Increase in BrAO1 gene expression and aldehyde oxidase activity during clubroot development in Chinese cabbage (Brassica rapa L.)

Plasmodiophora brassicae affects host gene expression by secreting the transcription factor-type effector PbZFE1

The protist pathogen Plasmodiophora brassicae hijacks the metabolism and development of host cruciferous plants and induces clubroot formation, but little is known about its regulatory mechanisms. Previously, the Pnit2int2 sequence, a sequence around the second intron of the nitrilase gene (BrNIT2) involved in auxin biosynthesis in Brassica rapa ssp. pekinensis, was identified as a specific promoter activated during clubroot formation. In this study, we hypothesized that analysis of the transcriptional regulation of Pnit2int2 could reveal how P. brassicae affects the host gene regulatory system during clubroot development. By yeast one-hybrid screening, the pathogen zinc finger protein PbZFE1 was identified to specifically bind to Pnit2int2. Specific binding of PbZFE1 to Pnit2int2 was also confirmed by electrophoretic mobility shift assay. The binding site of PbZFE1 is essential for promoter activity of Pnit2int2 in clubbed roots of transgenic Arabidopsis thaliana (Pnit2int2-2::GUS), indicating that PbZFE1 is secreted from P. brassicae and functions within plant cells. Ectopic expression of PbZEF1 in A. thaliana delayed growth and flowering time, suggesting that PbZFE1 has significant impacts on host development and metabolic systems. Thus, P. brassicae appears to secrete PbZFE1 into host cells as a transcription factor-type effector during pathogenesis.

Multi-Omics Approaches to Improve Clubroot Resistance in Brassica with a Special Focus on Brassica oleracea L

Brassica oleracea is an agronomically important species of the Brassicaceae family, including several nutrient-rich vegetables grown and consumed across the continents. But its sustainability is heavily constrained by a range of destructive pathogens, among which, clubroot disease, caused by a biotrophic protist Plasmodiophora brassicae, has caused significant yield and economic losses worldwide, thereby threatening global food security. To counter the pathogen attack, it demands a better understanding of the complex phenomenon of Brassica-P. brassicae pathosystem at the physiological, biochemical, molecular, and cellular levels. In recent years, multiple omics technologies with high-throughput techniques have emerged as successful in elucidating the responses to biotic and abiotic stresses. In Brassica spp., omics technologies such as genomics, transcriptomics, ncRNAomics, proteomics, and metabolomics are well documented, allowing us to gain insights into the dynamic changes that transpired during host-pathogen interactions at a deeper level. So, it is critical that we must review the recent advances in omics approaches and discuss how the current knowledge in multi-omics technologies has been able to breed high-quality clubroot-resistant B. oleracea. This review highlights the recent advances made in utilizing various omics approaches to understand the host resistance mechanisms adopted by Brassica crops in response to the P. brassicae attack. Finally, we have discussed the bottlenecks and the way forward to overcome the persisting knowledge gaps in delivering solutions to breed clubroot-resistant Brassica crops in a holistic, targeted, and precise way.

Research progress of aldehyde oxidases in plants

Plant aldehyde oxidases (AOs) are multi-functional enzymes, and they could oxidize abscisic aldehyde into ABA (abscisic acid) or indole acetaldehyde into IAA (indoleacetic acid) as the last step, respectively. AOs can be divided into four groups based on their biochemical and physiological functions. In this review, we summarized the recent studies about AOs in plants including the motif information, biochemical, and physiological functions. Besides their role in phytohormones biosynthesis and stress response, AOs could also involve in reactive oxygen species homeostasis, aldehyde detoxification and stress tolerance.

SYNERGISTIC ON AUXIN AND CYTOKININ 1 positively regulates growth and attenuates soil pathogen resistance

Plants as non-mobile organisms constantly integrate varying environmental signals to flexibly adapt their growth and development. Local fluctuations in water and nutrient availability, sudden changes in temperature or other abiotic and biotic stresses can trigger changes in the growth of plant organs. Multiple mutually interconnected hormonal signaling cascades act as essential endogenous translators of these exogenous signals in the adaptive responses of plants. Although the molecular backbones of hormone transduction pathways have been identified, the mechanisms underlying their interactions are largely unknown. Here, using genome wide transcriptome profiling we identify an auxin and cytokinin cross-talk component; SYNERGISTIC ON AUXIN AND CYTOKININ 1 (SYAC1), whose expression in roots is strictly dependent on both of these hormonal pathways. We show that SYAC1 is a regulator of secretory pathway, whose enhanced activity interferes with deposition of cell wall components and can fine-tune organ growth and sensitivity to soil pathogens.

