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Tapetal Cell Fate, Lineage and Proliferation in the Arabidopsis Anther Xiaoqi Feng and Hugh G

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Tapetal Cell Fate, Lineage and Proliferation in the Arabidopsis Anther Xiaoqi Feng and Hugh G RESEARCH ARTICLE 2409 Development 137, 2409-2416 (2010) doi:10.1242/dev.049320 © 2010. Published by The Company of Biologists Ltd Tapetal cell fate, lineage and proliferation in the Arabidopsis anther Xiaoqi Feng and Hugh G. Dickinson* SUMMARY The four microsporangia of the flowering plant anther develop from archesporial cells in the L2 of the primordium. Within each microsporangium, developing microsporocytes are surrounded by concentric monolayers of tapetal, middle layer and endothecial cells. How this intricate array of tissues, each containing relatively few cells, is established in an organ possessing no formal meristems is poorly understood. We describe here the pivotal role of the LRR receptor kinase EXCESS MICROSPOROCYTES 1 (EMS1) in forming the monolayer of tapetal nurse cells in Arabidopsis. Unusually for plants, tapetal cells are specified very early in development, and are subsequently stimulated to proliferate by a receptor-like kinase (RLK) complex that includes EMS1. Mutations in members of this EMS1 signalling complex and its putative ligand result in male-sterile plants in which tapetal initials fail to proliferate. Surprisingly, these cells continue to develop, isolated at the locular periphery. Mutant and wild-type microsporangia expand at similar rates and the ‘tapetal’ space at the periphery of mutant locules becomes occupied by microsporocytes. However, induction of late expression of EMS1 in the few tapetal initials in ems1 plants results in their proliferation to generate a functional tapetum, and this proliferation suppresses microsporocyte number. Our experiments also show that integrity of the tapetal monolayer is crucial for the maintenance of the polarity of divisions within it. This unexpected autonomy of the tapetal ‘lineage’ is discussed in the context of tissue development in complex plant organs, where constancy in size, shape and cell number is crucial. KEY WORDS: Anther tapetum, Arabidopsis, Cell fate establishment, EMS1, Reproductive cell lineage INTRODUCTION downstream of the floral ‘C’ gene AGAMOUS (AG) (Ito et al., Unlike animals, which can initiate germlines as early as 2004) to promote sporogenous cell identity (Schiefthaler et al., embryogenesis, plant reproductive cells form late in development. 1999; Yang et al., 1999) and is reported to interact with the leucine- All male reproductive cells of Arabidopsis develop within rich repeat receptor-like kinases (LRR-RLKs) BARELY ANY concentrically organised microsporangia derived from archesporial MERISTEM 1 and 2 (BAM1 and BAM2) (Hord et al., 2006). cells in the L2 of the stamen primordium (Fig. 1A,E). These BAM1 and BAM2 are held to promote somatic cell fates, first in archesporial cells divide periclinally to form an inner primary the outward-facing products of the archesporial cell division, and sporogenous cell and an outer primary parietal cell (Fig. 1B,E). subsequently during microsporangial expansion by restricting the Primary sporogenous cells continue division to give rise to a central expression domain of SPL to the locular centre (Hord et al., 2006). sporogenous mass, and consecutive divisions of neighbouring cells In turn, SPL appears to promote BAM1 expression, possibly result in the formation of three concentric parietal layers – the creating a regulatory loop similar to that involving CLAVATA 1 tapetum adjacent to the sporogenous cells, a middle layer, and the (CLV1) and WUSCHEL (WUS) in shoot apical and floral meristems endothecium subjacent to the epidermis (Fig. 1C-E) (Canales et al., (Hord et al., 2006). 2002; Feng and Dickinson, 2007; Scott et al., 2004; Sorensen et al., Also playing an important part in microsporangial patterning, the 2002). In common with the development of other plant LRR-RLK EXCESS MICROSPOROCYTES 1 (EMS1; also reproductive organs, where consistency in size and shape is known as EXTRA SPOROGENOUS CELLS, EXS) (Canales et essential for effective function, microsporangial formation does not al., 2002; Zhao et al., 2002) is reported to form a receptor complex involve ‘classical’ plant meristems, but rather finite numbers of with two further LRR-RLKs – SOMATIC EMBRYOGENESIS divisions within apparent cell lineages – reminiscent of RECEPTOR-LIKE KINASE1 and 2 (SERK1 and SERK2) – in the development in some animal tissues. plasma membrane of tapetal cells (Albrecht et al., 2005; Colcombet Patterning of the concentric microsporangia involves the et al., 2005). This complex is held to bind TAPETUM interplay of many genes (Feng and Dickinson, 2007; Ma, 2005; DETERMINANT 1 (TPD1) (Yang et al., 2005; Yang et al., 2003), Wilson and Yang, 2004). The transcription factor a putative ligand synthesised by the microsporocyte mass, and is SPOROCYTELESS (SPL; also known as NOZZLE, NZZ) reported to specify tapetal cell fate (Albrecht et al., 2005; Feng and (Schiefthaler et al., 1999; Yang et al., 1999) acts directly Dickinson, 2007; Jia et al., 2008; Ma, 2005; Yang et al., 2005). ems1 microsporangia have no