Genetic and Biochemical Mechanisms of Pollen Wall Development

Pollen wall development exhibits conserved and diversified features. Genes associated with pollen wall development are coordinately regulated. The synthesis of exine and anther cutin may share common pathways in rice. The pollen wall is a specialized extracellular cell wall matrix that surrounds male gametophytes and plays an essential role in plant reproduction. Uncovering the mechanisms that control the synthesis and polymerization of the precursors of pollen wall components has been a major research focus in plant biology. We review current knowledge on the genetic and biochemical mechanisms underlying pollen wall development in eudicot model Arabidopsis thaliana and monocot model rice (Oryza sativa), focusing on the genes involved in the biosynthesis, transport, and assembly of various precursors of pollen wall components. The conserved and divergent aspects of the genes involved as well as their regulation are addressed. Current challenges and future perspectives are also highlighted. The pollen wall is a specialized extracellular cell wall matrix that surrounds male gametophytes and plays an essential role in plant reproduction. Uncovering the mechanisms that control the synthesis and polymerization of the precursors of pollen wall components has been a major research focus in plant biology. We review current knowledge on the genetic and biochemical mechanisms underlying pollen wall development in eudicot model Arabidopsis thaliana and monocot model rice (Oryza sativa), focusing on the genes involved in the biosynthesis, transport, and assembly of various precursors of pollen wall components. The conserved and divergent aspects of the genes involved as well as their regulation are addressed. Current challenges and future perspectives are also highlighted. the white–brown complex (WBC) subfamily of ATP- binding cassette (ABC) membrane-transport proteins that facilitate the transport of specific substrates across the membrane in an ATP-dependent manner. They are half-size ABC proteins and form hetero- or homodimers that function as active membrane transporters. the cuticular lipidic layer that covers the anther surface and protects anthers from biotic and abiotic stresses. It is composed predominantly of a polymer matrix cutin consisting mainly of C16 and C18-hydroxy fatty acids that is covered and embedded with distinct very-long-chain (C24–C34) saturated and unsaturated non-polar waxes. Cuticle in the surface of plant tissues other than anther is synthesized exclusively by the epidermal cells. By contrast, anther cuticle seems to be synthesized mainly in the tapetum, the innermost layer of the anther wall. also known as β-glucan, is synthesized at the cell wall by callose synthases and degraded by β-1,3-glucanases. This specific cell wall polymer functions in several fundamental biological processes, ranging from plant development to the response to abiotic and biotic stresses. During angiosperm microsporogenesis, callose serves as a temporary wall to separate newly formed microspores in the tetrad from microsporocytes. The development of pollen wall originates from the callose wall, and the callose wall around the tetrad may recruit the primexine and together provide a structural support for exine formation. Abnormal callose deposition and dissolution often leads to male sterility. a type of non-pigmented and specialized plastid (leucoplast) derived from proplastids for the storage of steryl esters, free polar lipids, and plastid lipid-associated proteins in Brassicaceae species. the most complex and important outer layer of the pollen wall. Generally contains two layers, the inner nexine and the outer sexine. The nexine has a bilayer structure, consisting of nexine I and nexine II, and the sexine is further composed of tectum and bacula. The portions between sexine and nexine sculpt the species-specific structure of the pollen grains, while the chemically- and physically-resistant properties of the exine allow plants to adapt to and colonize the land environment. an innermost layer of the pollen wall underlying the exine that consists of the exintine and the endintine. The major components of intine are assumed to consist of pectin, cellulose, hemicellulose, hydrolytic enzymes, and hydrophobic proteins. It is required for the maintenance of the structural integrity of pollen grains, pollen germination, and pollen tube growth into the stigma. small proteins found in abundance in higher plants that play various roles in plant biology, such as cutin formation, embryogenesis, defense reactions against phytopathogens, symbiosis, and the adaptation of plants to various environmental conditions. LTPs facilitate the movement of lipids between membranes via their binding and solubilizing capacities. a microfibrillar matrix composed largely of neutral and acidic polysaccharide materials, some proteins, and cellulose components. It serves as a template for initial sporopollenin deposition following polymerization and patterning initiated by a enzymes termed sporopollenin acceptor particles (SAPs). a highly-resistant biopolymer of phenylpropanoid and lipidic monomers covalently coupled by ether and ester linkages. Sporopollenin provides the rigid and sculptured framework of the exine, functions to encapsulate and protect the pollen contents, and aids stigmatic capture. a type of lipid-accumulating ER-derived spherical organelles in tapetal cells of Brassicaceae species. Tapetosomes accumulate ER-derived flavonoids, alkanes, and oleosins for the formation of the pollen wall, as well as regulating tapetal programmed cell death. the innermost layer of the anther wall, which functions as a source for synthesis, storage, and transport of various nutrients and structural components for exine and tryphine formation. also known as the pollen coat, the outermost stratum of the pollen surface, tryphine is extremely hydrophobic and can be eluted from the exine using organic solvents. It is mainly composed of lipids, flavonoids, proteins, pigments, aromatic substances, and other unknown compounds. The pollen coat fills in the depressions or the interspace of the pollen exine, protecting male gametophytes from dehydration and facilitating subsequent pollen–stigma communication and adhesion. specialized lipidic structures which are proposed to arise from the pro-obicule and that carry tapetum-derived sporopollenin precursors across the hydrophilic cell wall to the outer surface of the microspores in the anther locule. Ubisch bodies are present in cereals and have an electron-dense outer layer with an undulating surface, and the center is hollow or is composed of lipids; Ubisch bodies are readily observable by scanning electron microscopy of mature anthers.