Regulation of xylem fiber differentiation by gibberellins through DELLA-KNAT1 interaction.

The thickening of plant organs is supported by secondary growth, a process by which new vascular tissues (xylem and phloem) are produced. Xylem is composed of several cell types, including xylary fibers, parenchyma and vessel elements. In Arabidopsis, it has been shown that fibers are promoted by the class-I KNOX gene KNAT1 and the plant hormones gibberellins, and are repressed by a small set of receptor-like kinases; however, we lack a mechanistic framework to integrate their relative contributions. Here, we show that DELLAs, negative elements of the gibberellin signaling pathway, physically interact with KNAT1 and impair its binding to KNAT1-binding sites. Our analysis also indicates that at least 37% of the transcriptome mobilized by KNAT1 is potentially dependent on this interaction, and includes genes involved in secondary cell wall modifications and phenylpropanoid biosynthesis. Moreover, the promotion by constitutive overexpression of KNAT1 of fiber formation and the expression of genes required for fiber differentiation were still reverted by DELLA accumulation, in agreement with post-translational regulation of KNAT1 by DELLA proteins. These results suggest that gibberellins enhance fiber development by promoting KNAT1 activity.

INDETERMINATE-DOMAIN 4 (IDD4) coordinates immune responses with plant-growth in Arabidopsis thaliana.

INDETERMINATE DOMAIN (IDD)/ BIRD proteins are a highly conserved plant-specific family of transcription factors which play multiple roles in plant development and physiology. Here, we show that mutation in IDD4/IMPERIAL EAGLE increases resistance to the hemi-biotrophic pathogen Pseudomonas syringae, indicating that IDD4 may act as a repressor of basal immune response and PAMP-triggered immunity. Furthermore, the idd4 mutant exhibits enhanced plant-growth indicating IDD4 as suppressor of growth and development. Transcriptome comparison of idd4 mutants and IDD4ox lines aligned to genome-wide IDD4 DNA-binding studies revealed major target genes related to defense and developmental-biological processes. IDD4 is a phospho-protein that interacts and becomes phosphorylated on two conserved sites by the MAP kinase MPK6. DNA-binding studies of IDD4 after flg22 treatment and with IDD4 phosphosite mutants show enhanced binding affinity to ID1 motif-containing promoters and its function as a transcriptional regulator. In contrast to the IDD4-phospho-dead mutant, the IDD4 phospho-mimicking mutant shows altered susceptibility to PstDC3000, salicylic acid levels and transcriptome reprogramming. In summary, we found that IDD4 regulates various hormonal pathways thereby coordinating growth and development with basal immunity.

Outgrowth of the axillary bud in rose is controlled by sugar metabolism and signalling.

Shoot branching is a pivotal process during plant growth and development, and is antagonistically orchestrated by auxin and sugars. In contrast to extensive investigations on hormonal regulatory networks, our current knowledge on the role of sugar signalling pathways in bud outgrowth is scarce. Based on a comprehensive stepwise strategy, we investigated the role of glycolysis/the tricarboxylic acid (TCA) cycle and the oxidative pentose phosphate pathway (OPPP) in the control of bud outgrowth. We demonstrated that these pathways are necessary for bud outgrowth promotion upon plant decapitation and in response to sugar availability. They are also targets of the antagonistic crosstalk between auxin and sugar availability. The two pathways act synergistically to down-regulate the expression of BRC1, a conserved inhibitor of shoot branching. Using Rosa calluses stably transformed with GFP-fused promoter sequences of RhBRC1 (pRhBRC1), glycolysis/TCA cycle and the OPPP were found to repress the transcriptional activity of pRhBRC1 cooperatively. Glycolysis/TCA cycle- and OPPP-dependent regulations involve the -1973/-1611 bp and -1206/-709 bp regions of pRhBRC1, respectively. Our findings indicate that glycolysis/TCA cycle and the OPPP are integrative parts of shoot branching control and can link endogenous factors to the developmental programme of bud outgrowth, likely through two distinct mechanisms.

Gibberellin signaling in plants.

