The function of plant PR1 and other members of the CAP protein superfamily in plant–pathogen interactions

The pathogenesis-related (PR) proteins of plants have originally been identified as proteins that are strongly induced upon biotic and abiotic stress. These proteins fall into 17 distinct classes (PR1–PR17). The mode of action of most of these PR proteins has been well characterized, except for PR1, which belongs to a widespread superfamily of proteins that share a common CAP domain. Proteins of this family are not only expressed in plants but also in humans and in many different pathogens, including phytopathogenic nematodes and fungi. These proteins are associated with a diverse range of physiological functions. However, their precise mode of action has remained elusive. The importance of these proteins in immune defence is illustrated by the fact that PR1 overexpression in plants results in increased resistance against pathogens. However, PR1-like CAP proteins are also produced by pathogens and deletion of these genes results in reduced virulence, suggesting that CAP proteins can exert both defensive and offensive functions. Recent progress has revealed that plant PR1 is proteolytically cleaved to release a C-terminal CAPE1 peptide, which is sufficient to activate an immune response. The release of this signalling peptide is blocked by pathogenic effectors to evade immune defence. Moreover, plant PR1 forms complexes with other PR family members, including PR5, also known as thaumatin, and PR14, a lipid transfer protein, to enhance the host's immune response. Here, we discuss possible functions of PR1 proteins and their interactors, particularly in light of the fact that these proteins can bind lipids, which have important immune signalling functions.     

The Arabidopsis AtNPR1 inversely modulates defense responses against fungal, bacterial, or viral pathogens while conferring hypersensitivity to abiotic stresses in transgenic rice

The nonexpressor of pathogenesis-related (PR) genes (NPR1) protein plays an important role in mediating defense responses activated by pathogens in Arabidopsis. In rice, a disease-resistance pathway similar to the Arabidopsis NPR1-mediated signaling pathway one has been described. Here, we show that constitutive expression of the Arabidopsis NPR1 (AtNPR1) gene in rice confers resistance against fungal and bacterial pathogens. AtNPR1 exerts its protective effects against fungal pathogens by priming the expression of salicylic acid (SA)-responsive endogenous genes, such as the PR1b, TLP (PR5), PR10, and PBZ1. However, expression of AtNPR1 in rice has negative effects on viral infections. The AtNPR1-expressing rice plants showed a higher susceptibility to infection by the Rice yellow mottle virus (RYMV) which correlated well with a misregulation of RYMV-responsive genes, including expression of the SA-regulated RNA-dependent RNA polymerase 1 gene (OsRDR1). Moreover, AtNPR1 negatively regulates the expression of genes playing a role in the plant response to salt and drought stress (rab21, salT, and dip1), which results in a higher sensitivity of AtNPR1 rice to the two types of abiotic stress. These observations suggest that AtNPR1 has both positive and negative regulatory roles in mediating defense responses against biotic and abiotic stresses.     

转录因子WRKY6和PR1在拟南芥胁迫记忆中的表达模式

植物暴露在细菌或其它微生物病原体下,会形成全身防御,称为系统获得性抗性SAR（Systemic Acquired Resistance）,该系统可以在病原体二次侵染时有效抑制病原体对植物的伤害。其中,WRKY转录因子和病程相关蛋白PRs（Pathogenesis-related proteins）在植物抗病信号调控途径中起着重要作用。本研究以模式植物拟南芥为实验材料,对WRKY6和PR1（PATHOGENESIS RELATED）两个转录因子进行初步研究。首先,从拟南芥eFP数据库中获得WRKY6和PR1的基因表达数据,进行生物信息学分析,获得WRKY6和PR1基因在不同胁迫条件下的表达热图。其次,通过实时荧光定量PCR技术,比较了经过生物胁迫和非生物胁迫处理后WRKY6和PR1的基因表达水平。结果表明,拟南芥经过生物胁迫丁香假单胞菌[Pseudomonas syringae pv.tomato（P_st） DC3000]处理后,WRKY6和PR1的基因表达模式具有一定的相似性,然而经过非生物胁迫和机械损伤组合处理后,WRKY6和PR1基因又呈现出不同的表达模式。本研究初步探索了WRKY6和PR1基因的表达模式及其关系,为今后进一步研究系统性获得抗性应答机制提供了思路。     

Molecular and functional profiling of Arabidopsis pathogenesis-related genes: insights into their roles in salt response of seed germination

