Developmental switches that hold the key to a revolution in crop biotechnology

Kaufmann et al. (Regulation of transcription in plants: mechanisms controlling developmental switches. Nature Rev. Genet. 11, 830–842 (2010))¹ have provided valuable insights into transcriptional regulation during well-known developmental switches in Arabidopsis thaliana — including the development of the inflorescence meristem from the apical meristem and the establishment of floral organ identities. Similar intensive studies of the developmental switches in the angiosperm nucellus are urgently needed to improve understanding of developmental reprogramming towards apomixis; characterizing the regulation of apomixis could lead to important biotechnological applications.

A nucellar cell normally undergoes meiosis to produce a haploid female gametophyte (FG, which includes the egg) during angiosperm sexual reproduction. In apomictic angiosperms, nucellar cells can avoid meiosis to produce an unreduced, diploid FG or even directly transdifferentiate into a somatic embryo (adventitious embryony) and eventually produce a clone. Manipulating these apomictic developmental switches to produce 'clones-through-seeds' has been a cherished objective in crop biotechnology². As Kaufmann et al. discuss, transcriptional regulators are known to initiate correct developmental transitions or cell fate specifications by coordinating the expressions of required genes¹. However, little is known about these aspects of apomictic developmental transitions.

For agriculture, the aim would be to produce asexual/adventitious embryos with genetic stability through generations. To achieve this, greater understanding of regulatory processes is required. Genetic variation among the progeny could be avoided by eliminating or suppressing meiotic recombination or meiosis itself. Although basic meiotic processes are conserved among species, diverse mechanisms control the mitosis-to-meiosis transition³. For example, the genes IME1 and IME2 in budding yeast, mei2 in fission yeast, stimulated by retinoic acid 8 (Stra8) in mice and ameiotic1 (am1) in maize regulate the initiation of meiosis in different ways³. In A. thaliana, mutants in which female meiosis is abolished — such as dyad (also known as swi1) mutants^4,5, spo11 rec8 osd1 triple mutants⁶ or ago9 mutants that fail to silence a transposable element-derived small RNA pathway⁷ — produce a functional but unreduced, diploid FG. Elimination of meiosis, however, may increase ploidy of both the embryo and endosperm over successive generations, and the implication of this for crop productivity is uncertain. Another approach that has been proposed is chromosome engineering to suppress meiotic recombination by using balancer chromosomes carrying localized inversions⁸.

A challenge in using adventitious embryony is how to combine it with a normally developed endosperm, either through fertilization (single fertilization) or autonomous development. The molecular mechanisms that underlie the developmental switch that directly transforms a somatic plant cell to an embryo are still elusive⁹. Several genes, such as WUSCHEL (WUS), SOMATIC EMBRYO RECEPTOR-LIKE KINASE 1 (SERK1), LEAFY COTYLEDON 1 (LEC1), LEC2 and BABY BOOM (BBM), can induce embryonic growth in plant somatic cells¹⁰, and Polycomb group proteins are also implicated in somatic embryogenesis¹¹. However, it is uncertain whether these pathways for reprogramming of plant somatic cells towards embryogenesis are conserved among species.

Given their immense biological and biotechnological significance, apomictic developmental switches deserve greater attention in future research.