Knockout of GH3 genes in the moss Physcomitrella patens leads to increased IAA levels at elevated temperature and in darkness

Ethylene controls three-dimensional growth involving reduced auxin levels in the moss Physcomitrium patens

The conquest of land by plants was concomitant with, and possibly enabled by, the evolution of three-dimensional (3D) growth. The moss Physcomitrium patens provides a model system for elucidating molecular mechanisms in the initiation of 3D growth. Here, we investigate whether the phytohormone ethylene, which is believed to have been a signal before land plant emergence, plays a role in 3D growth regulation in P. patens. We report ethylene controls 3D gametophore formation, based on results from exogenously applied ethylene and genetic manipulation of PpEIN2, which is a central component in the ethylene signaling pathway. Overexpression (OE) of PpEIN2 activates ethylene responses and leads to earlier formation of gametophores with fewer gametophores produced thereafter, phenocopying ethylene-treated wild-type. Conversely, Ppein2 knockout mutants, which are ethylene insensitive, show initially delayed gametophore formation with more gametophores produced later. Furthermore, pharmacological and biochemical analyses reveal auxin levels are decreased in the OE lines but increased in the knockout mutants. Our results suggest that evolutionarily, ethylene and auxin molecular networks were recruited to build the plant body plan in ancestral land plants. This might have played a role in enabling ancient plants to acclimate to the continental surfaces of the planet.

The Roles of GRETCHEN HAGEN3 (GH3)-Dependent Auxin Conjugation in the Regulation of Plant Development and Stress Adaptation

The precise control of free auxin (indole-3-acetic acid, IAA) gradient, which is orchestrated by biosynthesis, conjugation, degradation, hydrolyzation, and transport, is critical for all aspects of plant growth and development. Of these, the GRETCHEN HAGEN 3 (GH3) acyl acid amido synthetase family, pivotal in conjugating IAA with amino acids, has garnered significant interest. Recent advances in understanding GH3-dependent IAA conjugation have positioned GH3 functional elucidation as a hot topic of research. This review aims to consolidate and discuss recent findings on (i) the enzymatic mechanisms driving GH3 activity, (ii) the influence of chemical inhibitor on GH3 function, and (iii) the transcriptional regulation of GH3 and its impact on plant development and stress response. Additionally, we explore the distinct biological functions attributed to IAA-amino acid conjugates.

Developmental regulation of leaf venation patterns: monocot versus eudicots and the role of auxin

Organisation and patterning of the vascular network in land plants varies in different taxonomic, developmental and environmental contexts. In leaves, the degree of vascular strand connectivity influences both light and CO₂ harvesting capabilities as well as hydraulic capacity. As such, developmental mechanisms that regulate leaf venation patterning have a direct impact on physiological performance. Development of the leaf venation network requires the specification of procambial cells within the ground meristem of the primordium and subsequent proliferation and differentiation of the procambial lineage to form vascular strands. An understanding of how diverse venation patterns are manifest therefore requires mechanistic insight into how procambium is dynamically specified in a growing leaf. A role for auxin in this process was identified many years ago, but questions remain. In this review we first provide an overview of the diverse venation patterns that exist in land plants, providing an evolutionary perspective. We then focus on the developmental regulation of leaf venation patterns in angiosperms, comparing patterning in eudicots and monocots, and the role of auxin in each case. Although common themes emerge, we conclude that the developmental mechanisms elucidated in eudicots are unlikely to fully explain how parallel venation patterns in monocot leaves are elaborated.

Physcomitrium patens Mutants in Auxin Conjugating GH3 Proteins Show Salt Stress Tolerance but Auxin Homeostasis Is Not Involved in Regulation of Oxidative Stress Factors