How Auxin and Cytokinin Phytohormones Modulate Root Microbe Interactions

A large range of microorganisms can associate with plants, resulting in neutral, friendly or hostile interactions. The ability of plants to recognize compatible and incompatible microorganisms and to limit or promote their colonization is therefore crucial for their survival. Elaborated communication networks determine the degree of association between the host plant and the invading microorganism. Central to these regulations of plant microbe interactions, phytohormones modulate microorganism plant associations and coordinate cellular and metabolic responses associated to the progression of microorganisms across different plant tissues. We review here hormonal regulations, focusing on auxin and cytokinin phytohormones, involved in the interactions between plant roots and soil microorganisms, including bacterial and fungi associations, either beneficial (symbiotic) or detrimental (pathogenic). The aim is to highlight similarities and differences in cytokinin/auxin functions amongst various compatible versus incompatible associations.

Heteroconium chaetospira induces resistance to clubroot via upregulation of host genes involved in jasmonic acid, ethylene, and auxin biosynthesis

An endophytic fungus, Heteroconium chaetospira isolate BC2HB1 (Hc), suppressed clubroot (Plasmodiophora brassicae -Pb) on canola in growth-cabinet trials. Confocal microscopy demonstrated that Hc penetrated canola roots and colonized cortical tissues. Based on qPCR analysis, the amount of Hc DNA found in canola roots at 14 days after treatment was negatively correlated (r = 0.92, P<0.001) with the severity of clubroot at 5 weeks after treatment at a low (2×10(5) spores pot(-1)) but not high (2×10(5) spores pot(-1)) dose of pathogen inoculum. Transcript levels of nine B. napus (Bn) genes in roots treated with Hc plus Pb, Pb alone and a nontreated control were analyzed using qPCR supplemented with biochemical analysis for the activity of phenylalanine ammonia lyases (PAL). These genes encode enzymes involved in several biosynthetic pathways related potentially to plant defence. Hc plus Pb increased the activity of PAL but not that of the other two genes (BnCCR and BnOPCL) involved also in phenylpropanoid biosynthesis, relative to Pb inoculation alone. In contrast, expression of several genes involved in the jasmonic acid (BnOPR2), ethylene (BnACO), auxin (BnAAO1), and PR-2 protein (BnPR-2) biosynthesis were upregulated by 63, 48, 3, and 3 fold, respectively, by Hc plus Pb over Pb alone. This indicates that these genes may be involved in inducing resistance in canola by Hc against clubroot. The upregulation of BnAAO1 appears to be related to both pathogenesis of clubroot and induced defence mechanisms in canola roots. This is the first report on regulation of specific host genes involved in induced plant resistance by a non-mycorrhizal endophyte.

The activity pattern and gene expression profile of aldehyde oxidase during the development of Pisum sativum seeds

Aldehyde oxidase (AO, EC 1.2.3.1) is a molybdenohydroxylase that is considered to catalyze the final step in the synthesis of abscisic acid (ABA) and possibly of indole-3-acetic acid (IAA). Five AO activity bands were detected after native PAGE with indole-3-aldehyde (PsAO-α, -β, -γ, -δ, -κ) and three with abscisic aldehyde (PsAO-γ, -δ, -κ) in developing seeds of Pisum sativum. At early and mid-development, PsAO-α, -β, -γ and only PsAO-γ were observed, respectively, and their localization as well as the expression of PsAOs genes was almost exclusively restricted to the maternal fruit tissues, the seed coat and pericarp. Towards the end of rapid reserve synthesis, two additional isoforms (PsAO-δ, -κ) appeared in cotyledons, coinciding with a high transcript level of PsAO2. At this developmental stage, the activity level of PsAO-γ, was still considerable in the testa, and was higher than at earlier stages in the embryonic axis, which correlated with the PsAO3 transcript level. In mature dry seeds, AO activity and the expression of PsAOs became restricted to the embryonic tissues. The possible involvement of AO isoforms in ABA or IAA synthesis during pea seed development as well as the contribution of particular PsAO genes to the formation of the dimeric pea AO isoforms is discussed.

Alternative transcription initiation of the nitrilase gene (BrNIT2) caused by infection with Plasmodiophora brassicae Woron. in Chinese cabbage (Brassica rapa L.)

