Plant response to drought and hyperosmosis is mediated by the phytohormone abscisic acid (ABA), a sesquiterpene compound widely distributed in various embryophyte groups. Exogenous ABA as well as ...hyperosmosis activates the sucrose nonfermenting 1 (SNF1)-related protein kinase2 (SnRK2), which plays a central role in cellular responses against drought and dehydration, although the details of the activation mechanism are not understood. Analysis of a mutant of the mossPhyscomitrella patenswith reduced ABA sensitivity and reduced hyperosmosis tolerance revealed that a protein kinase designated “ARK” (for “ABA and abiotic stress-responsive Raf-like kinase”) plays an essential role in the activation of SnRK2. ARK encoded by a single gene inP. patensbelongs to the family of group B3 Raf-like MAP kinase kinase kinases (B3-MAPKKKs) mediating ethylene, disease resistance, and salt and sugar responses in angiosperms. Our findings indicate that ARK, as a novel regulatory component integrating ABA and hyperosmosis signals, represents the ancestral B3-MAPKKKs, which multiplied, diversified, and came to have specific functions in angiosperms.
Plants acclimate to environmental stress signals such as cold, drought and hypersalinity, and provoke internal protective mechanisms. Abscisic acid (ABA), a carotenoid‐derived phytohormone, which ...increases in response to the stress signals above, has been suggested to play a key role in the acclimation process in angiosperms, but the role of ABA in basal land plants such as mosses, including its biosynthetic pathways, has not been clarified. Targeted gene disruption of PpABA1, encoding zeaxanthin epoxidase in the moss Physcomitrella patens was conducted to determine the role of endogenous ABA in acclimation processes in mosses. The generated ppaba1 plants were found to accumulate only a small amount of endogenous ABA. The ppaba1 plants showed reduced osmotic acclimation capacity in correlation with reduced dehydration tolerance and accumulation of late embryogenesis abundant proteins. By contrast, cold‐induced freezing tolerance was less affected in ppaba1, indicating that endogenous ABA does not play a major role in the regulation of cold acclimation in the moss. Our results suggest that the mechanisms for osmotic acclimation mediated by carotenoid‐derived synthesis of ABA are conserved in embryophytes and that acquisition of the mechanisms played a crucial role in terrestrial adaptation and colonization by land plant ancestors.
The phytohormone ABA and the transcription factor ABSCISIC ACID INSENSITIVE 3 (ABI3)/VIVIPAROUS 1 (VP1) function in protecting embryos during the desiccation stage of seed development. In a similar ...signaling pathway, vegetative tissue of the moss Physcomitrella patens survives desiccation by activating downstream genes (e.g. LEA1) in response to ABA and ABI3.
We show that the PpLEA1 promoter responds to PpABI3 primarily through the ACTT-core element (5′-TCCACTTGTC-3′), while the ACGT-core ABA-responsive element (ABRE) appears to respond to ABA alone. We also found by yeast-two-hybrid screening that PpABI3A interacts with PpNF-YC1, a subunit of CCAAT box binding factor (CBF)/nuclear factor Y (NF-Y). PpNF-YC1 increased the activation of the PpLEA1 promoter when incubated with PpABI3A, as did NF-YB, NF-YC, and ABI3 from Arabidopsis.
This new response element (ACTT) is responsible for activating the ABI3-dependent ABA response pathway cooperatively with the nuclear factor Y (NF-Y) complex. These results further define the regulatory interactions at the transcriptional level for the expression of this network of genes required for drought/desiccation tolerance.
This gene regulatory set is in large part conserved between vegetative tissue of bryophytes and seeds of angiosperms and will shed light on the evolution of this pathway in the green plant lineage.
Desiccation tolerance is an ancestral feature of land plants and is still retained in non-vascular plants such as bryophytes and some vascular plants. However, except for seeds and spores, this trait ...is absent in vegetative tissues of vascular plants. Although many studies have focused on understanding the molecular basis underlying desiccation tolerance using transcriptome and proteome approaches, the critical molecular differences between desiccation tolerant plants and non-desiccation plants are still not clear. The moss Physcomitrella patens cannot survive rapid desiccation under laboratory conditions, but if cells of the protonemata are treated by the phytohormone abscisic acid (ABA) prior to desiccation, it can survive 24 h exposure to desiccation and regrow after rehydration. The desiccation tolerance induced by ABA (AiDT) is specific to this hormone, but also depends on a plant transcription factor ABSCISIC ACID INSENSITIVE3 (ABI3). Here we report the comparative proteomic analysis of AiDT between wild type and ABI3 deleted mutant (Δabi3) of P. patens using iTRAQ (Isobaric Tags for Relative and Absolute Quantification). From a total of 1980 unique proteins that we identified, only 16 proteins are significantly altered in Δabi3 compared to wild type after desiccation following ABA treatment. Among this group, three of the four proteins that were severely affected in Δabi3 tissue were Arabidopsis orthologous genes, which were expressed in maturing seeds under the regulation of ABI3. These included a Group 1 late embryogenesis abundant (LEA) protein, a short-chain dehydrogenase, and a desiccation-related protein. Our results suggest that at least three of these proteins expressed in desiccation tolerant cells of both Arabidopsis and the moss are very likely to play important roles in acquisition of desiccation tolerance in land plants. Furthermore, our results suggest that the regulatory machinery of ABA- and ABI3-mediated gene expression for desiccation tolerance might have evolved in ancestral land plants before the separation of bryophytes and vascular plants.
•Large-scale proteomics highlighted proteins related to plant desiccation tolerance.•The proteins were regulated by both the phytohormone ABA and ABI3.•The proteins accumulated in desiccation tolerant cells of both Arabidopsis and moss.•Evolutionary origin of regulatory machinery for desiccation tolerance is proposed.
