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Arabidopsis development in fast-forward or slow-motion: shifts in developmental timing are revealed by the transition from juvenile leaves (round, smooth margins) to adult leaves (elongated, serrated margins).
Developmental timing governs when, where, how fast, and how long each step of development occurs.
Development isn’t just about what forms in an organism—it’s also about when it happens.
While chronological time progresses uniformly, developmental timing operates as a complex biological clock controlling the timing, location, speed, and duration of each step within the developmental program during growth and differentiation. It can speed up or slow down depending on various factors and is regulated by internal genetic programs responding to both internal signals and environmental cues.
This temporal regulation coordinates cell division, tissue morphogenesis, differentiation, and organ formation, ensuring the developmental events happen in the correct spatial and temporal order to support proper organismal function and optimal adaptation to the environment.
So, what exactly is Developmental Timing?
You’ve likely observed how developmental timing regulates the plasticity of plant growth to adapt to the environment.
Take, for instance, the English Ivy (Hedera helix) climbing up a brick wall. The leaves growing on the lower portions of the plant (juvenile leaves) are typically lighter green, thinner, and deeply lobed. These juvenile leaves are adapted for low-light environments found closer to the ground or in shaded areas. The lobed structures give them a higher surface-to-mass ratio, which is beneficial for capturing diffuse light under a canopy.
In contrast, those higher up (adult leaves) are darker green, much thicker, and unlobed or ovate. The adult leaves are more robust and have higher photosynthetic capacity, enabling the plants to excel in a brighter environment. The higher photosynthetic efficiency supports energy-demanding processes, like flowering and fruit production.
The morphological transition between these two leaf forms represents an evolutionarily conserved developmental strategy switch within a single plant. Interesting, the transition point (timing) varies from plants growing in different environments. In full sun, plants transition from juvenile to adult leaf forms closer to the ground, as abundant light accelerates development and reduces the need for prolonged juvenile growth. In contrast, in deep shade (e.g., under dense vegetation or on the north side of buildings), plants may retain juvenile traits over longer distances, delaying the shift to adult morphology until they reach brighter areas.
Therefore, in ivy (Hedera helix), variability in the timing of leaf identity transition reflects how the juvenile-to-adult vegetative phase change fine-tunes plant development timing in response to both internal age and external cues. Beyond regulating vegetative phase change, developmental timing orchestrates at least the following critical processes throughout the plant life cycle:
Embryogenesis and seed germination
Organ initiation and patterning
Acquisition of reproductive competence
Fruit development, ripening, and seed maturation
Cyclical physiological processes (e.g., leaf movement, stomatal opening, hormonal cycles)
Seasonal and dormancy transition
Changes in Developmental Timing is a Significant Part of Evolution
Ontogeny and Phylogeny by Stephen Jay Gould
Alterations in the timing, rate, or duration of developmental processes compared to ancestors is a major mechanism driving evolutionary change, influencing the shape, size, and the features of organisms by shifting when key developmental events occur. Therefore, changes in Developmental Timing can help explain much of the morphological diversity observed across both the animal and plant kingdoms. This suggests that evolution can act by modifying not just genes, but also the schedules of developmental itself. Such shifts can lead to increased diversity among species and even sometimes to the emergence of new species.
At DePaul University, our lab investigates the molecular and genetic mechanisms that regulate developmental timing using plants (e.g., Arabidopsis thaliana) as the model organisms. Unlike animals, whose development ends at maturity with a fixed body plan, plants offer a unique advantage to scientists studying development timing: plants continue producing new organs throughout their lives, therefore their organ number and structure are clearly visible even in the adult form. By simply counting and observing these structures, we can gain insights into the rate and timing of developmental processes. This makes plants an excellent model for understanding how organisms regulate not just what they become, but when they become it.
Long-Lasting Questions that Drive Our Lab Research
What are the molecular and genetic bases that precisely control the timing of developmental phase transitions in plants?
How do microRNAs function in orchestrating these timing events, and are their roles conserved across different species and developmental stages?
What is the influence of environmental signals on developmental timing, and how is this integrated at the molecular level?
How do changes in developmental timing contribute to plant adaptation and evolution?
Can understanding these timing mechanisms provide practical tools for agricultural improvement and biotechnology?
How is Plant Developmental Timing Measured?
Leaf Heteroblasty: (Left) 3-week-old Arabidopsis thaliana plant. The number represents the emergence sequence of the rosette leaves generated during vegetative growth. (Right) Leaf 1&2: cotyledon, embryonic leaves. Leaf 3-6, juvenile rosette leaves. Leaf 7-10, adult rosette leaves. Copyright@Jianfei Zhao.
The above figure represents the leaf heteroblasty in Arabidopsis thaliana growing in the long-day condition (16-hour light: 8-hour dark). This plant ongoing growth, regulated by the shoot apical meristem, follows a predictable sequence known as heteroblasty. As a plant ages, it generates different types of above-ground organs in a specific order. First come the cotyledons (Leaf 1&2, embryonic leaves), followed by juvenile leaves (Leaf 3-5), then adult leaves (Leaf 7-10), and eventually bracts and floral organs like sepals and petals. Even stamens and carpels, which don’t look like leaves, are thought to be highly modified versions of them, tracing back to a shared developmental origin.
The timing and rate of new organ formation in plants—measured by parameters like the plastochron (the interval between successive leaf initiations) or the number of organs produced over time—are regulated by genetic programs responsive to both internal and environmental cues. Because these structures are easily counted and observed, plants serve as excellent models for unraveling how organisms control not only their form but also the timing of developmental transitions.
Developmental timing can move faster or slower depending on various factors and is regulated by internal genetic programs that respond to both internal signals and environmental cues. In plants, one way to measure this is by looking at the plastochron, which is the interval between the formation of two successive leaves. Another is to track the developmental rate, or the number of new organs (like leaves or flowers) produced over a given period.
Through Regulation of Their Target Genes, microRNAs Orchestrate multiple critical timing points of plant growth and development, including the Shift from Juvenile to Adult Vegetative Phase and When Plants Become Reproductively Competent. Please check the Research webpage for details.
Benefits from Studying Developmental Timing in Plants
Understanding the mechanisms of vegetative phase change offers farmers powerful insights to improve crop management and productivity. This developmental transition—from juvenile to adult growth stages—affects traits such as leaf shape, plant architecture, and the timing of flowering. By learning how phase change is regulated, farmers can better control when crops mature, enhance stress resilience, and increase yield quality and consistency. For example, delaying or accelerating phase transitions can help crops better match local growing seasons or environmental conditions. Knowledge of these mechanisms also supports more targeted breeding strategies, enabling the development of crop varieties with improved performance, adaptability, and market value.
By deciphering these timing mechanisms in plants—using easily measurable traits like plastochron or organ formation rates—scientists gain insight into universal principles of developmental biology, applicable to both plant and animal systems. This connects plant developmental timing research directly to broader fields such as evolutionary biology, genetics, and systems biology, as similar regulatory networks often control growth and differentiation across life forms.
Furthermore, there are clear links to ecology and environmental science, as understanding how plants adjust their developmental timing in response to climate or stress helps predict ecosystem responses to environmental change.
Biotechnological applications also stand to benefit: manipulating developmental timing could accelerate breeding programs, optimize crop yield, enhance stress resilience, or enable precision agriculture using gene-editing technologies. Thus, plant developmental timing research not only addresses foundational scientific questions but also fuels innovation across agriculture, ecology, and biotechnology.
Our lab is in the beautiful Lincoln Park Campus of DePaul University in Chicago.
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