Ivan Cordova

Is abscisic acid responsible for abnormal stomatal closure in coconut palms showing lethal yellowing

Summary Lethal yellowing induced permanent stomatal closure in coconut palms ( Cocos nucifera L.) is central to the development of the diseases symptoms such as decreased photosynthesis, leaf yellowing and palm death, but the cause of this abnormal permanent stomatal closure is unknown. Since abscisic acid (ABA) can cause stomatal closure, ABA concentrations were measured in the roots, leaves, and leaf xylem sap to test for a correlation with the degree of stomatal closure as disease severity advanced. Concentrations of ABA in leaf and xylem sap extracts increased with disease progression but not until the later stages of the disease, well after abnormal stomatal closure. ABA concentrations in the roots were also not correlated with stomatal closure. Transpiration declined proportionally in response to increasing ABA concentrations (10 −8 , 10 −6 , 10 −4 mol L −1 ) in a detached coconut leaf bioassay but all extracts from healthy and diseased coconut palms reduced transpiration to similar levels. Likewise, all coconut palm leaf extracts reduced stomatal aperture 60–70 % in Commelina communis leaf epidermal strips with no notable differences between healthy and diseased palms. Thus, the results indicate that bulk ABA concentrations in leaf xylem sap or leaf tissue are not responsible for the abnormal stomatal closure in lethal yellowing disease.

Practical use of DNA probes for the detection of lethal yellowing of Cocos nucifera L. in Mexico

Lethal yellowing (LY) is a fatal disease of palms [5], and is believed to be caused by a mycoplasmalike organism (MLO) [1,4, 6]. Harrison et al. [3] isolated and cloned fragments of LY-MLO DNA from palms affected by the disease in Florida and used them as diagnostic probes. These probes have since proved useful for the detection of the LY pathogen in affected palms in the Yucatan Peninsula of Mexico [2]. LY is a disease that presently cannot be controlled, but measures, such as quarantine, may be taken to reduce its rate of spread (Been, this volume). Effective quarantine requires a constant evaluation of the geographic occurrence of the disease. To achieve this, a reliable diagnostic method is required. This paper reports the detection of LY in symptomatic and symptomless coconut palms (Cocos nucifera L.) using a DNA probe, and also reports the use of this technique as a tool for monitoring the spread of LY in the Yucatan Peninsula.

Biochemical Changes in Roots of Coconut Palms (Cocos nucifera L.) Affected by Lethal Yellowing

Summary Respiration rates and protein concentrations of lateral and subtending roots of coconut palms were studied at various stages of development of lethal yellowing (LY) disease, including LYpositive symptom-less palms, and compared to respiration rates and protein concentrations of healthy palms. No visual symptoms were apparent in the roots of LYpositive symptomless palms. However, respiration rates and protein concentrations were already lower in LYpositive symptomless palms and they continued to decrease as the disease developed. The appearance of root necrosis coincided with the onset of leaf yellowing in above ground parts. Implications of these findings are discussed in the context of root damage, as a central process in the mode of pathogenicity of the disease.

GA3 induces expression of E2F-like genes and CDKA during in vitro germination of zygotic embryos of Cocos nucifera (L.)

Previous reports have demonstrated that gibberellins promote seed germination, including in vitro germination of zygotic embryos of coconut. Genes that control the cell cycle seem to play important roles during germination. In this study, complete full-length sequences of two E2F-like genes, CnE2F1 and CnE2F2, were obtained. Deduced amino acid sequences of both genes showed conserved domains characteristic of E2F genes. Both genes showed high homology with Arabidopsis thalianaE2Fa and E2Fb, which are transcriptional activators. The expression profiles of CnE2F genes were evaluated in in vitro zygotic embryos with or without gibberellic acid. In control embryos, expression of CnE2F1 showed a peak at day 2, and, gradually this dropped to similar levels as those at day 0. Gibberellic acid (GA3) treatment promoted expression at day 2, higher than that of the control. For CnE2F2, peak expression was detected at day 8 of culture. However, with GA treatment, CnE2F2 expression profile increased by day 2. The expression profile of a cyclin-dependent kinase (CnCDKA), another gene related to the cell cycle control, was also studied. In control embryos, expression of this gene showed two peaks at days 2 and 8, compared with day 0. A different profile was shown with GA treatment. In general, major changes in expression profiles were observed from days 2–8 when morphological changes in zygotic embryos were not yet observed.

Preliminary molecular detection of the somatic embryogenesis receptor-like kinase (VpSERK) and knotted-like homeobox (VpKNOX1) genes during in vitro morphogenesis of Vanilla planifolia Jacks

This work aimed to evaluate the embryogenic competence of different tissues from different stages (friable callus, bud-regenerating callus, and whole buds) of Vanilla planifolia, through the molecular detection of the somatic embryogenesis receptor-like kinase (VpSERK) and knotted-like homeobox (VpKNOX1) genes. RNA was extracted with Trizol®, cDNA was obtained, and the studied transcripts were amplified. Using non-specific primers, VpSERK and VpSTM gene expression was detected in the three stages evaluated. This study might contribute to providing an explanation for the recalcitrance of this Vanilla species to somatic embryogenesis.

New insights into the evolutionary history of resistance gene candidates in coconut palms and their expression profiles in palms affected by lethal yellowing disease

The nucleotide binding site and leucine rich repeat (NBS–LRR) class of R genes is the most comprehensively studied in terms of sequence evolution; however, in coconut palm and, more generally, in the family of Arecaceae, our understanding of the evolution of these genes is rather limited. In this study, disease resistance gene candidates (RGCs) of the nucleotide binding site (NBS) type of coconut palm were used to investigate evolutionary relationships in Arecaceae, Poaceae and Brassicaceae species. The results indicate a species-specific evolution of RGCs in coconut palm. However, strikingly similar RGCs between species of Arecales indicate a high conservation of specific RGCs of this family, suggesting a monophyletic origin of three genera. The phylogenetic relationship between RGCs of Arecales and Brassicales suggests that these sequences possibly emerged before being divided between monocots and dicots. Finally the comparative analysis of the expression of four RGCs in healthy coconut palm and those affected with lethal yellowing disease revealed differences in their expression profiles. This study provides new insights for future efforts towards the improvement of disease resistance in coconut palm and other species of Arecaceae.