Figure 1.

Pre-meiotic anther development. (A) The four-lobed anther typical of flowering plants with a central column of vasculature that extends into the stamen filament surrounded by connective tissue [stage 8]. (B) Tracings of confocal images of single lobes of the W23 maize inbred are colorized to show the progression of cell fate specification and anther lobe patterning. At stage [1] the lobe consists of pluripotent Layer1- and Layer2-derived cells, colored in beige and light gray, respectively. For all cell types, just-specified cells are colorized in a pale shade, which gradually darkens as the cells acquire stereotyped differentiated shapes, volumes, and staining properties. The first specification event results in visible archesporial (AR) cells centrally within each lobe. In maize, the glutaredoxin encoded by Msca1 responds to growth-generated hypoxia to initiate AR differentiation, marked by secretion of the MAC1 protein, which is required for cell specification of the subepidermal L2-d cells to Primary Parietal Cells (PPC) [stage 2]. PPC divide periclinally generating the subepidermal Endothecium (EN) and the bipotent Secondary Parietal Cells (SPC). In the same timeframe, Epidermal (EPI) cells differentiate; signals controlled by expression of the OCL4 epidermal-specific transcription factor suppress excess periclinal divisions in the EN (Vernoud et al., 2009) [stage 3]. Following these early patterning events that result in a three-layered wall surrounding the AR, there is a period of anticlinal division that expands anther cell number and organ size [stage 4]. Subsequently, each SPC divides once periclinally to generate the ML and TAP and the final four somatic walled architecture of the pre-meiotic anther lobe is achieved [stages 5–7]. Prior to meiosis, anticlinal divisions occur to increase anther size, and the individual cell types acquire differentiated properties [stages 6–8], including dramatic enlargement of AR as they mature into Pollen Mother Cells (PMC) capable of meiosis [stage 8]. Comparison of anther lengths at the 8 stages plus meiotic entry in Arabidopsis (Smyth et al., 1990; Sanders et al., 1999), maize (Kelliher and Walbot, 2014; Zhang et al., 2014), and rice (Zhang et al., 2011) is summarized in the table; lengths marked with an (^*) are inferred assuming linear growth in length in between known stages. (C) Longitudinal view of an anther lobe [stage 3] illustrates how AR column formation occurs simultaneously within both the tip and base of the lobe and that periclinal division of the PPC is stochastic. Parts of the illustrations in 1A,B are based on figures published in Zhang et al. (2014).