The plant hormone gibberellin (GA) regulates major aspects of plant growth and development. The role of GA in determining plant stature had major impacts on agriculture in the 1960s, and the development of semi-dwarf varieties that show altered GA responses contributed to a huge increase in grain yields during the 'green revolution'. The past decade has brought great progress in understanding the molecular basis of GA action, with the cloning and characterization of GA signaling components. Here, we review the molecular basis of the GA signaling pathway, from the perception of GA to the regulation of downstream genes.

The gibberellin biosynthetic genes AtKAO1 and AtKAO2 have overlapping roles throughout Arabidopsis development.

Ent-kaurenoic acid oxidase (KAO), a class of cytochrome P450 monooxygenases of the subfamily CYP88A, catalyzes the conversion of ent-kaurenoic acid (KA) to gibberellin (GA) GA12 , the precursor of all GAs, thereby playing an important role in determining GA concentration in plants. Past work has demonstrated the importance of KAO activity for growth in various plant species. In Arabidopsis, this enzyme is encoded by two genes designated KAO1 and KAO2. In this study, we used various approaches to determine the physiological roles of KAO1 and KAO2 throughout plant development. Analysis of gene expression pattern reveals that both genes are mainly expressed in germinating seeds and young developing organs, thus suggesting functional redundancy. Consistent with this, kao1 and kao2 single mutants are indistinguishable from wild-type plants. By contrast, the kao1 kao2 double mutant exhibits typical non-germinating GA-dwarf phenotypes, similar to those observed in the severely GA-deficient ga1-3 mutant. Phenotypic characterization and quantitative analysis of endogenous GA contents of single and double kao mutants further confirm an overlapping role of KAO1 and KAO2 throughout Arabidopsis development.

The Arabidopsis DELLA RGA-LIKE3 is a direct target of MYC2 and modulates jasmonate signaling responses.

Gibberellins (GAs) are plant hormones involved in the regulation of plant growth in response to endogenous and environmental signals. GA promotes growth by stimulating the degradation of nuclear growth-repressing DELLA proteins. In Arabidopsis thaliana, DELLAs consist of a small family of five proteins that display distinct but also overlapping functions in repressing GA responses. This study reveals that DELLA RGA-LIKE3 (RGL3) protein is essential to fully enhance the jasmonate (JA)-mediated responses. We show that JA rapidly induces RGL3 expression in a CORONATINE INSENSITIVE1 (COI1)- and JASMONATE INSENSITIVE1 (JIN1/MYC2)-dependent manner. In addition, we demonstrate that MYC2 binds directly to RGL3 promoter. Furthermore, we show that RGL3 (like the other DELLAs) interacts with JA ZIM-domain (JAZ) proteins, key repressors of JA signaling. These findings suggest that JA/MYC2-dependent accumulation of RGL3 represses JAZ activity, which in turn enhances the expression of JA-responsive genes. Accordingly, we show that induction of primary JA-responsive genes is reduced in the rgl3-5 mutant and enhanced in transgenic lines overexpressing RGL3. Hence, RGL3 positively regulates JA-mediated resistance to the necrotroph Botrytis cinerea and susceptibility to the hemibiotroph Pseudomonas syringae. We propose that JA-mediated induction of RGL3 expression is of adaptive significance and might represent a recent functional diversification of the DELLAs.

DELLA proteins modulate Arabidopsis defences induced in response to caterpillar herbivory.

Upon insect herbivory, many plant species change the direction of metabolic flux from growth into defence. Two key pathways modulating these processes are the gibberellin (GA)/DELLA pathway and the jasmonate pathway. In this study, the effect of caterpillar herbivory on plant-induced responses was compared between wild-type Arabidopsis thaliana (L.) Heynh. and quad-della mutants that have constitutively elevated GA responses. The labial saliva (LS) of caterpillars of the beet armyworm, Spodoptera exigua, is known to influence induced plant defence responses. To determine the role of this herbivore cue in determining metabolic shifts, plants were subject to herbivory by caterpillars with intact or impaired LS secretions. In both wild-type and quad-della plants, a jasmonate burst is an early response to caterpillar herbivory. Negative growth regulator DELLA proteins are required for the LS-mediated suppression of hormone levels. Jasmonate-dependent marker genes are induced in response to herbivory independently of LS, with the exception of AtPDF1.2 that showed LS-dependent expression in the quad-della mutant. Early expression of the salicylic acid (SA)-marker gene, AtPR1, was not affected by herbivory which also reflected SA hormone levels; however, this gene showed LS-dependent expression in the quad-della mutant. DELLA proteins may positively regulate glucosinolate levels and suppress laccase-like multicopper oxidase activity in response to herbivory. The present results show a link between DELLA proteins and early, induced plant defences in response to insect herbivory; in particular, these proteins are necessary for caterpillar LS-associated attenuation of defence hormones.

