Abstract
Plant growth is commonly regulated by external cues such as light, temperature, water availability, and internal cues generated by the circadian clock. Changes in the rate of growth within the course of a day have been observed in the leaves, stems, and roots of numerous species. However, the relative impact of the circadian clock on the growth of grasses has not been thoroughly characterized. We examined the influence of diurnal temperature and light changes, and that of the circadian clock on leaf length growth patterns in Brachypodium distachyon using high-resolution time-lapse imaging. Pronounced changes in growth rate were observed under combined photocyles and thermocycles or with thermocycles alone. A considerably more rapid growth rate was observed at 28°C than 12°C, irrespective of the presence or absence of light. In spite of clear circadian clock regulated gene expression, plants exhibited no change in growth rate under conditions of constant light and temperature, and little or no effect under photocycles alone. Therefore, temperature appears to be the primary cue influencing observed oscillations in growth rate and not the circadian clock or photoreceptor activity. Furthermore, the size of the leaf meristem and final cell length did not change in response to changes in temperature. Therefore, the nearly five-fold difference in growth rate observed across thermocycles can be attributed to proportionate changes in the rate of cell division and expansion. A better understanding of the growth cues in B. distachyon will further our ability to model metabolism and biomass accumulation in grasses.
Highlights
Primary growth in plants is the product of cell division and elongation
Growth rate dramatically increased within the first hour of the day and the growth rate decreased rapidly following the transition from light to dark (Fig. 2A, Movie S1–2)
Growth rhythms of B. distachyon under diurnal conditions were typical of other grass species grown both in the field and controlled environments with a peak growth rate observed during the day [15,25,28]
Summary
Primary growth in plants is the product of cell division and elongation. Cell division occurs in the meristems where pluripotent undifferentiated cells are located. The elongation process is largely caused by changes in turgor pressure via water intake and storage in the central vacuole accompanied by loosening of the cell wall. This process results in increases in leaf, stem, and root length. As with myriad behavioral and physiological traits in plants, growth rhythms are initiated by endogenous mechanisms and external cues [1,2,3]. Two external cues implicated in driving growth rhythms are photocycles and thermocycles
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