Polarity and signalling in plant embryogenesis.

The establishment of the apical-basal axis is a critical event in plant embryogenesis, evident from the earliest stages onwards. Polarity is evident in the embryo sac, egg cell, zygote, and embryo-suspensor complex. In the embryo-proper, two functionally distinct meristems form at each pole, through the localized expression of key genes. A number of mutants, notably of the model genetic organism Arabidopsis thaliana, have revealed new gene functions that are required for patterning of the apical-basal axis. There is now increasing evidence that two particular modes of signalling, via auxin and cell wall components, play important roles in co-ordinating the gene expression programmes that define determinative roles in the establishment of polarity.