DELLAs regulate chlorophyll and carotenoid biosynthesis to prevent photooxidative damage during seedling deetiolation in Arabidopsis.

In plants, light represents an important environmental signal that triggers the production of photosynthetically active chloroplasts. This developmental switch is critical for plant survival because chlorophyll precursors that accumulate in darkness can be extremely destructive when illuminated. Thus, plants have evolved mechanisms to adaptively control plastid development during the transition into light. Here, we report that the gibberellin (GA)-regulated DELLA proteins play a crucial role in the formation of functional chloroplasts during deetiolation. We show that Arabidopsis thaliana DELLAs accumulating in etiolated cotyledons derepress chlorophyll and carotenoid biosynthetic pathways in the dark by repressing the transcriptional activity of the phytochrome-interacting factor proteins. Accordingly, dark-grown GA-deficient ga1-3 mutants (that accumulate DELLAs) display a similar gene expression pattern to wild-type seedlings grown in the light. Consistent with this, ga1-3 seedlings accumulate higher amounts of protochlorophyllide (a phototoxic chlorophyll precursor) in darkness but, surprisingly, are substantially more resistant to photooxidative damage following transfer into light. This is due to the DELLA-dependent upregulation of the photoprotective enzyme protochlorophyllide oxidoreductase (POR) in the dark. Our results emphasize the role of DELLAs in regulating the levels of POR, protochlorophyllide, and carotenoids in the dark and in protecting etiolated seedlings against photooxidative damage during initial light exposure.

A quantitative gibberellin signaling biosensor reveals a role for gibberellins in internode specification at the shoot apical meristem.

Growth at the shoot apical meristem (SAM) is essential for shoot architecture construction. The phytohormones gibberellins (GA) play a pivotal role in coordinating plant growth, but their role in the SAM remains mostly unknown. Here, we developed a ratiometric GA signaling biosensor by engineering one of the DELLA proteins, to suppress its master regulatory function in GA transcriptional responses while preserving its degradation upon GA sensing. We demonstrate that this degradation-based biosensor accurately reports on cellular changes in GA levels and perception during development. We used this biosensor to map GA signaling activity in the SAM. We show that high GA signaling is found primarily in cells located between organ primordia that are the precursors of internodes. By gain- and loss-of-function approaches, we further demonstrate that GAs regulate cell division plane orientation to establish the typical cellular organization of internodes, thus contributing to internode specification in the SAM.

Gibberellin and abscisic acid transporters facilitate endodermal suberin formation in Arabidopsis.

The plant hormone gibberellin (GA) regulates multiple developmental processes. It accumulates in the root elongating endodermis, but how it moves into this cell file and the significance of this accumulation are unclear. Here we identify three NITRATE TRANSPORTER1/PEPTIDE TRANSPORTER (NPF) transporters required for GA and abscisic acid (ABA) translocation. We demonstrate that NPF2.14 is a subcellular GA/ABA transporter, presumably the first to be identified in plants, facilitating GA and ABA accumulation in the root endodermis to regulate suberization. Further, NPF2.12 and NPF2.13, closely related proteins, are plasma membrane-localized GA and ABA importers that facilitate shoot-to-root GA12 translocation, regulating endodermal hormone accumulation. This work reveals that GA is required for root suberization and that GA and ABA can act non-antagonistically. We demonstrate how the clade of transporters mediates hormone flow with cell-file-specific vacuolar storage at the phloem unloading zone, and slow release of hormone to induce suberin formation in the maturation zone.

Plant DELLAs restrain growth and promote survival of adversity by reducing the levels of reactive oxygen species.

