Roles of Arabidopsis WRKY3 and WRKY4 Transcription Factors in Plant Responses to Pathogens

<p>Abstract</p> <p>Background</p> <p>Plant WRKY DNA-binding transcription factors are involved in plant responses to biotic and abiotic responses. It has been previously shown that <it>Arabidopsis WRKY3 </it>and <it>WRKY4</it>, which encode two structurally similar WRKY transcription factors, are induced by pathogen infection and salicylic acid (SA). However, the role of the two WRKY transcription factors in plant disease resistance has not been directly analyzed.</p> <p>Results</p> <p>Both WRKY3 and WRKY4 are nuclear-localized and specifically recognize the TTGACC W-box sequences <it>in vitro</it>. Expression of <it>WRKY3 </it>and <it>WRKY4 </it>was induced rapidly by stress conditions generated by liquid infiltration or spraying. Stress-induced expression of <it>WRKY4 </it>was further elevated by pathogen infection and SA treatment. To determine directly their role in plant disease resistance, we have isolated T-DNA insertion mutants and generated transgenic overexpression lines for <it>WRKY3 </it>and <it>WRKY4</it>. Both the loss-of-function mutants and transgenic overexpression lines were examined for responses to the biotrophic bacterial pathogen <it>Pseudomonas syringae </it>and the necrotrophic fungal pathogen <it>Botrytis cinerea</it>. The <it>wrky3 </it>and <it>wrky4 </it>single and double mutants exhibited more severe disease symptoms and support higher fungal growth than wild-type plants after <it>Botrytis </it>infection. Although disruption of <it>WRKY3 </it>and <it>WRKY4 </it>did not have a major effect on plant response to <it>P. syringae</it>, overexpression of <it>WRKY4 </it>greatly enhanced plant susceptibility to the bacterial pathogen and suppressed pathogen-induced <it>PR1 </it>gene expression.</p> <p>Conclusion</p> <p>The nuclear localization and sequence-specific DNA-binding activity support that WRKY3 and WRKY4 function as transcription factors. Functional analysis based on T-DNA insertion mutants and transgenic overexpression lines indicates that WRKY3 and WRKY4 have a positive role in plant resistance to necrotrophic pathogens and WRKY4 has a negative effect on plant resistance to biotrophic pathogens.</p

Functional analysis of Arabidopsis WRKY25 transcription factor in plant defense against Pseudomonas syringae

BACKGROUND: A common feature of plant defense responses is the transcriptional regulation of a large number of genes upon pathogen infection or treatment with pathogen elicitors. A large body of evidence suggests that plant WRKY transcription factors are involved in plant defense including transcriptional regulation of plant host genes in response to pathogen infection. However, there is only limited information about the roles of specific WRKY DNA-binding transcription factors in plant defense. RESULTS: We analyzed the role of the WRKY25 transcription factor from Arabidopsis in plant defense against the bacterial pathogen Pseudomonas syringae. WRKY25 protein recognizes the TTGACC W-box sequences and its translational fusion with green fluorescent protein is localized to the nucleus. WRKY25 expression is responsive to general environmental stress. Analysis of stress-induced WRKY25 in the defense signaling mutants npr1, sid2, ein2 and coi1 further indicated that this gene is positively regulated by the salicylic acid (SA) signaling pathway and negatively regulated by the jasmonic acid signaling pathway. Two independent T-DNA insertion mutants for WRKY25 supported normal growth of a virulent strain of P. syringae but developed reduced disease symptoms after infection. By contrast, Arabidopsis constitutively overexpressing WRKY25 supported enhanced growth of P. syringae and displayed increased disease symptom severity as compared to wild-type plants. These WRKY25-overexpressing plants also displayed reduced expression of the SA-regulated PR1 gene after the pathogen infection, despite normal levels of free SA. CONCLUSION: The nuclear localization and sequence-specific DNA-binding activity support that WRKY25 functions as a transcription factor. Based on analysis of both T-DNA insertion mutants and transgenic overexpression lines, stress-induced WRKY25 functions as a negative regulator of SA-mediated defense responses to P. syringae. This proposed role is consistent with the recent finding that WRKY25 is a substrate of Arabidopsis MAP kinase 4, a repressor of SA-dependent defense responses

LIP5, a MVB biogenesis regulator, is required for rice growth

LYST-INTERACTING PROTEIN5 (LIP5) is a conserved regulator of multivesicular body (MVB) biogenesis in eukaryotes. In Arabidopsis, AtLIP5 is a target of stress-responsive MITOGEN-ACTIVATED PROTEIN KINASE3 and 6 and mediates stress-induced MVB biogenesis to promote stress responses. However, Arabidopsis atlip5 knockout mutants are normal in growth and development. Here we report that rice OsLIP5 gene could fully restore both the disease resistance and salt tolerance of the Arabidopsis oslip5 mutant plants to the wild-type levels. Unlike Arabidopsis atlip5 mutants, rice oslip5 mutants were severely stunted, developed necrotic lesions and all died before flowering. Unlike in Arabidopsis, LIP5 regulated endocytosis under both stress and normal conditions in rice. These findings indicate that there is strong evolutionary divergence among different plants in the role of the conserved LIP5-regulated MVB pathway in normal plant growth

