Ultrastructure of microsporogenesis and microgametogenesis in Brachypodium distachyon

Abstract

Brachypodium distachyon has emerged as a model system for forage grass and cereal grain species. Here, we report B. distachyon pollen development at the ultrastructural level. The process of microsporogenesis and microgametogenesis in B. distachyon follows the typical angiosperm pollen development sequence. Pronounced evaginations of the nuclear envelope are observed prior to meiosis, indicating active nucleocytoplasmic exchange processes. The microspore mother cells undergo meiosis and subsequent cytokinesis, forming isobilateral tetrads. Following dissolution of the callose wall and release of free and vacuolated microspores, mitotic divisions lead to the formation of mature, three-celled pollen grains. In B. distachyon, pollen wall formation begins at the tetrad stage by the formation of the exine template (primexine). The exine is tectate-columellate, comprising a foot layer and endexine. Development of the tectum and the foot layer is complete by the free microspore stage of development, with the tectum formed discontinuously. The endexine initiates in the free microspore stage but becomes compressed in mature grains. The intine layer is deposited after mitosis and comprises three layers during the mature pollen stage of development. Pore development initiates during early free microspore development stage and Brachypodium pollen has a single germination pore consisting of a slightly raised annulus surrounding a central operculum. The tapetum is of the secretory type with loss of the tapetal cell walls beginning at about the time of microsporocyte meiosis. This is the first report on ultrastructure of microsporogenesis and microgametogenesis in B. distachyon. In general, Brachypodium microsporogenesis and microgametogenesis conform to a typical grass pollen development pattern.

Male functionality inGarcinia celebicaL., a candidate ancestor species of mangosteen (G. mangostanaL.)

“…This degeneration of the tapetum is normally classified as a type of programmed cell death (PCD) (Papini et al 1999), D r a f t and it has been recently reported that PCD in the tapetum of Brassica napus and Nicotiana tabacum are regulated by epigenetic changes, including an increase in global DNA methylation and MET1 expression (Solı´s et al 2014). As a result, G. celebica provides a normal pattern of pollen and tapetum development because the onset of tapetum degradation shares similarities with observations of most angiosperms, including as Brachypodium distachyon (Sharma et al 2015), Oryza sativa (Raghavan 1988), and Arabidopsis thaliana (Owen and Makaroff 1995). In contrast, the tapetum of G. mangostana began to degrade prior to meiosis, premature microspore mother cell degeneration was observed, microspore tetrads with complete callose deposition were rarely detected, and no pollen was observed at anthesis (Yapwattanaphun et al 2008;Nuanjunkong and Meesawat 2010).…”

“…The tapetum is of the secretory type, defined by the absence of a cell wall, and the tapetal protoplasts remaining in the parietal position of the locule until its complete degeneration (Pacini and Juniper 1979;Shivanna 2003). This type is considered to be a primitive characteristic due to its widespread occurrence in gymnosperms and angiosperms (Sharma et al 2015). The breakdown of tapetum took place after meiosis II, which corresponds with the complete encasement of callose walls around microspore tetrads.…”

Dynamics of Polysaccharides and Neutral Lipids during Anther Development in Castor (Ricinus communis)

“…Therefore, starch grains were involved in the formation of the callose wall and pollen intine. However, previous investigations demonstrated that the intine layer is deposited after mitosis (Sharma et al, 2015b). At the late microspore stage, starch grains began to accumulate in the microspores, and plentiful starch grains were present in the bicellular pollen.…”

Genotyping of Individually Isolated gametes from Gossypium hirsutum Chromosome Substitution Lines

“…Pollen grains were individually isolated in PCR tubes and germinated for PEP amplifications of whole genomes. The individual microspores, when treated with REPLI-g Single Cell Kit (QIAGEN) release genomic DNAs for subsequent amplifications [9]. All individually isolated cotton early free microspores released their contents giving adequate yields of amplified DNA products through multiple displacement amplification (MDA) technology.…”

“…Microsporogenesis stages can allow the isolation of free microspores (immediately after the tetrad separation) when their tectum formation is discontinuous and before the initiation stage of endexine development [8,9]. Such microspores can easily be lysed with reagents used for DNA extractions from animal cells that only have cell membranes [10].…”