Genetic Control of Male Germ Unit Organization in Arabidopsis

Abstract

In flowering plants, the vegetative nucleus and the two sperm cells are proposed to form a functional assemblage, the male germ unit (MGU). Here, we describe the developmental pathway of MGU assembly in Arabidopsis and report two classes of mutations that affect the integrity and/or the positioning of the MGU in the mature pollen grain. In germ unit malformed (gum) mutants, the vegetative nucleus is positioned adjacent to the pollen grain wall, separate from the two sperm cells, whereas in MGU displaced (mud) mutants, the intact MGU is displaced to the pollen grain wall. mud and gum mutants correspond to male-specific gametophytic mutations that also reduce pollen fitness. Genetic mapping showed that the gum1 and gum2 mutations are genetically linked, possibly allelic, whereas the mud1 and mud2 mutations correspond to two unlinked loci mapping on different chromosomes. The hierarchical relationship between mud and gum mutations was investigated by phenotypic analysis of double mutants. gum1 appeared to act earlier than mud1 and mud2, affecting initial MGU assembly and its stability during pollen maturation. In contrast, mud1 and mud2 mutations appear to act only on MGU positioning during final maturation. From in planta analyses of pollen germination in mud and gum mutants, we conclude that the initial proximity and positioning of MGU components is not required for their entrance into the pollen tube, but the efficiency of MGU translocation is reduced.

Pollen and Stigma Structure and Function: The Role of Diversity in Pollination

“…Other visual screens have identified mutations that allow tubes to grow without emerging from the grain or disrupt the accumulation and deposition of callose before pollen tube emergence (Johnson and McCormick, 2001). In addition, screens for pollen gametophytic mutations have yielded mutants altered in germination, polarity, and tube emergence (Ryan et al, 1998;Grini et al, 1999;Johnson and McCormick, 2001;Procissi et al, 2001;Johnson and Preuss, 2002;Lalanne and Twell, 2002). Similar approaches have not yet yielded stigma-specific mutations, despite numerous screens to isolate sporophytic sterile mutants.…”

“…Whether tubes emerge on a dry stigma surface, from a grain submerged in stigma exudates, or from pollen germinated in vitro, the grain organizes its cytoplasm and cytoskeleton to support the extension of a single tube. These changes occur within minutes after hydration and include the formation of filamentous cytoskeletal structures that wrap around the nuclei, actin cytoskeleton polarization toward the site of tube emergence (Tiwari and Polito, 1988;Heslop-Harrison and Heslop-Harrison, 1992), reorientation of the large vegetative nucleus so that it enters the extending tube before the generative cells (Heslop-Harrison et al, 1986a;Heslop-Harrison and Heslop-Harrison, 1989;Lalanne and Twell, 2002), assembly of mitochondria and polysaccharide particles at the site of the elongating tube tip (Cresti et al, 1977(Cresti et al, , 1985Mazina et al, 2002), and selection of the pollen plasma membrane for secretory vesicle targeting and deposition of callose (b-1!3 glycan) at the site of tube emergence (Johnson and McCormick, 2001).…”

“…For example, mutations that upset the physical association between the two Arabidopsis sperm cells and the vegetative pollen nucleus have been identified. These mutations disrupt the polarized transport of the male germ unit to the pollen tube tip by (1) separating the vegetative nucleus from the two sperm cells (germ unit malformed1 [gum1] and gum2) or (2) mislocalizing an intact male germ unit to the pollen wall (mgu displaced1 [mud1] and mud2) (Lalanne and Twell, 2002). Other visual screens have identified mutations that allow tubes to grow without emerging from the grain or disrupt the accumulation and deposition of callose before pollen tube emergence (Johnson and McCormick, 2001).…”

Sperm dimorphism in terms of nuclear shape and microtubule accumulation in Cyrtanthus mackenii

“…Further, proteins colocalized with microtubules in male gametes may play an important role in the development of the gametes (Lee et al 2007;Mori et al 2003;Twell et al 2002), in mediating the relationship between microtubules and in the plasma membrane (Cai et al 2005). Microtubule distribution in male gametes and their correlation with the distribution of microtubule-associated proteins may also be demonstrated in C. mackenii and are possibly related to the biological roles of sperm cells in double fertilization and in mediating the functions of microtubule-associated proteins.…”

“…This association forms the male germ unit (MGU), which is proposed to function as a vehicle for the transmission of male gametes to female gametes and to participate in the fusion with the target female cell during fertilization (Dumas et al 1984). In Arabidopsis thaliana L. Heynh, the position or organization of the MGU in mature pollen has been suggested to influence the efficiency of male gamete delivery (Lalanne and Twell 2002).…”