Spatial patterning in most multicellular organisms requires genes to both establish regions of cell differentiation and specify cellular fate. In the early Drosophila
embryo, for example, cells are organized into boundaries by the pair rule and segment polarity genes, then they acquire distinct fates through homeotic gene expression [1
]. Homeotic genes are also required to establish boundaries during temporal patterning, whereas heterochronic genes define the timing of the cell fate decisions within those boundaries [2
]. One challenge in developmental biology is to identify and understand the overall developmental role of genes involved in the temporal patterning of genetic programs.
Higher plants are well-suited for identifying genes involved in developmental timing because they continually produce easily distinguishable organs throughout the life cycle, whose fates are dependent on the time of emergence [3
]. The types of leaves that emerge over time often show distinctive developmental changes that allow them to be classified into juvenile and adult leaves. Later, when a plant enters reproductive development, the vegetative meristematic region switches to an inflorescence meristem that produces flower bracts with floral meristems in their axils [4
]. Genetic analysis in Arabidopsis thaliana
has identified a myriad of genes that converge to control the juvenile to adult leaf transitions and the switch of the vegetative meristem to reproductive development [5
Unlike flowers and leaves, which form from a shoot apical meristem, the developmental relationship between embryonic leaves (or cotyledons) and adult foliar organs is complicated by cotyledon formation during embryonic patterning. Furthermore, in many plants such as Arabidopsis, cotyledons switch from a storage organ to a more leaflike photosynthetic organ soon after germination. Despite these complexities, single loss-of-function mutations in Arabidopsis have been identified in three genes, LEAFY COTYLEDON1
), LEAFY COTYLEDON2
) and FUSCA3
), whose mutations result in the replacement of cotyledons with organs more similar to vegetative leaves [6
]. In lec1
mutants, genes that encode markers of late embryogenesis are reduced or missing [9
]. By contrast, germination markers that normally proceed late embryogenesis are precociously activated. These expression patterns suggest that LEC1
may establish temporal boundaries. Although little is known about how these genes contribute to temporal patterns, it is known that FUS3
regulates and is regulated itself by the synthesis of two terpenoid hormones, abscisic acid (ABA) and gibberellins (GA) [10
]. The ratio of these two hormones contributes to proper cotyledon patterning by regulating the rates of cell cycling [11
Although extensive analyses of LEC1, LEC2
gene action have been carried out with respect to embryogenesis, the effects of these mutations on vegetative leaf development have not been studied extensively [11
]. It has been shown that after germination, the first juvenile leaves of lec1
seedlings are shifted toward later leaf identities; however, this shift is not maintained, and successive leaves and flowering time were corrected back to a wild-type pattern [15
]. This suggests that embryonic leaf development can have a restricted impact on future vegetative leaf identities. What remains unclear, however, is how cotyledon development impinges on later vegetative development, which is temporally and spatially distinct.
To address such questions, we decided to use a combination of controlled FUS3 activation during vegetative development with whole-genome transcript profiling. Using this approach, we discovered that FUS3 downregulates a collection of genes involved in ethylene biosynthesis and signaling. Consistent with this finding, loss-of-function fus3 mutants show ectopic ethylene responses at both the developmental and molecular levels. The fus3 plants also show precocious vegetative phase change; however, unlike the lec1 mutants, this change is not corrected at later adult stages. More importantly, the accelerated vegetative phase transition can be suppressed by inhibiting ethylene action either genetically or pharmacologically. Thus it appears that this previously defined embryonic regulator also has roles in vegetative development. One role of FUS3 during early seedling growth is to dampen ethylene action, which in turn contributes to a slowing of subsequent vegetative phase transitions. These results add ethylene to the list of hormones that contribute to temporal patterning in Arabidopsis.