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"Burns, Jacqueline"
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Gene expression in Citrus sinensis fruit tissues harvested from huanglongbing-infected trees: comparison with girdled fruit
Distribution of viable Candidatus Liberibacter asiaticus (CaLas) in sweet orange fruit and leaves ('Hamlin' and 'Valencia') and transcriptomic changes associated with huanglongbing (HLB) infection in fruit tissues are reported. Viable CaLas was present in most fruit tissues tested in HLB trees, with the highest titre detected in vascular tissue near the calyx abscission zone. Transcriptomic changes associated with HLB infection were analysed in flavedo (FF), vascular tissue (VT), and juice vesicles (JV) from symptomatic (SY), asymptomatic (AS), and healthy (H) fruit. In SY 'Hamlin', HLB altered the expression of more genes in FF and VT than in JV, whereas in SY 'Valencia', the number of genes whose expression was changed by HLB was similar in these tissues. The expression of more genes was altered in SY 'Valencia' JV than in SY 'Hamlin' JV. More genes were also affected in AS 'Valencia' FF and VT than in AS 'Valencia' JV. Most genes whose expression was changed by HLB were classified as transporters or involved in carbohydrate metabolism. Physiological characteristics of HLB-infected and girdled fruit were compared to differentiate between HLB-specific and carbohydrate metabolism-related symptoms. SY and girdled fruit were smaller than H and ungirdled fruit, respectively, with poor juice quality. However, girdling did not cause misshapen fruit or differential peel coloration. Quantitative PCR analysis indicated that many selected genes changed their expression significantly in SY flavedo but not in girdled flavedo. Mechanisms regulating development of HLB symptoms may lie in the host disease response rather than being a direct consequence of carbohydrate starvation.
Journal Article
Phytohormone Changes and Carbohydrate Status in Sweet Orange Fruit from Huanglongbing-infected Trees
Huanglongbing (HLB) infection alters citrus fruit growth and development, resulting in small, misshapen, and poorly colored fruit containing aborted or partially developed seeds. Typically, symptomatic fruit have delayed maturation and abscise prematurely. We studied carbohydrate and phytohormone changes in HLB-affected fruit to explain symptom development because (1) carbohydrate shortage has been linked to fruit growth arrest and eventually abscission and (2) hormonal signals regulate, at least partially, fruit set and development. Symptomatic fruit (S), asymptomatic fruit (AS) from symptomatic trees, and healthy fruit (H) from asymptomatic trees were harvested from ‘Valencia’ sweet orange trees [Citrus sinensis (L.) Osbeck] infected with the HLB pathogen or not, as verified by PCR. Mature S weighed less, had lower °Brix, were smaller, had more aborted seeds, and were greener than AS or H. Starch and sucrose contents were lower in mature S flavedo compared with that of H and AS. S and AS harvested 7 and 12 months after full bloom produced significantly less ethylene than H. Indole-3-acetic acid (IAA) and abscisic acid (ABA) contents in flavedo removed from the stylar end, middle section, or stem end of fruit generally were higher in S flavedo than in AS and H. ABA content was fourfold higher in flavedo from the middle section of S than in AS and H. Flavedo excised from the large shoulder of misshapen S had significantly higher IAA content when compared with the normal-sized area of the same fruit on the opposite side. This increase corresponded to an increase in hypodermal cell area in S flavedo. Overall, these data reveal an imbalance of carbohydrate and phytohormone status in fruit from HLB-infected trees and suggest a role of such changes in fruit symptom development.
Journal Article
Profiling ethylene-regulated gene expression in Arabidopsis thaliana by microarray analysis
Ethylene-regulated gene expression in leaves of Arabidopsis thaliana was investigated with an expressed sequence tag-based microarray containing about 6000 unique genes. Comparing expression profiles of the ethylene-insensitive mutant etr1-1, the ethylene-constitutive mutant ctr1-1, ethylene-treated wild-type and untreated wild-type plants identified ca. 7% of the investigated genes as ethylene-regulated. Exogenous ethylene treatment and ctr1-1 had similar changes in gene expression, but differences were noted. Ethylene-regulated genes involved in its own biosynthesis and signal transduction pathway were identified. A large number of transcription factors and some putative signaling components were highly regulated by ethylene. Chloroplast structural protein and photosynthetic genes were generally down-regulated. Ethylene appeared to regulate other primary metabolic genes. Plant defense and PR protein genes were differentially regulated, with some genes within this class highly up-regulated. Other ethylene-regulated genes identified were known sugar-, auxin-, wounding- and jasmonic acid-related genes, suggesting the existence of coordinated interactions between ethylene and other hormonal and defense signaling pathways. Although hundreds of potentially important transcriptome changes were identified, the functions of many ethylene-regulated genes remain unknown.