Salt stress is among the most challenging abiotic stress situations that a plant can experience. High salt levels do not only occur in areas with obvious salty water, but also during drought periods where salt accumulates in the soil. The moss Physcomitrium patens became a model for studying abiotic stress in non-vascular plants. Here, we show that high salt concentrations can be tolerated in vitro, and that auxin homeostasis is connected to the performance of P. patens under these stress conditions. The auxin levels can be regulated by conjugating IAA to amino acids by two members of the family of GH3 protein auxin amino acid-synthetases that are present in P. patens. Double GH3 gene knock-out mutants were more tolerant to high salt concentrations. Furthermore, free IAA levels were differentially altered during the time points investigated. Since, among the mutant lines, an increase in IAA on at least one NaCl concentration tested was observed, we treated wild type (WT) plants concomitantly with NaCl and IAA. This experiment showed that the salt tolerance to 100 mM NaCl together with 1 and 10 µM IAA was enhanced during the earlier time points. This is an additional indication that the high IAA levels in the double GH3-KO lines could be responsible for survival in high salt conditions. While the high salt concentrations induced several selected stress metabolites including phenols, flavonoids, and enzymes such as peroxidase and superoxide dismutase, the GH3-KO genotype did not generally participate in this upregulation. While we showed that the GH3 double KO mutants were more tolerant of high (250 mM) NaCl concentrations, the altered auxin homeostasis was not directly involved in the upregulation of stress metabolites.

Auxin Biology in Bryophyta: A Simple Platform with Versatile Functions

Bryophytes, including liverworts, mosses, and hornworts, are gametophyte-dominant land plants that are derived from a common ancestor and underwent independent evolution from the sporophyte-dominant vascular plants since their divergence. The plant hormone auxin has been shown to play pleiotropic roles in the haploid bodies of bryophytes. Pharmacological and chemical studies identified conserved auxin molecules, their inactivated forms, and auxin transport in bryophyte tissues. Recent genomic and molecular biological studies show deep conservation of components and their functions in auxin biosynthesis, inactivation, transport, and signaling in land plants. Low genetic redundancy in model bryophytes enable unique assays, which are elucidating the design principles of the auxin signaling pathway. In this article, the physiological roles of auxin and regulatory mechanisms of gene expression and development by auxin in Bryophyta are reviewed.

Convergence of sphingolipid desaturation across over 500 million years of plant evolution

For plants, acclimation to low temperatures is fundamental to survival. This process involves the modification of lipids to maintain membrane fluidity. We previously identified a new cold-induced putative desaturase in Physcomitrium (Physcomitrella) patens. Lipid profiles of null mutants of this gene lack sphingolipids containing monounsaturated C24 fatty acids, classifying the new protein as sphingolipid fatty acid denaturase (PpSFD). PpSFD mutants showed a cold-sensitive phenotype as well as higher susceptibility to the oomycete Pythium, assigning functions in stress tolerance for PpSFD. Ectopic expression of PpSFD in the Atads2.1 (acyl coenzyme A desaturase-like 2) Arabidopsis thaliana mutant functionally complemented its cold-sensitive phenotype. While these two enzymes catalyse a similar reaction, their evolutionary origin is clearly different since AtADS2 is a methyl-end desaturase whereas PpSFD is a cytochrome b₅ fusion desaturase. Altogether, we suggest that adjustment of membrane fluidity evolved independently in mosses and seed plants, which diverged more than 500 million years ago.

Light- and hormone-mediated development in non-flowering plants: An overview

Light, hormones and their interaction regulate different aspects of development in non-flowering plants. They might have played a role in the evolution of different plant groups by conferring specific adaptive evolutionary changes. Plants are sessile organisms. Unlike animals, they lack the opportunity to abandon their habitat in unfavorable conditions. They respond to different environmental cues and adapt accordingly to control their growth and developmental pattern. While phytohormones are known to be internal regulators of plant development, light is a major environmental signal that shapes plant processes. It is plausible that light-hormone crosstalk might have played an important role in plant evolution. But how the crosstalk between light and phytohormone signaling pathways might have shaped the plant evolution is unclear. One of the possible reasons is that flowering plants have been studied extensively in context of plant development, which cannot serve the purpose of evolutionary comparisons. In order to elucidate the role of light, hormone and their crosstalk in the evolutionary adaptation in plant kingdom, one needs to understand various light- and hormone-mediated processes in diverse non-flowering plants. This review is an attempt to outline major light- and phytohormone-mediated responses in non-flowering plant groups such as algae, bryophytes, pteridophytes and gymnosperms.

