The Arabidopsis small G-protein AtRAN1 is a positive regulator in chitin-induced stomatal closure and disease resistance

Chitin, a fungal microbial-associated molecular pattern, triggers various defence responses in several plant systems. Although it induces stomatal closure, the molecular mechanisms of its interactions with guard cell signalling pathways are unclear. Based on screening of public microarray data obtained from the ATH1 Affymetrix and Arabidopsis eFP browser, we isolated a cDNA encoding a Ras-related nuclear protein 1 AtRAN1. AtRAN1 expression was enriched in guard cells in a manner consistent with involvement in the control of the stomatal movement. AtRAN1 mutation impaired chitin-induced stomatal closure and accumulation of reactive oxygen species and nitric oxide in guard cells. In addition, Atran1 mutant plants exhibited compromised chitin-enhanced plant resistance to both bacterial and fungal pathogens due to changes in defence-related genes. Furthermore, Atran1 mutant plants were hypersensitive to drought stress compared to Col-0 plants, and had lower levels of stress-responsive genes. These data demonstrate a previously uncharacterized signalling role for AtRAN1, mediating chitin-induced signalling.     

A pair of light signaling factors FHY3 and FAR1 regulates plant immunity by modulating chlorophyll biosynthesis

Light and chloroplast function is known to affect the plant immune response; however, the underlying mechanism remains elusive. We previously demonstrated that two light signaling factors, FAR‐RED ELONGATED HYPOCOTYL 3 (FHY3) and FAR‐RED IMPAIRED RESPONSE 1 (FAR1), regulate chlorophyll biosynthesis and seedling growth via controlling HEMB1 expression in Arabidopsis thaliana. In this study, we reveal that FHY3 and FAR1 are involved in modulating plant immunity. We showed that the fhy3 far1 double null mutant displayed high levels of reactive oxygen species and salicylic acid (SA) and increased resistance to Pseudomonas syringae pathogen infection. Microarray analysis revealed that a large proportion of pathogen‐related genes, particularly genes encoding nucleotide‐binding and leucine‐rich repeat domain resistant proteins, are highly induced in fhy3 far1. Genetic studies indicated that the defects of fhy3 far1 can be largely rescued by reducing SA signaling or blocking SA accumulation, and by overexpression of HEMB1, which encodes a 5‐aminolevulinic acid dehydratase in the chlorophyll biosynthetic pathway. Furthermore, we found that transgenic plants with reduced expression of HEMB1 exhibit a phenotype similar to fhy3 far1. Taken together, this study demonstrates an important role of FHY3 and FAR1 in regulating plant immunity, through integrating chlorophyll biosynthesis and the SA signaling pathway.     

Superoxide anion regulates plant growth and tuber development of potato

A higher concentration of H₂O₂ was detected in the sense transgenic potato plant (SS4) with the lily chCu,ZnSOD sequence, whereas higher levels of O₂⁻ was detected in the antisense transgenic plant (SA1) than the WT plant. The elongation growth in SA1 was significantly inhibited by treatment with diphenyleneiodonium, an inhibitor of O₂⁻ generation, and promoted in the SS4 on treatment with herbicide methyl viologen, a generator of apoplastic O₂⁻. Higher concentrations of GAs were detected during plant growth and the early stage of tuberization in SA1. Complete recovery of the above elongation growth and microtuberization pattern in transgenic plants following treatment of GA₃ or an inhibitor of gibberellin synthesis, paclobutrazol, indicate that these changes were mainly caused by active GA levels. In conclusion, a specific ROS (O₂⁻ ) acts as a signal transducer via GA biosynthetic pathways for the regulation of plant growth and tuber development of potato.     

Evolutionarilv Conserved DELLA-mediated Gibberellin Signaling in Plants

Gibberellins (GAs) play important roles in many essential plant growth and development processes. A family of nuclear growth-repressing DELLA proteins is the key component in GA signaling. GA perception is mediated by GID1, and the key event of GA signaling is the degradation of DELLA proteins via the 26S proteasome pathway. DELLA proteins integrating other plant hormones signaling and environmental cue modulating plant growth and development have been revealed. GA turning on the de-DELLA-repressing system is conserved, and independently establishes step-by-step recruitment of GAstimulated GID1-DELLA interaction and DELLA growth-repression functions during land plant evolution. These discoveries open new prospects for the understanding of GA action and DELLA-mediated signaling in plants.     