tapetal layer but excess microsporocytes (Canales et al., 2002; Zhao et al., 2002), a Department of Plant Sciences, University of Oxford, South Parks Road, Oxford OX1 phenotype shared with serk1 serk2 double mutant lines (Albrecht 3RB, UK. et al., 2005; Colcombet et al., 2005) and tpd1 plants (Yang et al., 2003). How this EMS1 complex mediates the formation of a *Author for correspondence ([email protected]) functional tapetal layer is unknown; models range from a role in Accepted 14 May 2010 regulating the number of functional archesporial cells (Canales et DEVELOPMENT 2410 RESEARCH ARTICLE Development 137 (14) al., 2002) to the specification of tapetal fate in the inner division lines of Arabidopsis (Dilkes et al., 2008; Paul et al., 1992). These plants products of the inner secondary parietal cells, cells which, in ems1 form a full complement of microsporocytes and tapetal cells, but the latter mutant lines, are held to revert to a default microsporocyte fate – become highly vacuolated during the proliferation phase (S5; see Fig. S1 implying a switch in fate within a cell lineage (Zhao et al., 2002). in the supplementary material), expand rapidly and degenerate after S5, In an attempt to define further the role of the EMS1 complex in confirming pA9 to be tissue specific and active during the proliferation of the specification of the Arabidopsis tapetum and the mechanism by the tapetum. which it proliferates as a monolayer, we expressed EMS1 Light microscopy and measurements ectopically in ems1 plants after its normal period of expression. Material was prepared as previously described (Sorensen et al., 2002), These experiments show that a small population of founder cells is observed under an Olympus BX50 microscope and imaged using Image- committed to a tapetal fate very early in development, but that the Pro 6.2 software. The longitudinal mid-point of abaxial microsporangia EMS1 complex is not involved in this early specification. from the medial anthers of anther stage 6 was chosen for cell number However, the EMS1 complex is required for the subsequent measurement. Mean value and s.d. were calculated from 150 independent proliferation of these cells to form the tapetal monolayer, but not locules of each line. divisions in any other cell layer. We also show competition for Transmission electron microscopy space to exist between the products of the microsporocyte and Material was prepared as previously described (Sorensen et al., 2002). tapetal cell lineages. This early independence of the tapetal lineage and its ability to interact with neighboring cell layers provide the In situ hybridisation A 1157-bp fragment for EMS1 detection was amplified using primers first clues as to how these small but complex plant organs develop XH402F/XH402R (Zhao et al., 2002) using wild-type cDNA as template; – apparently in the absence of functional meristems. similarly, a 305-bp A9 fragment was amplified using primers XH401F/XH401R, and a fragment for A7 detection was made using DNA MATERIALS AND METHODS from vector pMC1577 kindly provided by Hong Ma (Zhao et al., 2002; Plant materials and growth conditions Rubinelli et al., 1998). Fragments were cloned into the pGEM T-Easy Wild-type (wt) Arabidopsis thaliana (Linneaus) Landsberg erecta (Ler) vector (Promega) and checked by sequencing. In situ hybridisation was plants were used unless otherwise specified. exs1, exs2, exs3 seeds were carried out on 8-mm paraffin wax-embedded sections using antisense and available in the laboratory (Canales et al., 2002). ems1 seeds were kindly sense probes for each gene as previously described (Lincoln et al., 1994). provided by Hong Ma (Pennsylvania State University, USA) (Zhao et al., 2002); tpd1 seeds by De Ye (China Agricultural University, China) (Yang Primers et al., 2003); and serk1 serk2 seeds by Sacco de Vries (Wageningen See Table S1 in the supplementary material for a list of primers used. University, Holland) (Albrecht et al., 2005). Rod Scott (Bath University, UK) provided pA9::Barnase lines in both Ler and Columbia-0 backgrounds (Dilkes et al., 2008). The ems1 line was genotyped using RESULTS XF301, XF504 and XF3R primers, and tpd1 (Yang et al., 2003) and serk1 Tapetal initials in the Arabidopsis anther are serk2 (Albrecht et al., 2005) mutants were genotyped as previously specified early, and require the EMS1 receptor described. Plants were grown at 21°C on a 16 hour light/8 hour dark cycle. kinase to proliferate RT-PCR analysis To investigate the role of the EMS1 complex in tapetal Biological replicates comprised floral buds at flower stages (Smyth et al., development, we first examined events immediately following the 1990) 1-14, collected from three plants, and three independent replicates division of the archesporial cell in wild-type Arabidopsis (Fig. 1A- were used from each line. Quantitative RT-PCR (qRT-PCR) was performed I). At this stage, and only in the region under the radius of the with an Applied Biosystems 7300 Real-Time PCR system, using anther lobe (distant from the connective), the primary parietal cell XG102F/XG102R primers for AtA9 and At/AhEF1a primers (Becher et al., and its adjacent two or three L2 cells (in transverse section) 2004) as a control.
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