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Redundant roles of photoreceptors and cytokinins in regulating photosynthetic acclimation to canopy density.

Boonman A, Prinsen E, Voesenek LA, Pons TL - J. Exp. Bot. (2009)

Bottom Line: No evidence was found for a role for sugars, or for nitrate.However, the reduction of the chlorophyll a/b ratio in response to local shade was completely absent in the Arabidopsis Ws-2 accession mutated in PHYTOCHROME D and in the triple phyAphyCphyD mutant.Moreover, cytokinin receptor mutants also showed a reduced response, suggesting a previously unrecognized function of phyD and cytokinins.

View Article: PubMed Central - PubMed

Affiliation: Plant Ecophysiology Group, Institute of Environmental Biology, Utrecht University, 3584 CA, Utrecht, The Netherlands.

ABSTRACT
The regulation of photosynthetic acclimation to canopy density was investigated in tobacco canopies and in tobacco and Arabidopsis plants with part of their foliage experimentally shaded. Both species acclimated to canopy light gradients and partial shading by allocating photosynthetic capacity to leaves in high light and adjusting chloroplast organization to the local light conditions. An investigation was carried out to determine whether signalling mediated by photoreceptors, sugars, cytokinin, and nitrate is involved in and necessary for proper photosynthetic acclimation. No evidence was found for a role for sugars, or for nitrate. The distribution of cytokinins in tobacco stands of contrasting density could be explained in part by irradiance-dependent delivery of cytokinins through the transpiration stream. Functional studies using a comprehensive selection of Arabidopsis mutants and transgenics showed that normal wild-type responses to partial shading were retained when signalling mediated by photoreceptors or cytokinins was disrupted. This indicates that these pathways probably operate in a redundant manner. However, the reduction of the chlorophyll a/b ratio in response to local shade was completely absent in the Arabidopsis Ws-2 accession mutated in PHYTOCHROME D and in the triple phyAphyCphyD mutant. Moreover, cytokinin receptor mutants also showed a reduced response, suggesting a previously unrecognized function of phyD and cytokinins.

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Canopy density effects on putative signal parameters and photosynthetic acclimation in tobacco. Shown are PPFD (A), R: FR (B), soluble sugars (C), and nitrate (D), photosynthetic capacity per unit chlorophyll (Amax/Chl) (E) and the chlorophyll a/b ratio (Chl a/b) (F) measured in open (3.6 plants m−2) and dense (35 plants m−2) tobacco canopies at three heights representative for maximal, intermediate, and minimal irradiance in each stand. Data are means±SE, n=6–12. Note the log-scale on the y-axis in A and B.
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fig1: Canopy density effects on putative signal parameters and photosynthetic acclimation in tobacco. Shown are PPFD (A), R: FR (B), soluble sugars (C), and nitrate (D), photosynthetic capacity per unit chlorophyll (Amax/Chl) (E) and the chlorophyll a/b ratio (Chl a/b) (F) measured in open (3.6 plants m−2) and dense (35 plants m−2) tobacco canopies at three heights representative for maximal, intermediate, and minimal irradiance in each stand. Data are means±SE, n=6–12. Note the log-scale on the y-axis in A and B.

Mentions: Tobacco plants growing in a dense stand were exposed to a much steeper gradient in irradiance (Fig. 1A) and R:FR (Fig. 1B) when compared with the open stand. Besides these primary signals, canopy density also affected the distribution of soluble sugars (Fig. 1C), nitrate (Fig. 1D), and cytokinins (Table 1). Soluble sugars accumulated to much higher levels in open stand plants at all canopy positions (Fig. 1C), whereas nitrate accumulated much more in dense stand plants and was only present at low concentrations in the open stand at all positions (Fig. 1D). In the dense stand, only upper leaves had significant sugar concentrations, and intermediate and lower leaves exposed to deep canopy shade contained almost no sugar (Fig. 1C). In contrast, nitrate accumulated in the lower leaves in dense stand plants, opposite to the light gradient (Fig. 1D). Eleven different cytokinins were detected using micro-LC MS/MS (Table 1). In general, the dominant cytokinins were cis-zeatin riboside monophosphate (ZRP) and isopentenyl adenosine monophosphate (iPRP), and concentrations were highest in the upper leaves and declined towards the bottom in both stands. High canopy density reduced the concentration of cis-ZRP in upper leaves by ∼50%. Trans-Z-type cytokinins were only detected in upper leaves in both stands, and dihydrozeatin (DHZ)-type cytokinins were below or close to the detection limit in all samples. For one of the cytokinins, isopentenyl adenosine (iPR), a significant height by density interaction was observed when only upper and intermediate leaves were considered (P <0.05; ANCOVA), that was consistent with the irradiance gradient. That is, iPR concentrations were reduced more strongly from the top of the canopy downwards in the dense stand than in the open stand (Table 1).