Journal Article
CsPLD alpha 1 and CsPLD gamma 1 are differentially induced during leaf and fruit abscission and diurnally regulated in Citrus sinensis
Understanding leaf and fruit abscission is essential in order to develop strategies for controlling the process in fruit crops. Mechanisms involved in signalling leaf and fruit abscission upon induction by abscission agents were investigated in Citrus sinensis cv. 'Valencia'. Previous studies have suggested a role for phospholipid signalling; hence, two phospholipase D cDNA sequences, CsPLD alpha 1 and CsPLD gamma 1, were isolated and their role was examined. CsPLD alpha 1 expression was reduced in leaves but unaltered in fruit peel tissue treated with an ethylene-releasing compound (ethephon), or a fruit-specific abscission agent, 5-chloro-3-methyl-4-nitro-1H-pyrazole (CMNP). By contrast, CsPLD gamma 1 expression was up-regulated within 6 h (leaves) and 24 h (fruit peel) after treatment with ethephon or CMNP, respectively. CsPLD alpha 1 expression was diurnally regulated in leaf blade but not fruit peel. CsPLD gamma 1 exhibited strong diurnal oscillation in expression in leaves and fruit peel with peak expression around midday. While diurnal fluctuation in CsPLD alpha 1 expression appeared to be light-entrained in leaves, CsPLD gamma 1 expression was regulated by light and the circadian clock. The diurnal expression of both genes was modulated by ethylene-signalling. The ethephon-induced leaf abscission and the ethephon- and CMNP-induced decrease in fruit detachment force were enhanced by application during rising diurnal expression of CsPLD gamma 1. The results indicate differential regulation of CsPLD alpha 1 and CsPLD gamma 1 in leaves and fruit, and suggest possible roles for PLD-dependent signalling in regulating abscission responses in citrus.
Journal Article
Light controls phospholipase A₂α and β gene expression in Citrus sinensis
The low-molecular weight secretory phospholipase A₂α (CssPLA₂α) and β (CsPLA₂β) cloned in this study exhibited diurnal rhythmicity in leaf tissue of Citrus sinensis. Only CssPLA₂α displayed distinct diurnal patterns in fruit tissues. CssPLA₂α and CsPLA₂β diurnal expression exhibited periods of approximately 24 h; CssPLA₂α amplitude averaged 990-fold in the leaf blades from field-grown trees, whereas CsPLA₂β amplitude averaged 6.4-fold. Diurnal oscillation of CssPLA₂α and CsPLA₂β gene expression in the growth chamber experiments was markedly dampened 24 h after transfer to continuous light or dark conditions. CssPLA₂α and CsPLA₂β expressions were redundantly mediated by blue, green, red and red/far-red light, but blue light was a major factor affecting CssPLA₂α and CsPLA₂β expression. Total and low molecular weight CsPLA₂ enzyme activity closely followed diurnal changes in CssPLA₂α transcript expression in leaf blades of seedlings treated with low intensity blue light (24 μmol m⁻² s⁻¹). Compared with CssPLA₂α basal expression, CsPLA₂β expression was at least 10-fold higher. Diurnal fluctuation and light regulation of PLA₂ gene expression and enzyme activity in citrus leaf and fruit tissues suggests that accompanying diurnal changes in lipophilic second messengers participate in the regulation of physiological processes associated with phospholipase A₂ action.
Journal Article
CsPLDα1 and CsPLDγ1 are differentially induced during leaf and fruit abscission and diurnally regulated in Citrus sinensis
Understanding leaf and fruit abscission is essential in order to develop strategies for controlling the process in fruit crops. Mechanisms involved in signalling leaf and fruit abscission upon induction by abscission agents were investigated in Citrus sinensis cv. 'Valencia'. Previous studies have suggested a role for phospholipid signalling; hence, two phospholipase D cDNA sequences, CsPLDα1 and CsPLDγ1, were isolated and their role was examined. CsPLDα1 expression was reduced in leaves but unaltered in fruit peel tissue treated with an ethylene-releasing compound (ethephon), or a fruit-specific abscission agent, 5-chloro-3-methyl-4-nitro-1H-pyrazole (CMNP). By contrast, CsPLDγ1 expression was up-regulated within 6 h (leaves) and 24 h (fruit peel) after treatment with ethephon or CMNP, respectively. CsPLDα1 expression was diurnally regulated in leaf blade but not fruit peel. CsPLDγ1 exhibited strong diurnal oscillation in expression in leaves and fruit peel with peak expression around midday. While diurnal fluctuation in CsPLDα1 expression appeared to be light-entrained in leaves, CsPLDγ1 expression was regulated by light and the circadian clock. The diurnal expression of both genes was modulated by ethylene-signalling. The ethephon-induced leaf abscission and the ethephon- and CMNP-induced decrease in fruit detachment force were enhanced by application during rising diurnal expression of CsPLDγ1. The results indicate differential regulation of CsPLDα1 and CsPLDγ1 in leaves and fruit, and suggest possible roles for PLD-dependent signalling in regulating abscission responses in citrus.
