Nehela, Yasser


Publications (7)

Not Just a Cycle: Three <i>gab</i> Genes Enable the Non-Cyclic Flux Toward Succinate via GABA Shunt in ‘<i>Candidatus</i> Liberibacter asiaticus’–Infected Citrus

Citation
Nehela, Killiny (2022). Molecular Plant-Microbe Interactions® 35 (3)
Names
Ca. Liberibacter asiaticus
Subjects
Agronomy and Crop Science General Medicine Physiology
Abstract
Although the mitochondria retain all required enzymes for an intact tricarboxylic acid (TCA) cycle, plants might shift the cyclic flux from the TCA cycle to an alternative noncyclic pathway via γ-aminobutyric acid (GABA) shunt under specific physiological conditions. We hypothesize that several genes may ease this noncyclic flux and contribute to the citrus response to the phytopathogenic bacterium ‘Candidatus Liberibacter asiaticus’, the causal agent of Huanglongbing in citrus. To test this hypothesis, we used multiomics techniques (metabolomics, fluxomics, and transcriptomics) to investigate the potential roles of putative gab homologies from Valencia sweet orange (Citrus sinensis). Our findings showed that ‘Ca. L. asiaticus’ significantly increased the endogenous GABA and succinate content but decreased ketoglutarate in infected citrus plants. Citrus genome harbors three putative gab genes, including amino-acid permease (also known as GABA permease; CsgabP), GABA transaminase (CsgabT), and succinate-semialdehyde dehydrogenase (also known as GABA dehydrogenase; CsgabD). The transcript levels of CsgabP, CsgabT, and CsgabD were upregulated in citrus leaves upon the infection with ‘Ca. L. asiaticus’ and after the exogenous application of GABA or deuterium-labeled GABA isotope (GABA-D6). Moreover, our finding showed that exogenously applied GABA is quickly converted to succinate and fed into the TCA cycle. Likewise, the fluxomics study showed that GABA-D6 is rapidly metabolized to succinate-D4. Our work proved that GABA shunt and three predicated gab genes from citrus, support the upstream noncyclic flux toward succinate rather than an intact TCA cycle and contribute to citrus defense responses to ‘Ca. L. asiaticus’. [Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .

Diaphorina citri Genome Possesses a Complete Melatonin Biosynthesis Pathway Differentially Expressed under the Influence of the Phytopathogenic Bacterium, Candidatus Liberibacter asiaticus

Citation
Nehela, Killiny (2021). Insects 12 (4)
Names
Ca. Liberibacter asiaticus
Subjects
Insect Science
Abstract
Melatonin is synthesized from the amino acid L-tryptophan via the shikimic acid pathway and ubiquitously distributed in both prokaryotes and eukaryotes. Although most of melatonin biosynthesis genes were characterized in several plants and animal species including the insect model, Drosophila melanogaster, none of these enzymes have been identified from the Asian citrus psyllid, Diaphorina citri. We used comprehensive in silico analysis and gene expression techniques to identify the melatonin biosynthesis-related genes of D. citri and to evaluate the expression patterns of these genes within the adults of D. citri with gradient infection rates (0, 28, 34, 50, 58, and 70%) of the phytopathogenic bacterium Candidatus Liberibacter asiaticus and after the treatment with exogenous melatonin. We showed that the D. citri genome possesses six putative melatonin biosynthesis-related genes including two putative tryptophan 5-hydroxylase (DcT5H-1 and DcT5H-2), a putative aromatic amino acid decarboxylase (DcAADC), two putative arylalkylamine N-acetyltransferase (DcAANAT-1 and DcAANAT-2), and putative N-acetylserotonin O-methyltransferase (DcASMT). The infection with Ca. L. asiaticus decreased the transcript levels of all predicted genes in the adults of D. citri. Moreover, melatonin supplementation induced their expression levels in both healthy and Ca. L. asiaticus-infected psyllids. These findings confirm the association of these genes with the melatonin biosynthesis pathway.

‘Candidatus Liberibacter asiaticus’ and Its Vector, Diaphorina citri, Augment the Tricarboxylic Acid Cycle of Their Host via the γ-Aminobutyric Acid Shunt and Polyamines Pathway

