The aesthetic quality of ornamental plants is affected by several criterions, including the global shape of the plant. It is the result of buds activity along stems and of the growth of the shoot produced. In many plants, environmental factors have a strong impact on plant architecture, which generates this form. Among these factors, light, both in intensity or in quality, may modulate the ability of bud to burst. Apart from this observation made in several species, only few studies have tried to identify the mechanisms involved in the light control of this process. To address this issue, we have started works on many rose bush varieties. These works are based on morphological, histological, biochemical and molecular studies. Our studies were carried out on experimental plants, defoliated and decapitated over the three basal buds of the main stem. Working on such plant model allows to limit the impact of correlative inhibitions between organs, which may block or interact with the mechanisms of photocontrol. Our results demonstrate that light is absolutely required to trigger bud burst in rose. Indeed, no bud burst occurred under darkness among the six varieties studied, which is not the case in most species like Arabidopsis thaliana, tomato or poplar. Shading experiments have shown that bud directly perceives the light signal necessary for burst, thus indicating that a short-distance signalling takes place between the receptive organ (bud scales) and the growing organs (shoot apical meristem, young leaves). Light, both in terms of intensity and quality, modulates bud burst. Thus, low light intensities (2 +mol.m-2.s-1) are sufficient to allow the growth of preformed leaves but not to trigger the shoot apical meristem organogenic activity. Blue and red lights, unlike far-red light, induce both these processes. A question comes up then: what are the key physiological processes involved in bud burst that are under light control ? Two physiological processes have been particularly studied : sugar metabolism and cell wall expansion. Our results show that, under light, bud bursting is associated with an influx of water and sugars in the bud, and with sucrose degradation into fructose and glucose. Light acts by promoting the transcription of genes and/or the activities of sucrose degradation enzymes such as vacuolar acid invertase and sucrose synthase. Cell elongation is also stimulated in light conditions as shown by our scanning electron microscopy experiment. Under darkness, these processes are reduced or inhibited and do not offer the optimal conditions necessary to bud burst. Thus, gene expression of an expansin, a protein involved in cell wall loosening during cell elongation, is highly reduced. In response to the stress caused by a long dark period, an increased transcription of a sorbitol dehydrogenase gene takes place, that could take over from sugar metabolism for cell survival. Thus, our study demonstrates that the photocontrol of bud burst occurs through at least two physiological processes : sugar metabolism and cell elongation. Given our results and those of the literature, we propose a model of control of bud burst by light.
