SUMMARY DNA of the lethal yellowing (LY) phytoplasma was detected in 13 of 72 embryos from fruits of four diseased Atlantic tall coconut palms by polymerase chain reaction (PCR) assays employing phytoplasma universal rRNA primer pair P1/P7, nested LY group‐specific rRNA primer pair 503f/LY16Sr or LY phytoplasma‐specific nonribosomal primer pair LYF1/R1. Phytoplasma distribution in sectioned tissues from six PCR positive embryos was determined by in situ PCR and digoxigenin‐11‐deoxy‐UTP (Dig) labelling of amplification products. Dig‐labeled DNA products detected by colourimetric assay were clearly evident on sections from the same three embryos investigated in detail by in situ PCRs employing primer pairs P1/P7 or LYF1/R1. Deposition of blue–green stain on sections as a result of each assay was restricted to areas of the embryos corresponding to the plumule and cells ensheathing it. By comparison, similarly treated embryo sections derived from fruits of a symptomless Atlantic tall coconut palm were consistently devoid of any stain. Presence of phytoplasma DNA in embryo tissues suggests the possible potential for seed transmission which remains to be demonstrated.
Lethal yellowing (LY), the most devastating disease affecting the coconut palm in America, is caused by phytoplasmas known to be distributed in different parts of infected plants. However, no comprehensive reports exist on the phytoplasma distribution. This study refers to the detection of LY phytoplasma DNA using PCR in different coconut plant parts, throughout the development of the disease. Sample analysis of positive palms taken at different stages of disease development (either symptomatic or symptomless) showed differences in the percentage of LY detection between plant parts. Some parts showed a very high level of LY DNA (stem, young leaves, inflorescences, stem apex and root apex), low levels were found in the intermediate leaves and roots without apex, whereas no LY phytoplasma DNA was detected in mature leaves. The detection percentage of LY phytoplasma DNA was lowest in symptomless‐infected palms for all parts, except the stem, where phytoplasma accumulations were consistently detected. This pattern of detection among parts is consistent with the hypothesis that phytoplasmas move from photosynthate source tissues to sink tissues via the phloem mass flow process. The accumulations in the (lower) stem, prior to the appearance of symptoms, suggest that this part of the palm is where phytoplasmas first move from leaves after foliar feeding by vectors and in which they probably multiply and distribute to other palm parts, including roots. Embryos from infected palms were analysed by nested‐PCR and 28% of 394 embryos were positive. Phytoplasma DNA was detected in embryos from fruit on any of the fruiting bunches regardless the age, but no pattern of quantitative distribution throughout the bunch developmental stages was observed. Germination of seeds from LY‐positive symptomatic palms was 58% and from LY‐negative symptomless palms were 71%. No phytoplasma was detected in seedlings tested from both symptomatic and non‐symptomatic palms. Seedlings tested after 2 years did not develop LY symptoms or eventually died.
The CDKA gene is linked to the cellular control. This gene was isolated from coconut palm (Cocos nucifera L) and a detailed expression analysis was done during somatic embryogenesis. Analysis of the deduced amino acid sequence showed the most important residues to be conserved. The highest homology was with Picea abies (96% similarity). Expression of the putative CnCDKA gene steadily increased during embryogenic callus formation phase when embryogenic competence is attained. In situ hybridization specified the localization of the transcripts as being mainly in a few cell layers within the meristematic centres in embryogenic calli at 90 day cultures. Analysis of CnCDKA expression at different somatic embryo formation stages showed that the expression was decreased progressively with the lowest expression in germinated somatic embryos. (Résumé d'auteur
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