Pathogenesis-related (PR) proteins are a group of heterogeneous proteins encoded by genes that are rapidly induced by pathogenic infections and by salicylic acid (SA), jasmonic acid (JA) and ethylene (ET). They are widely used as molecular markers for resistance response to pathogens and systemic acquired response (SAR). However, recent studies have shown that the PR genes are also regulated by environmental factors, including light and abiotic stresses, and by developmental cues, suggesting that they also play a role in certain stress responses and developmental processes. In this work, we systematically examined the expression patterns of Arabidopsis PR genes. We also investigated the effects of environmental stresses and growth hormones on the expression of PR genes. We found that individual PR genes are temporally and spatially regulated in distinct patterns. In addition, they are differentially regulated by plant growth hormones, including SA, ABA, JA, ET and brassinosteroid (BR), and by diverse abiotic stresses, supporting the contention that the PR proteins play a role in plant developmental processes other than disease resistance response. Interestingly, PR-3 was induced significantly by high salt in an ABA-dependent manner. Consistent with this, a T-DNA insertional knockout plant with disruption of the PR-3 gene showed a significantly reduced rate of seed germination in the presence of high salt. It is thus proposed that PR-3 mediates ABA-dependent salt stress signals that affect seed germination in Arabidopsis. PR-4 and PR-5 also contributed to salt regulation of seed germination, although their effects were not as evident as those of PR-3.     

Proteomic identification of differentially expressed proteins in Arabidopsis in response to methyl jasmonate

Jasmonates (JAs) are the well characterized fatty acid-derived cyclopentanone signals involved in the plant response to biotic and abiotic stresses. JAs have been shown to regulate many aspects of plant metabolism, including glucosinolate biosynthesis. Glucosinolates are natural plant products that function in defense against herbivores and pathogens. In this study, we applied a proteomic approach to gain insight into the physiological processes, including glucosinolate metabolism, in response to methyl jasmonate (MeJA). We identified 194 differentially expressed protein spots that contained proteins that participated in a wide range of physiological processes. Functional classification analysis showed that photosynthesis and carbohydrate anabolism were repressed after MeJA treatment, while carbohydrate catabolism was up-regulated. Additionally, proteins related to the JA biosynthesis pathway, stress and defense, and secondary metabolism were up-regulated. Among the differentially expressed proteins, many were involved in oxidative tolerance. The results indicate that MeJA elicited a defense response at the proteome level through a mechanism of redirecting growth-related metabolism to defense-related metabolism.     

In vivo role of nitric oxide in plant response to abiotic and biotic stress

Over the past few years, nitric oxide (NO) has emerged as an important regulator in many physiological events, especially in response to abiotic and biotic stress. However, the roles of NO were mostly derived from pharmacological studies or the mutants impaired NO synthesis unspecifically. In our recent study, we highlighted a novel strategy by expressing the rat neuronal NO synthase (nNOS) in Arabidopsis to explore the in vivo role of NO. Our results suggested that plants were able to perform well in the constitutive presence of nNOS, and provided a new class of plant experimental system with specific in vivo NO release. Furthermore, our findings also confirmed that the in vivo NO is essential for most of environmental abiotic stresses and disease resistance against pathogen infection. Proper level of NO may be necessary and beneficial, not only in plant response to the environmental abiotic stress, but also to biotic stress.     

植物中逆境反应相关的WRKY转录因子研究进展

WRKY转录因子是植物体内一类比较大的转录因子家族,它在植物的生长发育以及抗逆境反应中起着非常重要的作用。本文综述了WRKY转录因子在植物应对冻害、干旱、盐害等非生物胁迫与病原菌、虫害等生物胁迫反应中的重要调控功能,并概括了WRKY转录因子在调控这些逆境反应中的机制。     

Pathogenesis-related protein-4 (PR-4) gene family in Qingke (Hordeum vulgare L. var. nudum): genome-wide identification,structural analysis and expression profile under stresses

Molecular Biology Reports - Pathogenesis-related (PR) proteins are active participants of plant defense against biotic and abiotic stresses. The PR-4 family features a Barwin domain at the...     