Journal Article
citrus abscission agent induces anoxia- and senescence-related gene expression in Arabidopsis
The mechanisms of negative effects of 5-chloro-3-methyl-4-nitro-1H-pyrazole (CMNP), a pyrazole-derived plant growth regulator used as a citrus abscission agent, were explored in Arabidopsis by integrating transcriptomic, physiological, and ultrastructural analyses. CMNP promoted starch degradation and senescence-related symptoms, such as chloroplast membrane disruption, electrolyte leakage, and decreased chlorophyll and protein content. Symptoms of plant decline were evident 12 h after CMNP treatment. Microarray analysis revealed that CMNP influenced genes associated with stress, including those related to anoxia, senescence, and detoxification. Sucrose treatment arrested CMNP-induced plant decline. The results demonstrate that the plant response to CMNP shares common elements with various stresses and senescence at physiological and molecular levels.
Journal Article
Induction of phytohormones and differential gene expression in citrus flowers infected by the fungus Colletotrichum acutatum
Colletotrichum acutatum infects citrus petals and induces premature fruit drop and the formation of persistent calyces. The accumulation of hormones and other growth regulators, and differential gene expression in affected flowers and young fruit, was examined following fungal infection. Ethylene evolution increased threefold and indole-3-acetic acid (IAA) accumulation was as much as 140 times. Abscisic acid (ABA) levels showed no significant response. After infection, both trans- and cis-12-oxo-phytodienoic acid increased 8- to 10-fold. No significant difference of trans-jasmonic acid (JA) was observed in citrus flower petals or pistils. However, a fivefold increase of cis-JA was detected. The amount of salicylic acid (SA) was elevated twofold in affected petals, but not in pistils. Northern blot analyses revealed that the genes encoding ACC oxidase or ACC synthase, and 12-oxo-phytodienoic acid (12-oxo-PDA) reductase, were highly expressed in affected flowers. The genes encoding auxin-related proteins also were upregulated. Application of 2-(4-chlorophenoxy)-2-methyl-propionic acid (clofibrate; a putative auxin inhibitor), 2,3,5-triiodobenzolic acid (an auxin transport inhibitor), or SA after inoculation significantly decreased the accumulation of the gene transcripts of auxin-responsive, GH3-like protein and 12-oxo-PDA reductase, but resulted in higher percentages of young fruit retention. The results indicate that imbalance of IAA, ethylene, and JA in C. acutatum-infected flowers may be involved in symptom development and young fruit drop.
Journal Article
beta-galactosidase gene is expressed during mature fruit abscission of 'Valencia' orange (Citrus sinensis)
beta-galactosidases have been detected in a wide range of plants and are characterized by their ability to hydrolyse terminal non-reducing beta-D-galactosyl residues from beta-D-galactosides. These enzymes have been detected in a wide range of plant organs and tissues. In a search for differentially expressed genes during the abscission process in citrus, sequences encoding beta-galactosidase were identified. Three cDNA fragments of a beta-galactosidase gene were isolated from a cDNA subtraction library constructed from mature fruit abscission zones 48 h after the application of a mature fruit-specific abscission agent, 5-chloro-3-methyl-4-nitro-1 H-pyrazole (CMN-pyrazole). Based on sequence information derived from these fragments, a full-length cDNA of 2847 nucleotides (GenBank accession number AY029198) encoding beta-galactosidase was isolated from mature fruit abscission zones by 5'- and 3'-RACE approaches. The beta-galactosidase cDNA encoded a protein of 737 amino acid residues with a calculated molecular weight of 82 kDa. The deduced protein was highly homologous to plant beta-galactosidases expressed in fruit ripening. Southern blot analysis demonstrated that at least two closely related beta-galactosidase genes were present in 'Valencia' orange. Temporal expression patterns in mature fruit abscission zones indicated beta-galactosidase mRNA was detected 48 h after treatment of CMN-pyrazole and ethephon in mature fruit abscission zones. beta-galactosidase transcripts were detected in leaf abscission zones only after ethephon application. The citrus beta-galactosidase was expressed in stamens and petals of fully opened flowers and young fruitlets. The results suggest that this beta-galactosidase may play a role during abscission as well as early growth and development processes in flowers and fruitlets.
Journal Article