title: In response to partial plant shading, the lack of phytochrome A does not directly induce leaf senescence but alters the fine-tuning of chlorophyll biosynthesis.
abstract: Phytochrome is thought to control the induction of leaf senescence directly, however, the signalling and molecular mechanisms remain unclear. In the present study, an ecophysiological approach was used to establish a functional connection between phytochrome signalling and the physiological processes underlying the induction of leaf senescence in response to shade. With shade it is important to distinguish between complete and partial shading, during which either the whole or only a part of the plant is shaded, respectively. It is first shown here that, while PHYB is required to maintain chlorophyll content in a completely shaded plant, only PHYA is involved in maintaining the leaf chlorophyll content in response to partial plant shading. Second, it is shown that leaf yellowing associated with strong partial shading in phyA-mutant plants actually correlates to a decreased biosynthesis of chlorophyll rather than to an increase of its degradation. Third, it is shown that the physiological impact of this decreased biosynthesis of chlorophyll in strongly shaded phyA-mutant leaves is accompanied by a decreased capacity to adjust the Light Compensation Point. However, the increased leaf yellowing in phyA-mutant plants is not accompanied by an increase of senescence-specific molecular markers, which argues against a direct role of PHYA in inducing leaf senescence in response to partial shade. In conclusion, it is proposed that PHYA, but not PHYB, is essential for fine-tuning the chlorophyll biosynthetic pathway in response to partial shading. In turn, this mechanism allows the shaded leaf to adjust its photosynthetic machinery to very low irradiances, thus maintaining a positive carbon balance and repressing the induction of leaf senescence, which can occur under prolonged periods of shade.
authors.affiliation: Umeå Plant Science Centre, Department of Plant Physiology, Umeå University, S-90187 Umeå, Sweden.
authors.forename: Bastiaan
authors.initials: B
authors.lastname: Brouwer
authors.affiliation: Umeå Plant Science Centre, Department of Plant Physiology, Umeå University, S-90187 Umeå, Sweden.
authors.forename: Per
authors.initials: P
authors.lastname: Gardeström
authors.affiliation: Umeå Plant Science Centre, Department of Plant Physiology, Umeå University, S-90187 Umeå, Sweden olivier.keech@umu.se.
authors.forename: Olivier
authors.initials: O
authors.lastname: Keech
mesh_terms: D017360:Arabidopsis / Q000235:genetics / Q000502:physiology* / Q000528:radiation effects; D029681:Arabidopsis Proteins / Q000378:metabolism; D002245:Carbon Dioxide / Q000378:metabolism; D019069:Cell Respiration / Q000235:genetics / Q000528:radiation effects; D002734:Chlorophyll / Q000096:biosynthesis*; D018506:Gene Expression Regulation, Plant / Q000528:radiation effects; D017343:Genes, Plant; D008027:Light; D009154:Mutation / Q000235:genetics; D010788:Photosynthesis / Q000235:genetics / Q000528:radiation effects; D051942:Phytochrome A / Q000172:deficiency* / Q000378:metabolism; D051939:Phytochrome B / Q000378:metabolism; D018515:Plant Leaves / Q000235:genetics / Q000254:growth & development* / Q000528:radiation effects; D055550:Protein Stability / Q000528:radiation effects; D015398:Signal Transduction / Q000235:genetics / Q000528:radiation effects
chemical_list: D029681:Arabidopsis Proteins; C497973:PHYA protein, Arabidopsis; C497952:PHYB protein, Arabidopsis; D051942:Phytochrome A; D051939:Phytochrome B; D002734:Chlorophyll; D002245:Carbon Dioxide
keywords: Arabidopsis; chlorophyll; far-red light; phytochrome; senescence; shade.