The pepper extracellular peroxidase CaPO2 is required for salt, drought and oxidative stress tolerance as well as resistance to fungal pathogens

In plants, biotic and abiotic stresses regulate the expression and activity of various peroxidase isoforms. Capsicum annuum EXTRACELLULAR PEROXIDASE 2 (CaPO2) was previously shown to play a role in local and systemic reactive oxygen species bursts and disease resistance during bacterial pathogen infection. Here, we report CaPO2 expression patterns and functions during conditions of biotic and abiotic stress. In pepper plants, CaPO2 expression was strongly induced by abscisic acid, but not by defense-related plant hormones such as salicylic acid, ethylene and jasmonic acid. CaPO2 was also strongly induced by abiotic and biotic stress treatments, including drought, cold, high salinity and infection by the hemibiotrophic fungal pathogen Colletotrichum coccodes. Loss-of-function of CaPO2 in virus-induced gene silenced pepper plants led to increased susceptibility to salt- and osmotic-induced stress. In contrast, CaPO2 overexpression in transgenic Arabidopsis thaliana plants conferred enhanced tolerance to high salt, drought, and oxidative stress, while also enhancing resistance to infection by the necrotrophic fungal pathogen Alternaria brassicicola. Taken together, these results provide evidence for the involvement of pepper extracellular peroxidase CaPO2 in plant defense responses to various abiotic stresses and plant fungal pathogens.     

Priming for stress resistance: from the lab to the field

Upon treatment with necrotizing pathogens, many plants develop an enhanced capacity for activating defense responses to biotic and abiotic stress--a process called priming. The primed state can also be induced by colonization of plant roots with beneficial micro-organisms or by treatment of plants with various natural and synthetic compounds. Priming is thought to be the mechanism by which plants can show induced resistance against ostensibly virulent pathogens after a conditioning treatment. Although the phenomenon has been known for years, it has been appreciated just recently that priming for enhanced defense responses can result from plant-plant communication in nature and that priming can also boost the resistance of crops to biotic and abiotic stresses in the field.     

Molecular defense responses in roots and the rhizosphere against Fusarium oxysporum

Plants face many different concurrent and consecutive abiotic and biotic stresses during their lifetime. Roots can be infected by numerous pathogens and parasitic organisms. Unlike foliar pathogens, root pathogens have not been explored enough to fully understand root-pathogen interactions and the underlying mechanism of defense and resistance. PR gene expression, structural responses, secondary metabolite and root exudate production, as well as the recruitment of plant defense–assisting “soldier” rhizosphere microbes all assist in root defense against pathogens and herbivores. With new high-throughput molecular tools becoming available and more affordable, now is the opportune time to take a deep look below the ground. In this addendum, we focus on soil-borne Fusarium oxysporum as a pathogen and the options plants have to defend themselves against these hard-to-control pathogens.     

Modulation of nitric oxide bioactivity by plant haemoglobins

Nitric oxide (NO) is a highly reactive signalling molecule that has numerous targets in plants. Both enzymatic and non-enzymatic synthesis of NO has been detected in several plant species, and NO functions have been characterized during diverse physiological processes such as plant growth, development, and resistance to biotic and abiotic stresses. This wide variety of effects reflects the basic signalling mechanisms that are utilized by virtually all mammalian and plant cells and suggests the necessity of detoxification mechanisms to control the level and functions of NO. During the last two years an increasing number of reports have implicated non-symbiotic haemoglobins as the key enzymatic system for NO scavenging in plants, indicating that the primordial function of haemoglobins may well be to protect against nitrosative stress and to modulate NO signalling functions. The biological relevance of plant haemoglobins during specific conditions of plant growth and stress, and the existence of further enzymatic and non-enzymatic NO scavenging systems, suggest the existence of precise NO modulation mechanisms in plants, as observed for different NO sources.     

Methylglyoxal detoxification by a DJ-1 family protein provides dual abiotic and biotic stress tolerance in transgenic plants

Methylglyoxal (MG) is a key signaling molecule resulting from glycolysis and other metabolic pathways. During abiotic stress, MG levels accumulate to toxic levels in affected cells. However, MG is routinely detoxified through the action of DJ1/PARK7/Hsp31 proteins that are highly conserved across kingdoms and mutations in such genes are associated with neurodegenerative diseases. Here, we report for the first time that, similar to abiotic stresses, MG levels increase during biotic stresses in plants, likely contributing to enhanced susceptibility to a wide range of stresses. We show that overexpression of yeast Heat shock protein 31 (Hsp31), a DJ-1 homolog with robust MG detoxifying capabilities, confers dual biotic and abiotic stress tolerance in model plant Nicotiana tabacum. Strikingly, overexpression of Hsp31 in tobacco imparts robust stress tolerance against diverse biotic stress inducers such as viruses, bacteria and fungi, in addition to tolerance against a range of abiotic stress inducers. During stress, Hsp31 was targeted to mitochondria and induced expression of key stress-related genes. These results indicate that Hsp31 is a novel attractive tool to engineer plants against both biotic and abiotic stresses.     