Hairy root transformation of Brassica rapa with bacterial halogenase genes and regeneration to adult plants to modify production of indolic compounds

During the last years halogenated compounds have drawn a lot of attention. Metabolites with one or more halogen atoms are often more active than their non-halogenated derivatives like indole-3-acetic acid (IAA) and 4-Cl-IAA. Within this work, bacterial flavin-dependent tryptophan halogenase genes were inserted into Brassica rapa ssp. pekinensis (Chinese cabbage) with the aim to produce novel halogenated indole compounds. It was investigated which tryptophan-derived indole metabolites, such as indole glucosinolates or potential degradation products can be synthesized by the transgenic root cultures. In vivo and in vitro activity of halogenases heterologously produced was shown and the production of chlorinated tryptophan in transgenic root lines was confirmed. Furthermore, chlorinated indole-3-acetonitrile (Cl-IAN) was detected. Other tryptophan-derived indole metabolites, such as IAA or indole glucosinolates were not found in the transgenic roots in a chlorinated form. The influence of altered growth conditions on the amount of produced chlorinated compounds was evaluated. We found an increase in Cl-IAN production at low temperatures (8 °C), but otherwise no significant changes were observed. Furthermore, we were able to regenerate the wild type and transgenic root cultures to adult plants, of which the latter still produced chlorinated metabolites. Therefore, we conclude that the genetic information had been stably integrated. The transgenic plants showed a slightly altered phenotype compared to plants grown from seeds since they also still expressed the rol genes. By this approach we were able to generate various stably transformed plant materials from which it was possible to isolate chlorinated tryptophan and Cl-IAN.

Roles of phytohormone changes in the grain yield of rice plants exposed to heat: a review

During its reproductive phase, rice is susceptible to heat stress. Heat events will occur at all stages during the reproductive phase of rice as a result of global warming. Moreover, rice yield traits respond differently to heat stress during panicle initiation, flowering and grain filling. The reduction in the number of spikelets per panicle of heat-stressed plants is due to the attenuated differentiation of secondary branches and their attached florets as well as the promotion of their degradation during the panicle-initiation stage but is not affected by heat stress thereafter. Spikelet sterility as a result of heat stress is attributed not only to physiological abnormalities in the reproductive organs during the flowering stage but also to structural and morphological abnormalities in reproductive organs during the panicle-initiation stage. The reduced grain weight of heat-stressed plants is due to a reduction in nonstructural carbohydrates, undeveloped vascular bundles, and a reduction in glume size during the panicle-initiation stage, while a shortened grain-filling duration, reduced grain-filling rate, and decreased grain width contribute to reduced grain weight during the grain-filling stage. Thus, screening and breeding rice varieties that have comprehensive tolerance to heat stress at all time points during their reproductive stage may be possible to withstand unpredictable heat events in the future. The responses of yield traits to heat stress are regulated by phytohormone levels, which are determined by phytohormone homeostasis. Currently, the biosynthesis and transport of phytohormones are the key processes that determine phytohormone levels in and grain yield of rice under heat stress. Studies on phytohormone homeostatic responses are needed to further reveal the key processes that determine phytohormone levels under heat conditions.

Auxin Metabolism Controls Developmental Decisions in Land Plants

Unlike animals, whose body plans are set during embryo development, plants maintain the ability to initiate new organs throughout their life cycle. Auxin is a key regulator of almost all aspects of plant development, including morphogenesis and adaptive responses. Cellular auxin concentrations influence whether a cell will divide, grow, or differentiate, thereby contributing to organ formation, growth, and ultimately plant shape. Auxin gradients are established and maintained by a tightly regulated interplay between metabolism, signalling, and transport. Auxin is synthesised, stored, and inactivated by a multitude of parallel pathways that are all tightly regulated. Here we summarise the remarkable progress that has been achieved in identifying some key components of these pathways and the genetic complexity underlying their precise regulation.

Functional characterization of Gh_A08G1120 (GH3.5) gene reveal their significant role in enhancing drought and salt stress tolerance in cotton

BACKGROUND

Auxins play an important role in plant growth and development; the auxins responsive gene; auxin/indole-3-acetic acid (Aux/IAA), small auxin-up RNAs (SAUR) and Gretchen Hagen3 (GH3) control their mechanisms. The GH3 genes function in homeostasis by the catalytic activities in auxin conjugation and bounding free indole-3-acetic acid (IAA) to amino acids.