Redundant roles of photoreceptors and cytokinins in regulating photosynthetic acclimation to canopy density.

Boonman A, Prinsen E, Voesenek LA, Pons TL - J. Exp. Bot. (2009)

Canopy density effects on putative signal parameters and photosynthetic acclimation in tobacco. Shown are PPFD (A), R: FR (B), soluble sugars (C), and nitrate (D), photosynthetic capacity per unit chlorophyll (Amax/Chl) (E) and the chlorophyll a/b ratio (Chl a/b) (F) measured in open (3.6 plants m−2) and dense (35 plants m−2) tobacco canopies at three heights representative for maximal, intermediate, and minimal irradiance in each stand. Data are means±SE, n=6–12. Note the log-scale on the y-axis in A and B.
© Copyright Policy - open-access
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC2657547&req=5

fig1: Canopy density effects on putative signal parameters and photosynthetic acclimation in tobacco. Shown are PPFD (A), R: FR (B), soluble sugars (C), and nitrate (D), photosynthetic capacity per unit chlorophyll (Amax/Chl) (E) and the chlorophyll a/b ratio (Chl a/b) (F) measured in open (3.6 plants m−2) and dense (35 plants m−2) tobacco canopies at three heights representative for maximal, intermediate, and minimal irradiance in each stand. Data are means±SE, n=6–12. Note the log-scale on the y-axis in A and B.
Mentions: Tobacco plants growing in a dense stand were exposed to a much steeper gradient in irradiance (Fig. 1A) and R:FR (Fig. 1B) when compared with the open stand. Besides these primary signals, canopy density also affected the distribution of soluble sugars (Fig. 1C), nitrate (Fig. 1D), and cytokinins (Table 1). Soluble sugars accumulated to much higher levels in open stand plants at all canopy positions (Fig. 1C), whereas nitrate accumulated much more in dense stand plants and was only present at low concentrations in the open stand at all positions (Fig. 1D). In the dense stand, only upper leaves had significant sugar concentrations, and intermediate and lower leaves exposed to deep canopy shade contained almost no sugar (Fig. 1C). In contrast, nitrate accumulated in the lower leaves in dense stand plants, opposite to the light gradient (Fig. 1D). Eleven different cytokinins were detected using micro-LC MS/MS (Table 1). In general, the dominant cytokinins were cis-zeatin riboside monophosphate (ZRP) and isopentenyl adenosine monophosphate (iPRP), and concentrations were highest in the upper leaves and declined towards the bottom in both stands. High canopy density reduced the concentration of cis-ZRP in upper leaves by ∼50%. Trans-Z-type cytokinins were only detected in upper leaves in both stands, and dihydrozeatin (DHZ)-type cytokinins were below or close to the detection limit in all samples. For one of the cytokinins, isopentenyl adenosine (iPR), a significant height by density interaction was observed when only upper and intermediate leaves were considered (P <0.05; ANCOVA), that was consistent with the irradiance gradient. That is, iPR concentrations were reduced more strongly from the top of the canopy downwards in the dense stand than in the open stand (Table 1).

Bottom Line: No evidence was found for a role for sugars, or for nitrate.However, the reduction of the chlorophyll a/b ratio in response to local shade was completely absent in the Arabidopsis Ws-2 accession mutated in PHYTOCHROME D and in the triple phyAphyCphyD mutant.Moreover, cytokinin receptor mutants also showed a reduced response, suggesting a previously unrecognized function of phyD and cytokinins.

View Article: PubMed Central - PubMed

Affiliation: Plant Ecophysiology Group, Institute of Environmental Biology, Utrecht University, 3584 CA, Utrecht, The Netherlands.

ABSTRACT
The regulation of photosynthetic acclimation to canopy density was investigated in tobacco canopies and in tobacco and Arabidopsis plants with part of their foliage experimentally shaded. Both species acclimated to canopy light gradients and partial shading by allocating photosynthetic capacity to leaves in high light and adjusting chloroplast organization to the local light conditions. An investigation was carried out to determine whether signalling mediated by photoreceptors, sugars, cytokinin, and nitrate is involved in and necessary for proper photosynthetic acclimation. No evidence was found for a role for sugars, or for nitrate. The distribution of cytokinins in tobacco stands of contrasting density could be explained in part by irradiance-dependent delivery of cytokinins through the transpiration stream. Functional studies using a comprehensive selection of Arabidopsis mutants and transgenics showed that normal wild-type responses to partial shading were retained when signalling mediated by photoreceptors or cytokinins was disrupted. This indicates that these pathways probably operate in a redundant manner. However, the reduction of the chlorophyll a/b ratio in response to local shade was completely absent in the Arabidopsis Ws-2 accession mutated in PHYTOCHROME D and in the triple phyAphyCphyD mutant. Moreover, cytokinin receptor mutants also showed a reduced response, suggesting a previously unrecognized function of phyD and cytokinins.

Show MeSH