Evolutionarily conserved DELLA-mediated gibberellin signaling in plants

Gibberellins (GAs) play important roles in many essential plant growth and development processes. A family of nuclear growth-repressing DELLA proteins is the key component in GA signaling. GA perception is mediated by GID1, and the key event of GA signaling is the degradation of DELLA proteins via the 26S proteasome pathway. DELLA proteins integrating other plant hormones signaling and environmental cue modulating plant growth and development have been revealed. GA turning on the de-DELLA-repressing system is conserved, and independently establishes step-by-step recruitment of GA-stimulated GID1-DELLA interaction and DELLA growth-repression functions during land plant evolution. These discoveries open new prospects for the understanding of GA action and DELLA-mediated signaling in plants.     

Expression of a grape calcium-dependent protein kinase ACPK1 in Arabidopsis thaliana promotes plant growth and confers abscisic acid-hypersensitivity in germination, postgermination growth, and stomatal movement

Calcium is an important second messenger involved in abscisic acid (ABA) signal transduction. Calcium-dependent protein kinases (CDPKs) are the best characterized calcium sensor in plants and are believed to be important components in plant hormone signaling. However, in planta genetic evidence has been lacking to link CDPK with ABA-regulated biological functions. We previously identified an ABA-stimulated CDPK from grape berry, which is potentially involved in ABA signaling. Here we report that heterologous overexpression of ACPK1 in Arabidopsis promotes significantly plant growth and enhances ABA-sensitivity in seed germination, early seedling growth and stomatal movement, providing evidence that ACPK1 is involved in ABA signal transduction as a positive regulator, and suggesting that the ACPK1 gene may be potentially used for elevating plant biomass production. The authors Xiang-Chun Yu, Sai-Yong Zhu, and Gui-Feng Gao contributed equally to this work.     

An F‐box gene,CPR30, functions as a negative regulator of the defense response in Arabidopsis

Arabidopsis gain‐of‐resistance mutants, which show HR‐like lesion formation and SAR‐like constitutive defense responses, were used well as tools to unravel the plant defense mechanisms. We have identified a novel mutant, designated constitutive expresser of PR genes 30 (cpr30), that exhibited dwarf morphology, constitutive resistance to the bacterial pathogen Pseudomonas syringae and the dramatic induction of defense‐response gene expression. The cpr30‐conferred growth defect morphology and defense responses are dependent on ENHANCED DISEASE SUSCEPTIBILITY 1 (EDS1), PHYTOALEXIN DEFICIENT 4 (PAD4), and NONRACE‐SPECIFIC DISEASE RESISTANCE 1 (NDR1). Further studies demonstrated that salicylic acid (SA) could partially account for the cpr30‐conferred constitutive PR1 gene expression, but not for the growth defect, and that the cpr30‐conferred defense responses were NPR1 independent. We observed a widespread expression of CPR30 throughout the plant, and a localization of CPR30‐GFP fusion protein in the cytoplasm and nucleus. As an F‐box protein, CPR30 could interact with multiple Arabidopsis‐SKP1‐like (ASK) proteins in vivo. Co‐localization of CPR30 and ASK1 or ASK2 was observed in Arabidopsis protoplasts. Based on these results, we conclude that CPR30, a novel negative regulator, regulates both SA‐dependent and SA‐independent defense signaling, most likely through the ubiquitin‐proteasome pathway in Arabidopsis.     

OsDOG, a gibberellin-induced A20/AN1 zinc-finger protein, negatively regulates gibberellin-mediated cell elongation in rice

The A20/AN1 zinc-finger proteins (ZFPs) play pivotal roles in animal immune responses and plant stress responses. From previous gibberellin (GA) microarray data and A20/AN1 ZFP family member association, we chose Oryza sativa dwarf rice with overexpression of gibberellin-induced gene (OsDOG) to examine its function in the GA pathway. OsDOG was induced by gibberellic acid (GA₃) and repressed by the GA-synthesis inhibitor paclobutrazol. Different transgenic lines with constitutive expression of OsDOG showed dwarf phenotypes due to deficiency of cell elongation. Additional GA₁ and real-time PCR quantitative assay analyses confirmed that the decrease of GA₁ in the overexpression lines resulted from reduced expression of GA3ox2 and enhanced expression of GA2ox1 and GA2ox3. Adding exogenous GA rescued the constitutive expression phenotypes of the transgenic lines. OsDOG has a novel function in regulating GA homeostasis and in negative maintenance of plant cell elongation in rice.     