RESULTS

In our study, we identified the GH3 genes in three cotton species; Gossypium hirsutum, Gossypium arboreum and Gossypium raimondii, analyzed their chromosomal distribution, phylogenetic relationships, cis-regulatory element function and performed virus induced gene silencing of the novel Gh_A08G1120 (GH3.5) gene. The phylogenetic tree showed four clusters of genes with clade 1, 3 and 4 having mainly members of the GH3 of the cotton species while clade 2 was mainly members belonging to Arabidopsis. There were no paralogous genes, and few orthologous genes were observed between Gossypium and other species. All the GO terms were detected, but only 14 genes were found to have described GO terms in upland cotton, more biological functions were detected, as compared to the other functions. The GH3.17 subfamily harbored the highest number of the cis-regulatory elements, most having promoters towards dehydration-responsiveness. The RNA expression analysis revealed that 10 and 8 genes in drought and salinity stress conditions respectively were upregulated in G. hirsutum. All the genes that were upregulated in plants under salt stress conditions were also upregulated in drought stress; moreover, Gh_A08G1120 (GH3.5) exhibited a significant upregulation across the two stress factors. Functional characterization of Gh_A08G1120 (GH3.5) through virus-induced gene silencing (VIGS) revealed that the VIGS plants ability to tolerate drought and salt stresses was significantly reduced compared to the wild types. The chlorophyll content, relative leaf water content (RLWC), and superoxide dismutase (SOD) concentration level were reduced significantly while malondialdehyde concentration and ion leakage as a measure of cell membrane stability (CMS) increased in VIGS plants under drought and salt stress conditions.

CONCLUSION

This study revealed the significance of the GH3 genes in enabling the plant's adaptation to drought and salt stress conditions as evidenced by the VIGS results and RT-qPCR analysis.

Arabidopsis thaliana expressing PbBSMT, a gene encoding a SABATH-type methyltransferase from the plant pathogenic protist Plasmodiophora brassicae, show leaf chlorosis and altered host susceptibility

The plant pathogenic protist Plasmodiophora brassicae causes clubroot disease of Brassicaceae. This biotrophic organism can down-regulate plant defence responses. The previously characterised P. brassicae PbBSMT methyltransferase has substrate specificity for salicylic, benzoic and anthranilic acids. We therefore propose a role for the methylation of SA in attenuating plant defence response in infected roots as a novel strategy for intracellular parasitism. We overexpressed PbBSMT under the control of an inducible promoter in Arabidopsis thaliana and performed physiological, molecular and phytopathological analyses with the transgenic plants under control and induced conditions in comparison to the wild type. Upon induction, transcription of PbBSMT was associated with: (1) strong leaf phenotypes from anthocyanin accumulation and chlorosis followed by browning; (2) increased plant susceptibility after infection with P. brassicae that was manifested as more yellow leaves and reduced growth of upper plant parts; and (3) induced transgenic plants were not able to support large galls and had a brownish appearance of some clubs. Microarray data indicated that chlorophyll loss was accompanied by reduced transcription of genes involved in photosynthesis, while genes encoding glucose metabolism, mitochondrial functions and cell wall synthesis were up-regulated. Our results indicate a role for PbBSMT in attenuation of host defence responses in the roots by metabolising a plant defence signal.

Auxin-mediated developmental control in the moss Physcomitrella patens

The signalling molecule auxin regulates many fundamental aspects of growth and development in plants. We review and discuss what is known about auxin-regulated development in mosses, with special emphasis on the model species Physcomitrella patens. It is well established that mosses and other early diverging plants produce and respond to auxin. By sequencing the P. patens genome, it became clear that it encodes many core proteins important for auxin homeostasis, perception, and signalling, which have also been identified in flowering plants. This suggests that the auxin molecular network was present in the last common ancestor of flowering plants and mosses. Despite fundamental differences in their life cycles, key processes such as organ initiation and outgrowth, branching, tropic responses, as well as cell differentiation, division, and expansion appear to be regulated by auxin in the two lineages. This knowledge paves the way for studies aimed at a better understanding of the origin and evolution of auxin function and how auxin may have contributed to the evolution of land plants.