To survive fluctuating water availability on land, terrestrial plants must be able to sense water stresses, such as drought and flooding. The plant hormone abscisic acid (ABA) and plant-specific ...SNF1-related protein kinase 2 (SnRK2) play key roles in plant osmostress responses. We recently reported that, in the moss Physcomitrium patens, ABA and osmostress-dependent SnRK2 activation requires phosphorylation by an upstream RAF-like kinase (ARK). This RAF/SnRK2 module is an evolutionarily conserved mechanism of osmostress signaling in land plants. Surprisingly, ARK is also an ortholog of Arabidopsis CONSTITUTIVE RESPONSE 1 (CTR1), which negatively regulates the ethylene-mediated submergence response of P. patens, indicating a nexus for cross-talk between the two signaling pathways that regulate responses to water availability. However, the mechanism through which the ARK/SnRK2 module is activated in response to water stress remains to be elucidated. Here, we show that a group of ethylene-receptor-related sensor histidine kinases (ETR-HKs) is essential for ABA and osmostress responses in P. patens. The intracellular kinase domain of an ETR-HK from P. patens physically interacts with ARK at the endoplasmic reticulum in planta. Moreover, HK disruptants lack ABA-dependent autophosphorylation of the critical serine residue in the activation loop of ARK, leading to loss of SnRK2 activation in response to ABA and osmostress. Collectively with the notion that ETR-HKs participate in submergence responses, our present data suggest that the HK/ARK module functions as an integration unit for environmental water availability to elicit optimized water stress responses in the moss P. patens.
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•Ethylene-receptor-related histidine kinases are essential for moss stress responses•ETR-HKs interact with a RAF kinase at the endoplasmic reticulum•ETR-HKs regulate ABA-dependent activation of SnRK2 via RAF kinase activation
Toriyama et al. show that, in the moss Physcomitrium patens, ethylene-receptor-related sensor histidine kinases (HKs) interact with a Raf kinase (RAF) at the endoplasmic reticulum. This HK-RAF module is essential for ethylene-mediated submergence signaling and abscisic-acid-mediated osmostress signaling for optimal environmental stress responses.
Summary
The phytohormone
ABA
and the transcription factor
ABSCISIC ACID INSENSITIVE
3 (
ABI
3)/
VIVIPAROUS
1 (
VP
1) function in protecting embryos during the desiccation stage of seed development. ...In a similar signaling pathway, vegetative tissue of the moss
P
hyscomitrella patens
survives desiccation by activating downstream genes (e.g.
LEA
1) in response to
ABA
and
ABI
3.
We show that the
P
p
LEA
1
promoter responds to
P
p
ABI
3 primarily through the
ACTT
‐core element (5′‐
TCC
ACTT
GTC
‐3′), while the
ACGT
‐core
ABA
‐responsive element (
ABRE
) appears to respond to
ABA
alone. We also found by yeast‐two‐hybrid screening that
P
p
ABI
3A interacts with
P
p
NF
‐
YC
1, a subunit of
CCAAT
box binding factor (
CBF
)/nuclear factor
Y
(
NF
‐Y).
P
p
NF
‐
YC
1 increased the activation of the
P
p
LEA
1
promoter when incubated with
P
p
ABI
3
A
, as did
NF
‐
YB
,
NF
‐
YC
, and
ABI
3 from Arabidopsis.
This new response element (
ACTT
) is responsible for activating the
ABI
3‐dependent
ABA
response pathway cooperatively with the nuclear factor
Y
(
NF
‐
Y
) complex. These results further define the regulatory interactions at the transcriptional level for the expression of this network of genes required for drought/desiccation tolerance.
This gene regulatory set is in large part conserved between vegetative tissue of bryophytes and seeds of angiosperms and will shed light on the evolution of this pathway in the green plant lineage.
Abscisic acid (ABA) is a plant hormone important for environmental stress responses. We identified the Raf-like protein kinase ARK as an upstream regulator of subclass III sucrose ...non-fermenting1-related kinase2, known as a key activator of ABA signaling in Physcomitrella patens. Phosphopeptide mapping revealed that ARK is phosphorylated at a specific Ser residue in the activation loop of the kinase domain. Substitution of the Ser residue to Ala caused a decrease in ABA-induced gene expression while that to Asp caused an enhancement of the gene expression. From these results, we conclude that phosphorylation of ARK is important for its kinase activity and a response to ABA.
Abscisic acid (ABA) controls plants’ tolerance to environmental stress such as freezing and desiccation. Wepreviously reported that AR7, the ABA-insensitive mutant of Physcomitrella patens has a ...defect in the ARKgene encoding the group B3 MAP kinase kinase kinase, with a single amino acid change from serine tophenylalanine in the non-kinase domain having a putative regulatory function. By ultraviolet mutagenesis,we obtained over one hundred ABA-insensitive mutants, from which two ark nonsense mutants wereidentified. These mutants showed inhibited protonemal growth with elongated gametophore in the absence ofABA, in contrast to AR7 showing protonemal growth similar to wild type. These results indicate that ARK isinvolved in regulation of plant growth in addition to stress responses.
A series of new compounds comprising oleoyl, amino acid and d -glucamine moieties were synthesised using mild reaction conditions, and their hydrogelation ability was evaluated. The gels exhibited ...improved stiffness and thixotropic properties dependent on the chemical structure of the amino acid linker and the number of potential hydrogen bonding sites.
Hydrogels made from new gelators, composed of the alkyl chain and a d -glucamine moiety linked together with glycine(s), exhibit thixotropic properties, whereas the hydrogels formed with structurally ...similar N -alkyl- d -glucamide, which do not contain the glycine moiety, have a tendency to crystallize and do not show thixotropic behaviour.
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