Plant growth is adaptively modulated in response to environmental change. The phytohormone gibberellin (GA) promotes growth by stimulating destruction of the nuclear growth-repressing DELLA proteins [1-7], thus providing a mechanism for environmentally responsive growth regulation [8, 9]. Furthermore, DELLAs promote survival of adverse environments [8]. However, the relationship between these survival and growth-regulatory mechanisms was previously unknown. Here, we show that both mechanisms are dependent upon control of the accumulation of reactive oxygen species (ROS). ROS are small molecules generated during development and in response to stress that play diverse roles as eukaryotic intracellular second messengers [10]. We show that Arabidopsis DELLAs cause ROS levels to remain low after either biotic or abiotic stress, thus delaying cell death and promoting tolerance. In essence, stress-induced DELLA accumulation elevates the expression of genes encoding ROS-detoxification enzymes, thus reducing ROS levels. In accord with recent demonstrations that ROS control root cell expansion [11, 12], we also show that DELLAs regulate root-hair growth via a ROS-dependent mechanism. We therefore propose that environmental variability regulates DELLA activity [8] and that DELLAs in turn couple the downstream regulation of plant growth and stress tolerance through modulation of ROS levels.

DELLAs control plant immune responses by modulating the balance of jasmonic acid and salicylic acid signaling.

In Arabidopsis, the flagellin-derived peptide flg22 elevates antibacterial resistance [1] and inhibits growth [2] upon perception via the leucine-rich repeat receptor-like kinase Flagellin-Sensitive 2 (FLS2) [3]. DELLA proteins are plant growth repressors whose degradation is promoted by the phytohormone gibberellin [4]. Here, we show that DELLA stabilization contributes to flg22-induced growth inhibition. In addition, we show that DELLAs promote susceptibility to virulent biotrophs and resistance to necrotrophs, partly by altering the relative strength of salicylic acid and jasmonic acid (JA) signaling. A quadruple-DELLA mutant (which lacks four out of the five Arabidopsis DELLA proteins [5]) was partially insensitive to gene induction by Methyl-Jasmonate (MeJA), whereas the constitutively active dominant DELLA mutant gai[6] was sensitized for JA-responsive gene induction, implicating DELLAs in JA-signaling and/or perception. Accordingly, the elevated resistance of gai to the necrotrophic fungus Alternaria brassicicola and susceptibility to the hemibiotroph Pseudomonas syringae pv. tomato strain DC3000 (Pto DC3000) was attenuated in the JA-insensitive coi1-16 mutant [7]. These findings suggest an explanation for why the necrotrophic fungus Gibberella fujikuroi, causal agent of the foolish-seedling disease of rice, makes gibberellin.

Nitrate signaling promotes plant growth by upregulating gibberellin biosynthesis and destabilization of DELLA proteins.

Nitrate, one of the main nitrogen (N) sources for crops, acts as a nutrient and key signaling molecule coordinating gene expression, metabolism, and various growth processes throughout the plant life cycle. It is widely accepted that nitrate-triggered developmental programs cooperate with hormone synthesis and transport to finely adapt plant architecture to N availability. Here, we report that nitrate, acting through its signaling pathway, promotes growth in Arabidopsis and wheat, in part by modulating the accumulation of gibberellin (GA)-regulated DELLA growth repressors. We show that nitrate reduces the abundance of DELLAs by increasing GA contents through activation of GA metabolism gene expression. Consistently, the growth restraint conferred by nitrate deficiency is partially rescued in global-DELLA mutant that lacks all DELLAs. At the cellular level, we show that nitrate enhances both cell proliferation and elongation in a DELLA-dependent and -independent manner, respectively. Our findings establish a connection between nitrate and GA signaling pathways that allow plants to adapt their growth to nitrate availability.

Root-derived GA12 contributes to temperature-induced shoot growth in Arabidopsis.

Plants are able to sense a rise in temperature of several degrees, and appropriately adapt their metabolic and growth processes. To this end, plants produce various signalling molecules that act throughout the plant body. Here, we report that root-derived GA12, a precursor of the bioactive gibberellins, mediates thermo-responsive shoot growth in Arabidopsis. Our data suggest that root-to-shoot translocation of GA12 enables a flexible growth response to ambient temperature changes.

