Analysis of four maize mutants arrested in early embryogenesis reveals an irregular pattern of cell division

The process that leads to embryo formation appears to follow a defined pattern, whose sequential developmental steps - under strict genetic control - can be analysed through the study of mutants affecting embryogenesis. We present the analysis of four embryo-specific (emb) mutants of maize, characterised by abnormal development not overcoming the proembryo or early transition stage, that define three separate genes on the basis of their chromosomal location and complementation pattern. A common feature emerging from histological analysis is that suppression of morphogenesis is accompanied by an uncontrolled pattern of cell division. The block in embryo development is associated with abnormal suspensor proliferation, possibly due to the absence of a signal elaborated by the embryo proper and required for suspensor cell identity maintenance. Mutant endosperm morphogenesis is not impaired, as shown by the formation of the expected domains, i.e. aleurone, starchy endosperm, embryo-surrounding region and basal endosperm transfer layer. The program of cell death appears impaired in the mutants, as expected if this process is essential in determining the shape and morphology of the developing organs. An unexpected result is obtained when mutant embryo rescue is attempted. Immature embryos transferred to a basal medium germinated, yielding small but otherwise normal seedlings, an observation not consistent with the histological evidence of a complete absence of morphogenetic potential. The analysis of emb mutants appears a promising tool to elucidate crucial points of embryo development such as the coupling of cell division with morphogenesis, cell-to-cell interactions, the relationship between embryo and endosperm development, and the interaction between embryo proper and suspensor.