Proteins involved in peroxisome biogenesis are termed PEROXINS (PEXs). Generally, peroxisome biogenesis consists of three steps: biogenesis of the peroxisomal membrane, import of peroxisome matrix proteins, and peroxisome division. So far, several models have been proposed, including the ER vesiculation model, the autonomous peroxisome growth and division model, and the recent ER semiautonomous peroxisome model ( Mullen and Trelease, 2006 Hu et al., 2012). Recent advances have largely boosted our knowledge of peroxisome biogenesis although far from fully elucidating this process ( Hu et al., 2012 Kim and Mullen, 2013).
Peroxisomes are single membrane–bound organelles that function in diverse metabolic pathways. These findings indicate that JA is also involved in pollen maturation and germination, but the underlying molecular mechanisms are poorly understood. The dad1 grains are unable to germinate when manually laid on the stigmas. Pollen grains from opr3 and dad1 mutants develop normally up to the tricellular stage but are sterile after release. Among these mutants, phenotypic analysis of pollen was only conducted in opr3 and dad1 ( Stintzi and Browse, 2000 Ishiguro et al., 2001). Anther dehiscence–defective phenotypes have been reported in mutants defective in the JA biosynthetic pathway, including fatty acid desaturation ( McConn and Browse, 1996), defective in anther dehiscence1 ( dad1) ( Ishiguro et al., 2001), allene oxide synthase ( Park et al., 2002), 12-oxophytodienoic acid reductase3 ( opr3) ( Stintzi and Browse, 2000), delayed-dehiscence1 ( dde1), and dde2 ( Sanders et al., 2000 von Malek et al., 2002). Jasmonic acid ( JA) is an essential signal regulating anther development and pollen maturation ( Turner et al., 2002 Browse, 2009). The mature pollen grains released from the dehiscent anthers are greatly desiccated and metabolically dormant, which maintains pollen viability ( Swanson et al., 2004). The tricellular grains undergo a maturation process prior to release to prepare themselves for survival in terrestrial environment ( Taylor and Hepler, 1997). The tricellular pollen grains have gained pollination competence from this moment though still in undehisced anthers ( Kandasamy et al., 1994). Subsequently, the generative cell goes through a symmetric division to generate two sperm cells. In Arabidopsis thaliana anthers, microspores undergo an asymmetric division to produce a large vegetative cell and a small generative cell. During evolution, key innovations associated with the male gametophyte occurred, including the reversal of microspore polarity, the conversion from motile to immotile gametes, and the emergence of siphonogamy ( Rudall and Bateman, 2007). The life cycle of flowering plants alternates between a diploid, sporophytic generation and a haploid, gametophytic generation. Taken together, these data indicate that DAU/APEM9 plays critical roles in peroxisome biogenesis and function, which is essential for JA production and pollen maturation and germination.
In addition, the phenotypic survey of peroxin mutants indicates that the PEXs most likely play different roles in pollen germination. In addition, the jasmonic acid ( JA) level is significantly decreased in dau pollen, and the dau mutant phenotype is partially rescued by exogenous application of JA, indicating that the male sterility is mainly due to JA deficiency. Consistently, both peroxisome biogenesis and peroxisome protein import are impaired in dau pollen. DAU interacts with peroxisomal membrane proteins PEROXIN13 (PEX13) and PEX16 in planta. DAU is transiently expressed from bicellular pollen to mature pollen during male gametogenesis. Molecular analysis indicated that DAU encodes the peroxisomal membrane protein ABERRANT PEROXISOME MORPHOLOGY9 (APEM9). Here, we report an Arabidopsis thaliana mutant, dayu ( dau), which impairs pollen maturation and in vivo germination.
Molecular mechanisms controlling these processes remain largely unknown. Pollen undergo a maturation process to sustain pollen viability and prepare them for germination.
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