SIDECAR POLLENsuggests a plant-specific regulatory network underlying asymmetric microspore division in Arabidopsis

Abstract

The SIDECAR POLLEN gene encodes a microspore-specific LOB/AS2 domain protein required for the correct timing and orientation of asymmetric cell division. Plant J 2010; 64:839-50 A symmetric cell division is a universal strategy to generate diverse cell types necessary for patterning and proliferation of all eukaryotes. The development of haploid male gametophytes (pollen grains) in flowering plants is a remarkable example in which division asymmetry governs the functional specialization and germline differentiation essential for double fertilization. The male gametophyte is patterned via two mitotic divisions resulting in three highly differentiated daughter cells at maturity, a vegetative cell and two sperm cells. The first asymmetric division segregates a unique male germ cell from an undetermined haploid microspore and is executed in an elaborate sequence of cellular events. However the molecular mechanisms governing the division asymmetry in microspores are poorly understood. Recently we studied the phenotype of sidecar pollen (scp) mutants in detail, and demonstrated a requirement of SCP for both the correct timing and orientation of microspore division. SCP is a microspore-specific member of the LOB/AS2 domain family (LBD27/ASL29) showing that a plant-specific regulator plays a key role in oriented division of polarized microspores. Identification of SCP will serve as a new platform to further explore the largely unknown molecular networks regulating division asymmetry in microspores that establishes the male germline in flowering plants.Unlike animals, flowering plants do not set aside a distinct germline from an early stage of the life cycle. Instead the angiosperm germline or germ cells are only

Phylogeny and Functions of LOB Domain Proteins in Plants

“…AtLBD27 is required for correct initiation and orientation of the polarized microspore's asymmetric division to generate bicellular pollen, as the lbd27 mutant produces aberrant microspores with increased cell expansion, delayed mitosis entry and altered nuclear division orientation phenotypes. [112,113]. AtLBD10, which belongs to Class IA, can also affect the pollen development as mentioned above.…”

Analysis of gemini pollen 3 mutant suggests a broad function of AUGMIN in microtubule organization during sexual reproduction in Arabidopsis

“…A determinative process in male gametogenesis is the asymmetric microspore division (pollen mitosis I), which segregates the vegetative and generative (or germline) cells. In previous work we have characterized mutants that have helped to dissect the mechanisms that underlie asymmetric division at pollen mitosis I (Park et al ., ; Twell et al ., ; Oh et al ., , ). These can be grouped into classes that affect three consecutive steps including microspore polarization involving nuclear migration towards the future germ cell pole, oriented nuclear division at the germ cell pole and asymmetric cytokinesis (Oh et al ., ).…”

“…In previous work we have characterized mutants that have helped to dissect the mechanisms that underlie asymmetric division at pollen mitosis I (Park et al ., ; Twell et al ., ; Oh et al ., , ). These can be grouped into classes that affect three consecutive steps including microspore polarization involving nuclear migration towards the future germ cell pole, oriented nuclear division at the germ cell pole and asymmetric cytokinesis (Oh et al ., ). gemini pollen 1 ( gem1 ) mutants are affected in the first of these steps, giving rise to similar daughter cells and failed germ cell differentiation (Park et al ., ).…”

The male germline of angiosperms: repertoire of an inconspicuous but important cell lineage

“…Interestingly, the sidecar mutant can result in two equal-sized cells, initially retaining vegetative identity, but when one of these cells undergoes an asymmetric division, it forms a generative cell that divides to form two apparently completely normal sperm cells (Chen and McCormick, 1996 ). Dissimilar cell volumes presumably trigger the key transcription factors and activate the developmental program of the male germ lineage (Oh et al, 2011 ). Reactivation of the cell cycle in the generative cell appears to license the single mitotic division required to form the two sperm cells (Brownfield et al, 2009 ), whereas further cell cycle progression in the vegetative cell continues to be inhibited.…”