Plant–rhizobacteria interactions alleviate abiotic stress conditions

Root-colonizing non-pathogenic bacteria can increase plant resistance to biotic and abiotic stress factors. Bacterial inoculates have been applied as biofertilizers and can increase the effectiveness of phytoremediation. Inoculating plants with non-pathogenic bacteria can provide 'bioprotection' against biotic stresses, and some root-colonizing bacteria increase tolerance against abiotic stresses such as drought, salinity and metal toxicity. Systematic identification of bacterial strains providing cross-protection against multiple stressors would be highly valuable for agricultural production in changing environmental conditions. For bacterial cross-protection to be an effective tool, a better understanding of the underlying morphological, physiological and molecular mechanisms of bacterially mediated stress tolerance, and the phenomenon of cross-protection is critical. Beneficial bacteria-mediated plant gene expression studies under non-stress conditions or during pathogenic rhizobacteria–plant interactions are plentiful, but only few molecular studies on beneficial interactions under abiotic stress situations have been reported. Thus, here we attempt an overview of current knowledge on physiological impacts and modes of action of bacterial mitigation of abiotic stress symptoms in plants. Where available, molecular data will be provided to support physiological or morphological observations. We indicate further research avenues to enable better use of cross-protection capacities of root-colonizing non-pathogenic bacteria in agricultural production systems affected by a changing climate.     

Kinase-dead mutation: A novel strategy for improving soybean resistance to soybean cyst nematode Heterodera glycines

Protein kinases phosphorylate proteins for functional changes and are involved in nearly all cellular processes, thereby regulating almost all aspects of plant growth and development, and responses to biotic and abiotic stresses. We generated two independent co-expression networks of soybean genes using control and stress response gene expression data and identified 392 differentially highly interconnected kinase hub genes among the two networks. Of these 392 kinases, 90 genes were identified as “syncytium highly connected hubs”, potentially essential for activating kinase signalling pathways in the nematode feeding site. Overexpression of wild-type coding sequences of five syncytium highly connected kinase hub genes using transgenic soybean hairy roots enhanced plant susceptibility to soybean cyst nematode (SCN; Heterodera glycines) Hg Type 0 (race 3). In contrast, overexpression of kinase-dead variants of these five syncytium kinase hub genes significantly enhanced soybean resistance to SCN. Additionally, three of the five tested kinase hub genes enhanced soybean resistance to SCN Hg Type 1.2.5.7 (race 2), highlighting the potential of the kinase-dead approach to generate effective and durable resistance against a wide range of SCN Hg types. Subcellular localization analysis revealed that kinase-dead mutations do not alter protein cellular localization, confirming the structure–function of the kinase-inactive variants in producing loss-of-function phenotypes causing significant decrease in nematode susceptibility. Because many protein kinases are highly conserved and are involved in plant responses to various biotic and abiotic stresses, our approach of identifying kinase hub genes and their inactivation using kinase-dead mutation could be translated for biotic and abiotic stress tolerance.     

Identification and characterization of stress resistance related genes of Brassica rapa

Two biotic stress resistance related genes from the full-length cDNA library of Brassica rapa cv. Osome were identified from EST analysis and determined to be pathogenesis-related (PR) 12 Brassica defensin-like family protein (BrDLFP) and PR-10 Brassica Betv1 allergen family protein (BrBetv1AFP) after sequence analysis and homology study with other stress resistance related same family genes. In the expression analysis, both genes expressed in different organs and during all developmental growth stages in healthy plants. Expression of BrDLFP significantly increased and BrBetv1AFP gradually decreased after infection with Pectobacterium carotovorum subsp. carotovorum in Chinese cabbage. Expression of these two genes significantly changed after cold, salt, drought and ABA stress treatments. These two PR genes may therefore be involved in the plant resistance against biotic and abiotic stresses.     

Cytokinin action in response to abiotic and biotic stresses in plants

The phytohormone cytokinin was originally discovered as a regulator of cell division. Later, it was described to be involved in regulating numerous processes in plant growth and development including meristem activity, tissue patterning, and organ size. More recently, diverse functions for cytokinin in the response to abiotic and biotic stresses have been reported. Cytokinin is required for the defence against high light stress and to protect plants from a novel type of abiotic stress caused by an altered photoperiod. Additionally, cytokinin has a role in the response to temperature, drought, osmotic, salt, and nutrient stress. Similarly, the full response to certain plant pathogens and herbivores requires a functional cytokinin signalling pathway. Conversely, different types of stress impact cytokinin homeostasis. The diverse functions of cytokinin in responses to stress and crosstalk with other hormones are described. Its emerging roles as a priming agent and as a regulator of growth‐defence trade‐offs are discussed.