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.