Light, being indispensable for plant growth and photosynthesis, is an important factor that determines stem orientation. Plants have sophisticated mechanisms to interpret environmental stimuli so as to optimize resource allocation at any time. The primary gravitropic response is concomitant with a modification of the proteome, consisting of an over accumulation of energy and metabolism associated proteins, which may allow the stem to reorient rapidly after bending. Little or no response was observed at the stem bases of the same plants.
Our study demonstrates a rapid gravitropic response in apices of maritime pine seedlings inclined >30°. Degradation of Rubisco was implicated in some protein shifts. A group of thirty eight proteins were found to be involved in primary metabolism and energy related metabolic pathways. Early response gravitropic associated proteins were identified, which are known to function in energy related and primary metabolism. Significant changes were determined in the protein accumulation of 68 protein spots. A total of 486 spots were quantitatively analyzed using image analysis software. Two-dimensional gel electrophoresis coupled with mass spectrometry was then used to describe the molecular response of stem bending involved in photo- and gravi-tropism after 22 hr and 8 days of treatment. In plants inclined at 30° and 45°, the apex first reoriented in the vertical plane after 2 h, then turned towards the light source after 24 h. In control and 15° inclined plants, the apex turned directly towards the light source after only 2 h.
Depending on the initial angle of stem lean, the apical response to the lateral light source differed. A lateral light source illuminated the plants and stem movement over time was recorded. Seedlings were inclined at 15, 30 and 45 degrees with vertical plants as controls. We quantified the speed of maritime pine seedlings to reorient with regard to light and gravity over 22 days. Phototropism and gravitropism responses enable the plant to reorient with regard to light and gravity. Plants are subjected to continuous stimuli from the environment and have evolved an ability to respond through various growth and development processes.
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