Shared and Related Molecular Targets and Actions of Salicylic Acid in Plants and Humans

Salicylic acid (SA) is a phenolic compound produced by all plants that has an important role in diverse processes of plant growth and stress responses. SA is also the principal metabolite of aspirin and is responsible for many of the anti-inflammatory, cardioprotective and antitumor activities of aspirin. As a result, the number of identified SA targets in both plants and humans is large and continues to increase. These SA targets include catalases/peroxidases, metabolic enzymes, protein kinases and phosphatases, nucleosomal and ribosomal proteins and regulatory and signaling proteins, which mediate the diverse actions of SA in plants and humans. While some of these SA targets and actions are unique to plants or humans, many others are conserved or share striking similarities in the two types of organisms, which underlie a host of common biological processes that are regulated or impacted by SA. In this review, we compare shared and related SA targets and activities to highlight the common nature of actions by SA as a hormone in plants versus a therapeutic agent in humans. The cross examination of SA targets and activities can help identify new actions of SA and better explain their underlying mechanisms in plants and humans

Biosynthesis and Roles of Salicylic Acid in Balancing Stress Response and Growth in Plants

Salicylic acid (SA) is an important plant hormone with a critical role in plant defense against pathogen infection. Despite extensive research over the past 30 year or so, SA biosynthesis and its complex roles in plant defense are still not fully understood. Even though earlier biochemical studies suggested that plants synthesize SA from cinnamate produced by phenylalanine ammonia lyase (PAL), genetic analysis has indicated that in Arabidopsis, the bulk of SA is synthesized from isochorismate (IC) produced by IC synthase (ICS). Recent studies have further established the enzymes responsible for the conversion of IC to SA in Arabidopsis. However, it remains unclear whether other plants also rely on the ICS pathway for SA biosynthesis. SA induces defense genes against biotrophic pathogens, but represses genes involved in growth for balancing defense and growth to a great extent through crosstalk with the growth-promoting plant hormone auxin. Important progress has been made recently in understanding how SA attenuates plant growth by regulating the biosynthesis, transport, and signaling of auxin. In this review, we summarize recent progress in the biosynthesis and the broad roles of SA in regulating plant growth during defense responses. Further understanding of SA production and its regulation of both defense and growth will be critical for developing better knowledge to improve the disease resistance and fitness of crops

Regulation and Function of Defense-Related Callose Deposition in Plants

Plants are constantly exposed to a wide range of potential pathogens and to protect themselves, have developed a variety of chemical and physical defense mechanisms. Callose is a β-(1,3)-D-glucan that is widely distributed in higher plants. In addition to its role in normal growth and development, callose plays an important role in plant defense. Callose is deposited between the plasma membrane and the cell wall at the site of pathogen attack, at the plasmodesmata, and on other plant tissues to slow pathogen invasion and spread. Since it was first reported more than a century ago, defense-related callose deposition has been extensively studied in a wide-spectrum of plant-pathogen systems. Over the past 20 years or so, a large number of studies have been published that address the dynamic nature of pathogen-induced callose deposition, the complex regulation of synthesis and transport of defense-related callose and associated callose synthases, and its important roles in plant defense responses. In this review, we summarize our current understanding of the regulation and function of defense-related callose deposition in plants and discuss both the progresses and future challenges in addressing this complex defense mechanism as a critical component of a plant immune system

Roles of Arabidopsis Cyclin-Dependent Kinase C Complexes in Cauliflower Mosaic Virus Infection, Plant Growth, and Development[W]

The C-terminal domain (CTD) of RNA polymerase II is phosphorylated during the transcription cycle by three cyclin-dependent kinases (CDKs): CDK7, CDK8, and CDK9. CDK9 and its interacting cyclin T partners belong to the positive transcription elongation factor b (P-TEFb) complexes, which phosphorylate the CTD to promote transcription elongation. We report that Arabidopsis thaliana CDK9-like proteins, CDKC;1 and CDKC;2, and their interacting cyclin T partners, CYCT1;4 and CYCT1;5, play important roles in infection with Cauliflower mosaic virus (CaMV). cdkc;2 and cyct1;5 knockout mutants are highly resistant and cdkc;2 cyct1;5 double mutants are extremely resistant to CaMV. The mutants respond normally to other types of plant viruses that do not replicate by reverse transcription. Expression of a reporter gene driven by the CaMV 35S promoter is markedly reduced in the cdkc;2 and cyct1;5 mutants, indicating that the kinase complexes are important for transcription from the viral promoter. Loss of function of CDKC;1/CDKC;2 or CYCT1;4/CYCT1;5 results in complete resistance to CaMV as well as altered leaf and flower growth, trichome development, and delayed flowering. These results establish Arabidopsis CDKC kinase complexes as important host targets of CaMV for transcriptional activation of viral genes and critical regulators of plant growth and development