By Jia-wei Wang | May 29, 2015
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Introduction
Understanding the reproducibility of developmental programs between individuals of the same multicellular organism is a fundamental challenge in biology. This developmental precision reflects the conserved and intrinsic mechanisms that temporally regulate all the aspects of biological processes during development. The cues that guide these developmental events can also include extrinsic inputs, such as nutrients, light and temperature.
By contrast to animals, plants utilize different developmental programs from juvenility to maturity. After seed germination, the shoot apical meristem (SAM), a population of pluripotent stem cells at the shoot apex, begins to produce leaves. The plant reaches maturity when it becomes competent to exogenous or endogenous floral-inducing signals such as hormones, light and temperature . Upon the transition to reproductive growth, the SAM gives rise to flowers. Although different plants have dramatically different morphology, the program deployed to time developmental transitions is well conserved, which offers us an excellent system to study the reproducibility of developmental programs between individuals.
The plant developmental transitions are under genetic regulation and can be defined by the morphology of lateral organs. In principle, the mutations that affect developmental timing do not appreciably accelerate or retard the life cycle of the plant. Nor do they alter organ identity per se. Rather, they change the temporal identity of lateral organs to one normally expressed at a different time within the same lineage, but usually restricted to a distinct developmental stage. The observed temporal transformations in developmental timing mutants have been likened to the floral homeotic mutants, in which floral organ identities are spatially, rather than temporally, transformed .
The past decade has seen a great advance in our understanding of the molecular basis of plant developmental transitions. Mutant characterizations and genetic analyses in Arabidopsis thaliana reveal a central timing module governed by microRNA156 (miR156) and its regulator, sugar. In this review, we first describe the hallmarks of plant developmental transitions and then turn to summarize the roles of miR156 and sugars in plant developmental transitions. The related unanswered questions and future directions are then discussed.