53 The identification of several mutations and genes involved in sporogenesis and gametogenesis has initiated a genetic framework for understanding gametophyte biogenesis. Recent advances include the molecular characterization of genes required for sporocyte formation and meiosis. These studies have revealed some unexpected interactions linking development of sporophytic cells and tissues with initiation and progression of gametophyte development in angiosperms. Addresses Institute of Molecular Agrobiology, 1 Research Link, The National University of Singapore, Singapore 117604 *e-mail: weicai@ima.org.sg † e-mail: director@ima.org.sg Current Opinion in Plant Biology 2000, 3:53–57 1369-5266/00/$ — see front matter © 2000 Elsevier Science Ltd. All rights reserved. Abbreviations ant aintegumenta GC generative cell gem1 gemini pollen 1 gf/gfa gametophytic factor sap sterile apetala SPL SPOROCYTELESS VC vegetative cell Introduction The alternation between a diploid sporophytic generation and a haploid gametophytic generation is fundamental to the life cycle of plants. In higher plants, the formation of the gametophyte from the sporophyte is the result of two sequential processes, sporogenesis and gametogenesis. The molecular and genetic mechanisms controlling these processes are as yet poorly understood. In recent years, genetic studies have begun to reveal mutations and, in some cases, the corresponding genes that control these processes. Genes controlling ovule and pollen develop- ment have been reviewed extensively [1,2 • –5 • ,6 •• ]. In this article, we review progress over the past year in the area of male and female gametophyte development, focusing on genetic studies from the model plant Arabidopsis thaliana. Formation of sporocytes Sporogenesis starts with the differentiation of hypodermal cells of the ovules and anthers to form archesporial cells, which subsequently differentiate into sporocytes and under- go meiosis. In the ovule, the archesporial cell is a single cell that differentiates from the hypodermal layer at the distal end of the nucellus. In maize the recessive mac1 mutation results in the formation of multiple archesporial cells in ovules [7]. It is likely that Mac1 regulates archesporial cell specification in the ovule primordium through a signal that suppresses differ- entiation of neighboring hypodermal cells. Thus far, no similar mutation has been identified in Arabidopsis. Recently, a gene required for the initiation of sporogenesis, SPOROCYTELESS (SPL), has been identified in Arabidopsis [8 •• ]. In spl mutants, subepidermal cells of anther and ovule primordia are able to form archesporial cells but subsequently fail to form megasporocytes or microsporocytes. In spl ovules, integument development is unaffected; however, in spl anthers, formation of the anther walls and tapetum is also disrupted. The archesporial cells divide to form sporogenous cells and primary parietal cells but further development of both cell layers is arrested. The SPL gene has been cloned and shown to encode a nuclear protein with limited similarities to MADS box transcription factors. SPL expression is restricted to sporogenous cells and microsporocytes in anthers and to megasporocytes in the ovules. Thus the primary role of SPL may be to promote the formation of male and female sporocytes and the defect in anther wall development is likely to be indirect. The implication is that development of the anther walls and tapetum is dependent upon signals from the microsporocytes; in their absence, sporophytic development of the anther cannot proceed. Interactions between the megasporocyte and sporophytic tissues Sporophytic tissues such as integuments (Figure 1) play a role in the progression of the meiotic divisions of the megas- porocyte as defects in integument development in Arabidopsis result in meiotic arrest. This is demonstrated by the aintegumenta (ant) and bel1 mutants in which the integu- ments are either missing (ant) or abnormal (bel1) [9–11]. The exact nature of this role is unclear, however, and it is possible that integuments simply play a supportive (physiological or physical) role in promoting meiosis. Recently, a mutant called sap (sterile apetala) has been described in which megas- porogenesis is arrested during or just after the first meiotic division, although integument development is normal [12]. Surprisingly, while the SAP gene is expressed initially in the Genetics of gametophyte biogenesis in Arabidopsis Wei-Cai Yang* and Venkatesan Sundaresan † Figure 1 Schematic drawing of a 7-cell ovule near maturity in Arabidopsis. Chalaza Antipodals Funiculus Central cell Egg cell Micropyle Synergids Inner integument Outer integument Current Opinion in Plant Biology