Citation
Nehela, Killiny (2019). Molecular Plant-Microbe Interactions® 32 (4)
Names
Ca. Liberibacter asiaticus
Subjects
Agronomy and Crop Science General Medicine Physiology
Abstract
Huanglongbing (HLB), a destructive citrus disease, is associated with ‘Candidatus Liberibacter asiaticus’, which is transmitted by the Asian citrus psyllid Diaphorina citri. Both ‘Ca. L. asiaticus’ and its vector manipulate the host metabolism for their benefit, to meet their nutritional needs and neutralize the host defense responses. We used a targeted gas chromatography-mass spectrometry–based method to explore the connection between the tricarboxylic acid (TCA) cycle, γ-aminobutyric acid (GABA) shunt, and polyamines (PAs) pathways in citrus. ‘Ca. L. asiaticus’ and D. citri accelerated the conversion of α-ketoglutarate to glutamate, then to GABA, causing an accumulation of GABA in the cytosol. In silico analysis showed that the citrus genome possesses a putative GABA permease that connects the GABA shunt with the TCA cycle and supports the accumulation of succinate, fumarate, and citrate. Additionally, the PAs biosynthetic pathway might be connected directly to the TCA cycle, through the production of fumarate, or indirectly, via enhancement of GABA shunt. Taken together, we suggest that GABA shunt and PAs pathways are alternative pathways that contribute to the flux toward succinate rather than an intact TCA cycle in citrus. Both ‘Ca. L. asiaticus’ and its vector enhance these pathways. This study provides more insights into citrus responses to the HLB pathosystem and could be a further step toward clues for understanding the nutritional needs of ‘Ca. L. asiaticus’, which could help in culturing ‘Ca. L. asiaticus’.

Metabolomic Response to Huanglongbing: Role of Carboxylic Compounds in Citrus sinensis Response to ‘Candidatus Liberibacter asiaticus’ and Its Vector, Diaphorina citri

Citation
Killiny, Nehela (2017). Molecular Plant-Microbe Interactions® 30 (8)
Names
Ca. Liberibacter asiaticus
Subjects
Agronomy and Crop Science General Medicine Physiology
Abstract
Huanglongbing, a destructive disease of citrus, is caused by the fastidious bacterium ‘Candidatus Liberibacter asiaticus’ and transmitted by Asian citrus psyllid, Diaphorina citri. The impact of ‘Ca. L. asiaticus’ infection or D. citri infestation on Valencia sweet orange (Citrus sinensis) leaf metabolites was investigated using gas chromatography mass spectrometry, followed by gene expression analysis for 37 genes involved in jasmonic acid (JA), salicylic acid (SA), and proline-glutamine pathways. The total amino acid abundance increased after ‘Ca. L. asiaticus’ infection, while the total fatty acids increased dramatically after infestation with D. citri, compared with control plants. Seven amino acids (glycine, l-isoleucine, l-phenylalanine, l-proline, l-serine, l-threonine, and l-tryptophan) and five organic acids (benzoic acid, citric acid, fumaric acid, SA, and succinic acid) increased in ‘Ca. L. asiaticus’-infected plants. On the other hand, the abundance of trans-JA and its precursor α-linolenic increased in D. citri-infested plants. Surprisingly, the double attack of both D. citri infestation and ‘Ca. L. asiaticus’ infection moderated the metabolic changes in all chemical classes studied. In addition, the gene expression analysis supported these results. Based on these findings, we suggest that, although amino acids such as phenylalanine are involved in citrus defense against ‘Ca. L. asiaticus’ infection through the activation of an SA-mediated pathway, fatty acids, especially α-linolenic acid, are involved in defense against D. citri infestation via the induction of a JA-mediated pathway.

One Target, Two Mechanisms: The Impact of ‘Candidatus Liberibacter asiaticus’ and Its Vector, Diaphorina citri, on Citrus Leaf Pigments

Citation
Killiny, Nehela (2017). Molecular Plant-Microbe Interactions® 30 (7)
Names
Ca. Liberibacter asiaticus
Subjects
Agronomy and Crop Science General Medicine Physiology
Abstract
Huanglongbing (HLB) is currently the largest threat to global citrus production. We examined the effect of HLB pathogen ‘Candidatus Liberibacter asiaticus’ infection or infestation by its vector, Diaphorina citri, on ‘Valencia’ sweet orange leaf pigments using high-performance liquid chromatography, followed by gene expression analysis for 46 involved genes in carotenoid and chlorophyll biosynthesis pathways. Both ‘Ca. L. asiaticus’ and D. citri alter the total citrus leaf pigment balance with a greater impact by ‘Ca. L. asiaticus’. Although zeaxanthin was accumulated in ‘Ca. L. asiaticus’-infected leaves, chlorophyllide a was increased in D. citri-infested plants. Our findings support the idea that both ‘Ca. L. asiaticus’ and D. citri affect the citrus pigments and promote symptom development but using two different mechanisms. ‘Ca. L. asiaticus’ promotes chlorophyll degradation but accelerates the biosynthesis of carotenoid pigments, resulting in accumulation of abscisic acid and its precursor, zeaxanthin. Zeaxanthin also has a photoprotective role. By contrast, D. citri induced the degradation of most carotenoids and accelerated chlorophyll biosynthesis, leading to chlorophyllide a accumulation. Chlorophyllide a might have an antiherbivory role. Accordingly, we suggest that citrus plants try to defend themselves against ‘Ca. L. asiaticus’ or D. citri using multifaceted defense systems, based on the stressor type. These findings will help in better understanding the tritrophic interactions among plant, pathogen, and vector.