The Arabidopsis pi4kIIIβ1β2 double mutant is salicylic acid-overaccumulating: a new example of salicylic acid influence on plant stature

Growth is the best visible sign of plant comfort. If plants are under stress, a difference in growth with control conditions can indicate that something is going wrong (or better). Phytohormones such as auxin, cytokinins, brassinosteroids or giberellins, are important growth regulators and their role in plant growth was extensively studied. On the other hand the role of salicylic acid (SA), a phytohormone commonly connected with plant defense responses, in plant growth is under-rated. However, studies with SA-overaccumulating mutants directly showed an influence of SA on plant growth. Recently we characterized an Arabidopsis SA-overaccumulating mutant impaired in phosphatidylinositol-4-kinases (pi4kIIIβ1β2). This mutant is dwarf. The crossing with mutants impaired in SA signaling revealed that pi4kIIIβ1β2 stunted rosette is due to high SA, while the short root length is not. This brings into evidence that upper and lower parts of the plants, even though they may share common phenotypes, are differently regulated.     

The role of a class III gibberellin 2‐oxidase in tomato internode elongation

A network of environmental inputs and internal signaling controls plant growth, development and organ elongation. In particular, the growth‐promoting hormone gibberellin (GA) has been shown to play a significant role in organ elongation. The use of tomato as a model organism to study elongation presents an opportunity to study the genetic control of internode‐specific elongation in a eudicot species with a sympodial growth habit and substantial internodes that can and do respond to external stimuli. To investigate internode elongation, a mutant with an elongated hypocotyl and internodes but wild‐type petioles was identified through a forward genetic screen. In addition to stem‐specific elongation, this mutant, named tomato internode elongated ‐1 (tie‐1) is more sensitive to the GA biosynthetic inhibitor paclobutrazol and has altered levels of intermediate and bioactive GAs compared with wild‐type plants. The mutation responsible for the internode elongation phenotype was mapped to GA2oxidase 7, a class III GA 2‐oxidase in the GA biosynthetic pathway, through a bulked segregant analysis and bioinformatic pipeline, and confirmed by transgenic complementation. Furthermore, bacterially expressed recombinant TIE protein was shown to have bona fide GA 2‐oxidase activity. These results define a critical role for this gene in internode elongation and are significant because they further the understanding of the role of GA biosynthetic genes in organ‐specific elongation.     

Differential inducible defense mechanisms against bacterial speck pathogen in Arabidopsis thaliana by plant-growth-promoting-fungus Penicillium sp. GP16-2 and its cell free filtrate

Although a wealth of information is available regarding resistance induced by plant growth-promoting rhizobacteria (PGPR), not much is known about plant growth-promoting fungi (PGPF). Hence, the goal of the present research was to provide more information on this matter. In Arabidopsis thaliana L., root colonizing PGPF Penicillium sp. GP16-2 or its cell free filtrate (CF) elicited an induced systemic resistance (ISR) against infection by Pseudomonas syringae pv. tomato DC3000 (Pst), leading to a restriction of pathogen growth and disease development. We demonstrate that signal transduction leading to GP16-2-mediated ISR requires responsiveness to JA and ET in a NPR1-dependent manner, while CF-mediated ISR shows dispensability of SA, JA, ET and NPR1-dependent signaling (at least individually). In addition, root colonization by GP16-2 is not associated with a direct effect on expression of known defense-related genes, but potentiates the activation of JA/ET-inducible ChitB, which only becomes apparent after infection by Pst. However, CF-mediated ISR was partly associated with the direct activation of marker genes responsive to both SA and JA/ET signaling pathways and partly associated with priming, leading to activation of JA-/ET-inducible ChitB and Hel genes. These suggest that CF may contain one or more elicitors that induce resistance by way where at least SA, JA and ET may play a role in defense signaling in Arabidopsis. Therefore, defense gene changes and underlying signaling pathways induced by Penicillium sp. GP16-2 root colonization and its CF application are not the same and only partially overlap.