The Physcomitrella patens exocyst subunit EXO70.3d has distinct roles in growth and development, and is essential for completion of the moss life cycle

The exocyst, an evolutionarily conserved secretory vesicle-tethering complex, spatially controls exocytosis and membrane turnover in fungi, metazoans and plants. The exocyst subunit EXO70 exists in multiple paralogs in land plants, forming three conserved clades with assumed distinct roles. Here we report functional analysis of the first moss exocyst subunit to be studied, Physcomitrella patens PpEXO70.3d (Pp1s97_91V6), from the, as yet, poorly characterized EXO70.3 clade. Following phylogenetic analysis to confirm the presence of three ancestral land plant EXO70 clades outside angiosperms, we prepared and phenotypically characterized loss-of-function Ppexo70.3d mutants and localized PpEXO70.3d in vivo using green fluorescent protein-tagged protein expression. Disruption of PpEXO70.3d caused pleiotropic cell elongation and differentiation defects in protonemata, altered response towards exogenous auxin, increased endogenous IAA concentrations, along with defects in bud and gametophore development. During mid-archegonia development, an abnormal egg cell is formed and subsequently collapses, resulting in mutant sterility. Mutants exhibited altered cell wall and cuticle deposition, as well as compromised cytokinesis, consistent with the protein localization to the cell plate. Despite some functional redundancy allowing survival of moss lacking PpEXO70.3d, this subunit has an essential role in the moss life cycle, indicating sub-functionalization within the moss EXO70 family.

Tobacco TTG2 regulates vegetative growth and seed production via the predominant role of ARF8 in cooperation with ARF17 and ARF19

BACKGROUND

Plant TRANSPARENT TESTA GLABRA (TTG) proteins regulate various developmental activities via the auxin signaling pathway. Recently, we elucidated the developmental role of tobacco (Nicotiana tabacum L.) NtTTG2 in association with 12 genes that putatively encode AUXIN RESPONSIVE FACTOR (ARF) proteins, including NtARF8, NtARF17, and NtARF19. Here we show that NtTTG2 regulates tobacco growth and development by involving the NtARF8, NtARF17, and NtARF19 genes, with the NtARF8 gene playing a predominant contribution.

RESULTS

Independent silencing of the NtARF8 gene more strongly repressed tobacco growth than silencing the NtARF17 or NtARF19 gene and more effectively eradicated the growth enhancement effect of NtTTG2 overexpression. In contrast, plant growth was not affected by silencing additional nine NtTTG2-regulated NtARF genes. In double and triple gene silencing combinations, silencing the NtARF8 gene was more effective than silencing the NtARF17 or NtARF19 gene to repress growth as well as nullify growth enhancement. Therefore, the NtARF8 predominantly cooperated with the NtARF17 and NtAFR19 of the NtTTG2 functional pathway. NtARF8 also contributed to NtTTG2-regulated seed production as concurrent NtTTG2 and NtARF8 overexpression played a synergistic role in seed production quantity, whereas concurrent silencing of both genes caused more severe seed abortion than single gene silencing. In plant cells, the NtTTG2 protein facilitated the nuclear import of NtARF8 as well as increased its function as a transcription activator.

CONCLUSIONS

NtARF8 is an integral component of the NtTTG2 functional pathway, which regulates tobacco growth and development.

Local auxin metabolism regulates environment-induced hypocotyl elongation

A hallmark of plants is their adaptability of size and form in response to widely fluctuating environments. The metabolism and redistribution of the phytohormone auxin play pivotal roles in establishing active auxin gradients and resulting cellular differentiation. In Arabidopsis thaliana, cotyledons and leaves synthesize indole-3-acetic acid (IAA) from tryptophan through indole-3-pyruvic acid (3-IPA) in response to vegetational shade. This newly synthesized auxin moves to the hypocotyl where it induces elongation of hypocotyl cells. Here we show that loss of function of VAS2 (IAA-amido synthetase Gretchen Hagen 3 (GH3).17) leads to increases in free IAA at the expense of IAA-Glu (IAA-glutamate) in the hypocotyl epidermis. This active IAA elicits shade- and high temperature-induced hypocotyl elongation largely independently of 3-IPA-mediated IAA biosynthesis in cotyledons. Our results reveal an unexpected capacity of local auxin metabolism to modulate the homeostasis and spatial distribution of free auxin in specialized organs such as hypocotyls in response to shade and high temperature.