hormonal and molecular regulation of the constitutive and inducible adventitious root initiation and development is discussed. Recent data confirmed the crucial role of the auxin/cytokinin balance in adventitious rooting. Nevertheless, other hormones must be considered. At the genetic level, adventitious root formation integrates the transduction of external signals, as well as a core auxin-regulated developmental pathway that is shared with lateral root formation. The knowledge acquired from adventitious root development opens new perspectives to improve micropropagation by cutting in recalcitrant species, root system architecture of crops such as cereals, and to understand how plants adapted during evolution to the terrestrial environment by producing different post-embryonic root types.

Control of adventitious root formation: insights into synergistic and antagonistic hormonal interactions

Plants have evolved sophisticated root systems that help them to cope with harsh environmental conditions. They are typically composed of a primary root and lateral roots (LRs), but may also include adventitious roots (ARs). Unlike LRs, ARs may be initiated not only from pericycle cells, but from various cell types and tissues depending on the species. Phytohormones, together with many other internal and external stimuli, coordinate and guide every step of AR formation from the first event of cell reprogramming until emergence and outgrowth. In this review, we summarize recent advances in the molecular mechanisms controlling AR formation and highlight the main hormonal cross talk involved in its regulation under different conditions and in different model systems.

Cellular dynamics during maturation-related decline of adventitious root formation in forest tree species

Adventitious root formation is a process in which roots are induced, from determined or differentiated cells that have not been specified to develop a root, at positions where they do not normally occur during development. In forest tree species, a decline in the capacity to form adventitious roots from similar cell types in stem cuttings is associated with tree age and maturity. This decline limits the success of vegetative propagation of selected adult trees. The joint action of local signals and a dynamic cascade of regulatory changes in gene expression, resulting in stereotypical cell division patterns, regulate cell fate changes that enable a somatic differentiated cell to reactivate meristem programs toward the induction of an adventitious root meristem.

Jasmonate promotes auxin-induced adventitious rooting in dark-grown Arabidopsis thaliana seedlings and stem thin cell layers by a cross-talk with ethylene signalling and a modulation of xylogenesis

BACKGROUND

Adventitious roots (ARs) are often necessary for plant survival, and essential for successful micropropagation. In Arabidopsis thaliana dark-grown seedlings AR-formation occurs from the hypocotyl and is enhanced by application of indole-3-butyric acid (IBA) combined with kinetin (Kin). The same IBA + Kin-treatment induces AR-formation in thin cell layers (TCLs). Auxin is the main inducer of AR-formation and xylogenesis in numerous species and experimental systems. Xylogenesis is competitive to AR-formation in Arabidopsis hypocotyls and TCLs. Jasmonates (JAs) negatively affect AR-formation in de-etiolated Arabidopsis seedlings, but positively affect both AR-formation and xylogenesis in tobacco dark-grown IBA + Kin TCLs. In Arabidopsis the interplay between JAs and auxin in AR-formation vs xylogenesis needs investigation. In de-etiolated Arabidopsis seedlings, the Auxin Response Factors ARF6 and ARF8 positively regulate AR-formation and ARF17 negatively affects the process, but their role in xylogenesis is unknown. The cross-talk between auxin and ethylene (ET) is also important for AR-formation and xylogenesis, occurring through EIN3/EIL1 signalling pathway. EIN3/EIL1 is the direct link for JA and ET-signalling. The research investigated JA role on AR-formation and xylogenesis in Arabidopsis dark-grown seedlings and TCLs, and the relationship with ET and auxin. The JA-donor methyl-jasmonate (MeJA), and/or the ET precursor 1-aminocyclopropane-1-carboxylic acid were applied, and the response of mutants in JA-synthesis and -signalling, and ET-signalling investigated. Endogenous levels of auxin, JA and JA-related compounds, and ARF6, ARF8 and ARF17 expression were monitored.

RESULTS

MeJA, at 0.01 μM, enhances AR-formation, when combined with IBA + Kin, and the response of the early-JA-biosynthesis mutant dde2-2 and the JA-signalling mutant coi1-16 confirmed this result. JA levels early change during TCL-culture, and JA/JA-Ile is immunolocalized in AR-tips and xylogenic cells. The high AR-response of the late JA-biosynthesis mutant opr3 suggests a positive action also of 12-oxophytodienoic acid on AR-formation. The crosstalk between JA and ET-signalling by EIN3/EIL1 is critical for AR-formation, and involves a competitive modulation of xylogenesis. Xylogenesis is enhanced by a MeJA concentration repressing AR-formation, and is positively related to ARF17 expression.

CONCLUSIONS

The JA concentration-dependent role on AR-formation and xylogenesis, and the interaction with ET opens the way to applications in the micropropagation of recalcitrant species.