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Figure 1. 

The structure of anthers and pollen. In Arabidopsis, each anther has four anther locules (pollen sacs), and the anther wall around the anther locule is composed of the epidermis, endothecium, middle layer and tapetum. Mature pollen grains are produced inside the anther locule. A pollen grain has two sperm cells in the cytoplasm of the large vegetative cell and is covered with a complex pollen wall outside of the plasma membrane. Ep, epidermis; En, endothecium; ML, middle layer; T, tapetum; Vn, vegetative nucleus; Sc, sperm cell; PM, plasma membrane.

Figure 2. 

The cell wall undergoes a tremendous change during pollen development. The orange quadrilaterals represent the tapetal cells, and the corresponding microspores or pollen at specific anther stages are shown under the tapetum cells. At stage 7, the four microspores are enclosed inside the callose wall and the outer pectin wall. At late stage 7, the sexine and nexine precursors start to deposit outside the membrane. During stages 9 to 10, the tapetum begins to degenerate and becomes spongy. The intine layer appears between the plasma membrane and nexine layer at stage 10. At stage 11, the tapetum evidently degenerates, and the pollen coat precursor start to fill the sculptured cavities of the sexine. At stages 12 and 13, the tapetum cell degenerates completely, and all layers of the pollen wall are established.

Figure 3. 

Gene regulatory network in the tapetum of Arabidopsis and rice. Lines terminating in arrows represent positive regulation, lines with semicircle ends indicate interaction. Orange ovals and grey ovals mark the key transcription factors in Arabidopsis and rice respectively. In Arabidopsis, DYT1-TDF1-AMS are responsible for early tapetum development. AMS regulates nexine and sexine formation via TEK and MS188, respectively. MS1 is responsible for pollen coat formation. Abbreviations: TEK, transposable element silencing via AT-hook; BES1, BRI1 EMS SUPPRESSOR 1; UDT1, UNDEVELOPED TAPETUM 1; TDR, TAPETUM DEGENERATION RETARDATION; PTC1, PERSISTANT TAPETAL CELL 1.

Figure 4. 

Molecular pathways in tapetum contribution to pollen formation. The orange irregular shape represents the tapetal cell. The pathway regulates a large number of genes for pollen growth, which are shown below the tapetal cell, to provide Mg for pollen growth, to secrete enzymes for degradation of the pectin wall, for the callose wall to release microspores, to provide precursors of nexine and sexine, and to provide materials for pollen coat formation.