In clubroot disease, gall formation is induced by infection with the obligate biotroph Plasmodiophora brassicae, and cell hypertrophy is dependent on increased auxin levels. The enzyme nitrilase is suggested to play an important role in auxin biosynthesis in plants. Here, we investigated the expression of nitrilase genes in clubroot disease in Chinese cabbage (Brassica rapa L.). We isolated four isogenes of nitrilase (BrNIT1, BrNIT2, BrNIT3, and BrNIT4) from Chinese cabbage. When a BrNIT2-specific probe was used for Northern blot hybridization, enhanced accumulation of a 1.4 kb mRNA and additional shorter transcripts (1.1 kb) were only detected in clubbed roots 25 days postinoculation (dpi) onward. The expression of BrNIT1 was not strongly affected by infection with P. brassicae. BrNIT3 expression was detected in the roots at 10 and 20 dpi, and the expression was less in clubbed roots than in healthy roots at 20 dpi. Analysis of the transcription initiation point of the BrNIT2 gene suggests that 1.1 kb transcripts were generated by alternative transcription initiation between the second intron and the third exon. The sequence from the second intron to half of the third exon (+415 to +1037, 623 bp) had promoter activity in Arabidopsis during clubroot formation. Therefore, our results suggest that transcriptional regulation of BrNIT2 might be involved in auxin overproduction during clubroot development.

Molecular cloning, characterization and expression analysis of three aldehyde oxidase genes from Pisum sativum L

Aldehyde oxidase (AO, EC 1.2.3.1) is a molybdenohydroxylase that is considered to catalyze the last step of abscisic acid (ABA) and indole-3-acetic acid (IAA) synthesis. Three cDNAs encoding aldehyde oxidase proteins in Pisum sativum (cv. Little Marvel) were obtained based on RT-PCR (reverse transcriptase-polymerase chain reaction) strategy. The cloned genes, designated as PsAO1, PsAO2 and PsAO3, are 4630, 4347, 4600 bp in length, respectively, and show high sequence identity to each other and to aldehyde oxidases from other plant species. The deduced PsAO1, PsAO2, and PsAO3 proteins are 1373, 1367, 1367 amino acids in length, respectively, and contain consensus sequences for two iron-sulfur centers, a FAD binding domain, and a molybdenum cofactor (Moco) binding domain. PsAO1 and PsAO2 were mainly expressed in leaves of seedlings and young leaves of adult plants, while the highest PsAO3 transcript level was observed in aging leaves and matured seeds. PsAO2 mRNA was not affected by salinity or ammonium treatment, whereas the transcript level of PsAO3 increased significantly under both stress conditions, with the most pronounced changes in aging leaves, fully expanded leaves and roots. The PsAO1 transcript level was enhanced only in the presence of ammonium in the nutrient medium, but not under salinity. Based on the molecular mass of the deduced proteins and on organ-specific gene expression, studied both under control and stress conditions, the contribution of each PsAO cDNA in the formation of the previously described three dimeric pea AO isoforms and the possible involvement of the PsAO3 in abscisic acid (ABA) synthesis is discussed.

Molecular cloning of Brassica rapa nitrilases and their expression during clubroot development

SUMMARY Three isoforms of nitrilase were cloned from turnip, Brassica rapa L., and their expression during clubroot development caused by Plasmodiophora brassicae was investigated. The isoforms were designated BrNIT-T1, BrNIT-T2 and BrNIT-T4 based on homology to known nitrilases. BrNIT-T1 and BrNIT-T2 have 80% homology to three nitrilases from Arabidopsis thaliana (AtNIT1, AtNIT2 and AtNIT3). BrNIT-T4 showed 90% homology to AtNIT4. To confirm their enzyme activity, the recombinant proteins were expressed in Escherichia coli. The recombinant BrNIT-T1 and BrNIT-T2 but not BrNIT-T4 converted indole-3-acetonitrile to indole-3-acetic acid, an endogenous plant auxin, although kinetic analysis showed that indole-3-acetonitrile is a poor substrate compared with various aliphatic and aromatic nitriles. By contrast, the recombinant BrNIT-T4 specifically converted beta-cyano-l-alanine to aspartic acid and asparagine and these findings agree with the idea that it is involved in the cyanide detoxification pathway. Real-time PCR analysis clearly showed that these isoforms were differentially expressed during clubroot development. BrNIT-T1 transcripts were very low in non-infected roots but were enhanced up to 100-fold in infected roots exhibiting club growth. By contrast, BrNIT-T2 transcripts remained at a very low level during clubroot formation. All these results clearly indicate the specific involvement of BrNIT-T1 in clubroot formation. The BrNIT-T4 transcripts were substantially reduced in the clubroot-growing phase, but thereafter they increased rapidly to a level found in non-infected roots as the clubroot growth reached a plateau. These findings suggest the specific involvement of BrNIT-T4 in clubroot maturation. In fully developed clubs, the BrNIT-T1 and BrNIT-T2 transcripts also increased. Free indole-3-acetic acid (IAA) content increased in the early and the latest phase of infected roots compared with non-infected roots, but decreased substantially at the middle phase. Thus, free IAA may play a role in the initiation and maturation of clubroot. Total IAA content was significantly higher in infected roots than in non-infected roots throughout clubroot development and IAA conjugation/conjugate hydrolysis system as well as BrNIT-Ts appear to be involved in clubroot development.