F-box proteins everywhere.

The ubiquitin proteasome system is a key regulator of many biological processes in all eukaryotes. This mechanism employs several types of enzymes, the most important of which are the ubiquitin E3 ligases that catalyse the attachment of polyubiquitin chains to target proteins for their subsequent degradation by the 26S proteasome. Among the E3 families, the SCF is the best understood; it consists of a multi-protein complex in which the F-box protein plays a crucial role by recruiting the target substrate. Strikingly, nearly 700 F-box proteins have been predicted in Arabidopsis, suggesting that plants have the capacity to assemble a multitude of SCF complexes, possibly controlling the stability of hundreds of substrates involved in a plethora of biological processes. Interestingly, viruses and even pathogenic bacteria have also found ways to hijack the plant SCF and to reprogram it for their own purposes.

Long-distance transport of phytohormones through the plant vascular system.

Phytohormones are a group of low abundance molecules that activate various metabolic and developmental processes in response to environmental and endogenous signals. Like animal hormones, plant hormones often have distinct source and target tissues, hence ensuring long-range communication at the whole-plant level. Plants rely on various hormone distribution mechanisms depending on the distance and the direction of the transport. Here, we highlight the recent findings on the long-distance movement of plant hormones within the vasculature, from the physiological role to the molecular mechanism of the transport.

A Pivotal Role of DELLAs in Regulating Multiple Hormone Signals.

Plant phenotypic plasticity is controlled by diverse hormone pathways, which integrate and convey information from multiple developmental and environmental signals. Moreover, in plants many processes such as growth, development, and defense are regulated in similar ways by multiple hormones. Among them, gibberellins (GAs) are phytohormones with pleiotropic actions, regulating various growth processes throughout the plant life cycle. Previous work has revealed extensive interplay between GAs and other hormones, but the molecular mechanism became apparent only recently. Molecular and physiological studies have demonstrated that DELLA proteins, considered as master negative regulators of GA signaling, integrate multiple hormone signaling pathways through physical interactions with transcription factors or regulatory proteins from different families. In this review, we summarize the latest progress in GA signaling and its direct crosstalk with the main phytohormone signaling, emphasizing the multifaceted role of DELLA proteins with key components of major hormone signaling pathways.

Long-distance transport of endogenous gibberellins in Arabidopsis.

Gibberellins (GAs) are phytohormones controlling major aspects of plant growth and development. Although previous studies suggested the existence of a transport of GAs in plants, the nature and properties associated with this transport were unknown. We recently showed through micrografting and biochemical approaches that the GA12 precursor is the chemical form of GA undergoing long-distance transport across plant organs in Arabidopsis. Endogenous GA12 moves through the plant vascular system from production sites to recipient tissues, in which GA12 can be converted to bioactive forms to support growth via the activation of GA-dependent processes. GAs are also essential to promote seed germination; hence GA biosynthesis mutants do not germinate without exogenous GA treatment. Our results suggest that endogenous GAs are not (or not sufficiently) transmitted to the offspring to successfully complete the germination under permissive conditions.

Tissue-Specific Regulation of Gibberellin Signaling Fine-Tunes Arabidopsis Iron-Deficiency Responses.

Iron is an essential element for most living organisms. Plants acquire iron from the rhizosphere and have evolved different biochemical and developmental responses to adapt to a low-iron environment. In Arabidopsis, FIT encodes a basic helix-loop-helix transcription factor that activates the expression of iron-uptake genes in root epidermis upon iron deficiency. Here, we report that the gibberellin (GA)-signaling DELLA repressors contribute substantially in the adaptive responses to iron-deficient conditions. When iron availability decreases, DELLAs accumulate in the root meristem, thereby restraining root growth, while being progressively excluded from epidermal cells in the root differentiation zone. Such DELLA exclusion from the site of iron acquisition relieves FIT from DELLA-dependent inhibition and therefore promotes iron uptake. Consistent with this mechanism, expression of a non-GA-degradable DELLA mutant protein in root epidermis interferes with iron acquisition. Hence, spatial distribution of DELLAs in roots is essential to fine-tune the adaptive responses to iron availability.