Chromosomes associate premeiotically and in xylem vessel cells via their telomeres and centromeres in diploid rice ( Oryza sativa )

Abstract

Studies of the meiosis of diploid plants such as Arabidopsis, maize and diploid progenitors of wheat have revealed no premeiotic association of chromosomes. Premeiotic and somatic association of chromosomes has only been previously observed in the anther tissues and xylem vessel cells of developing roots in polyploid plants such as hexaploid and tetraploid wheat, polyploid relatives of wheat and artificial polyploids made from the progenitor diploids of wheat. This suggested that this association was confined specifically to polyploids or was induced by polyploidy. However, we developed procedures for in situ hybridization on structurally well-preserved tissue sections of rice, and analysed two diploid rice species ( Oryza sativa and O. punctata). Contrary to expectation, this has revealed that centromeres and telomeres also associate both in the xylem vessel cells of developing root and in undifferentiated anther cells in these diploids. However, in contrast to wheat and related polyploids, where the initial association in undifferentiated anthers is between either non-homologous or related chromosomes, and not homologous chromosomes, the initial association of rice chromosomes seems to be between homologues. Thus, in contrast to the diploid dicot model Arabidopsis, meiotic studies on the diploid model cereal, rice, will now need to take into account the effects of premeiotic chromosome association.

Chromosome Organization and Dynamics during Interphase, Mitosis, and Meiosis in Plants

“…In some of these species, such as wheat or barley (Hordeum vulgare), Rabl configuration persists in all cells throughout the plant (Anamthawat-Jonsson and Heslop- Harrison, 1990). In rice (Oryza sativa), on the other hand, Rabl is only observed in certain tissues, such as xylem cells in the root and undifferentiated cells in the anther (Prieto et al, 2004a;Santos and Shaw, 2004). Other plant species, such as maize (Zea mays) and sorghum (Sorghum bicolor), despite having fairly large genomes, are not known to exhibit Rabl configuration at all (Dong and Jiang, 1998).…”

Fluorescence in situ hybridization on vibratome sections of plant tissues

“…Before the telomeres form a cluster, PMCs and tapetal cells are not morphologically distinguishable, and can be classified only by their position in the anther. To label telomeres, we have used the telomeric sequence as a probe, amplified previously by PCR using the oligomer primers (5¢-TTTAGGG-3¢) 5 and (5¢-CCCTAAA-3¢) 5 in the absence of template DNA 19 . Telomeres associated as a telomere cluster are shown in Figure 1c and e. Figure 1f shows a microsporocyte at a stage before the telomeres form a cluster.…”

“…For the unequivocal identification and staging of the cells, the general morphology of the flower, the number of nucleoli present in the nucleus, the morphology of the nucleus, the position of the cell in the anther and the disposition of the telomeres can be used. This method has been used to analyze chromosome association during early meiosis in rice, wheat and related species 2,3,5,15,16 .…”

“…These requirements are met by the roots and florets and other organs of many plants. We have used it for both roots and florets of wheat (Triticum aestivum L.) and related species [1][2][3] , for rye (Secale cereale L.) florets 4 , for rice (Oryza sativa L.) roots and florets 5 and maize (Zea mays L.) roots 6 among the cereals, and for roots of pea (Pisum sativum L.), bean (Vicia faba L.) 7 and soya (Glycine max L.) 8 . The only plant species with which we have been unsuccessful in vibratome sectioning is Arabidopsis thaliana, because of its small size.…”

“…It provides a reliable tool to answer questions about the nature and timing of recognition events, chromosome associations and control mechanisms before and during meiosis, and to determine, for example, how universal meiotic events are in plants over a wide range of genome sizes and taxa 2,3,5,15,16 .…”