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.
Highlights
For most plants, survival and reproductive capacity depend on an ability to optimize photosynthetic yield and mobilize resources efficiently
To assess which phytochrome was involved in mediating the loss of chlorophyll in response to partial plant shading, mature Arabidopsis wild-type and the two null-mutant plants phytochrome A and phytochrome B were used
PhyA-5 leaves were indistinguishable from the Ws wt leaves, either under growth light or when darkened; instead they showed significantly lower chlorophyll content than Ws wt leaves when shaded below 10 μmol m–2 s–1 (Fig. 1b)
Summary
Survival and reproductive capacity depend on an ability to optimize photosynthetic yield and mobilize resources efficiently. Throughout evolutionary history plants have developed adaptive strategies to cope with a wide variety of stresses. One of these adaptive strategies is leaf senescence. This genetically controlled process (Yoshida, 1962) is characterized by leaf yellowing, which results from the active degradation of chlorophyll (Pružinská et al, 2005; Schelbert et al, 2009), proteins (Martínez et al, 2008), and nucleic acids (Buchanan-Wollaston et al, 2003). The senescence-associated degradation contributes strongly to the remobilization of growth-limiting nutrients such as nitrogen, phosphorus, and sulphur from senescing organs towards other parts of the plant (Snapp and Lynch, 1996; Masclaux-Daubresse et al, 2008). Leaf senescence can be induced and accelerated by a variety of biotic and abiotic stresses (Smart, 1994), including shade and darkness (Biswal and Biswal, 1984; Rousseaux et al, 1996; Weaver and Amasino, 2001)
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