Heterologous expression of Arabidopsis phytochrome B in transgenic potato influences photosynthetic performance and tuber development

Abstract

Transgenic potato (Solanum tuberosum) plants expressing Arabidopsis phytochrome B were characterized morphologically and physiologically under white light in a greenhouse to explore their potential for improved photosynthesis and higher tuber yields. As expected, overexpression of functional phytochrome B caused pleiotropic effects such as semidwarfism, decreased apical dominance, a higher number of smaller but thicker leaves, and increased pigmentation. Because of increased numbers of chloroplasts in elongated palisade cells, photosynthesis per leaf area and in each individual plant increased. In addition, photosynthesis was less sensitive to photoinactivation under prolonged light stress. The beginning of senescence was not delayed, but deceleration of chlorophyll degradation extended the lifetime of photosynthetically active plants. Both the higher photosynthetic performance and the longer lifespan of the transgenic plants allowed greater biomass production, resulting in extended underground organs with increased tuber yields.

Figure 1

Northern analysis showing Arabidopsis phyB (A.th. PhyB) and potato PhyB (S.t. PhyB) transcript levels in leaves of wild-type (Wt), Dara-5, and Dara-12 potato plants. Hybridization with a potato S4 probe was done to correct the signal for loading differences; 1.5 μg of poly(A⁺) RNA was loaded per lane.

Figure 2

Immunoblot showing expression levels of phyB in leaves of wild-type (Wt), Dara-5, and Dara-12 potato plants using a monoclonal antibody raised against Arabidopsis phyB. Both Arabidopsis phyB and endogenous potato phyB were equally recognized by this antibody; 100 μg of protein was loaded per lane.

Figure 3

a, Phenotype of a representative wild-type (Wt) potato. b, Phenotype of a 5-week-old Dara-12 potato plant showing shortened internodes and increased branching. Dara-5, which moderately expresses phyB, exhibits an intermediate phenotype (not shown; see data in Table I). c, Phenotype of leaves of wild-type (Wt), Dara-5, and Dara-12 plants grown under moderate light conditions (0.15–0.5 mmol m⁻² s⁻¹) in a greenhouse. Leaves 5 (counted from the apex) from 10-week-old plants are shown. Bar = 6 cm.

Figure 4

Light micrographs of leaf cross-sections of wild-type (a) and transgenic Dara-12 leaves (b). Representative sections were taken from leaves 8 of 6-week-old plants. Bar = 150 μm.

Figure 5

Mean chlorophyll content during the course of leaf maturation and senescence of wild-type (○), Dara-5 (▪), and Dara-12 (•) potato plants. The arrow indicates onset of flowering, which was at about the same time for all lines. Day 1 was October 26, 1997. Each point represents the mean chlorophyll content of 10 segments cut (two were taken at d 11) in regular distances from leaflets of apical to basal leaves. Data from two representative plants per line are depicted. Because of the sampling, aging was slightly (about 3 weeks) accelerated in all plants.

Figure 7

Photosynthesis rates at 1.8 mmol photons m⁻² s⁻¹ of wild-type, Dara-5, and Dara-12 before (0 h) and after high-light stress (under 1.8 mmol m⁻² s⁻¹) for 5 h. Leaf material was the same as for Figure 6. Data are from nine leaves of three plants from each line (sd as indicated).

Figure 6

Photosynthesis rates of mature, nonsenescent leaves (leaves 6–8) of 32- to 37-d-old wild-type, Dara-5, and Dara-12 plants. a, Rates of wild-type (○), Dara-5 (▪), and Dara-12 (•) at different photon fluxes (50–500 μmol m⁻² s⁻¹) normalized to leaf area (μmol CO₂ m⁻² s⁻¹). Data points are means of measurements of 4 to 15 leaves from eight plants of each line. b, Rates at 500 μmol photons m⁻² s⁻¹ normalized to chlorophyll contents at site of the investigated leaf (μmol CO₂ g⁻¹ s⁻¹). In a data points are for 4 to 15 leaves from eight plants of each line; in b data are from nine leaves of three plants of each line (sd = 0.5–2.6 μmol m⁻² s⁻¹ for Dara-5 or as indicated). chl, Chlorophyll.

Figure 8

Leaf phenotypes of wild-type, Dara-5, and Dara-12 plants grown under high intensities of white light (1.5–1.8 mmol m⁻² s⁻¹) in a greenhouse, demonstrating increased anthocyanin contents. Leaves 5 of 8-week-old plants are shown. Bar = 3 cm.

Figure 9

Expansion of underground organs and tuber yields of the wild type (Wt), Dara-5, and Dara-12 harvested upon decay of the aerial parts after 5 months of cultivation in a greenhouse. The transgenic lines showed enlarged underground organs and more but smaller tubers, resulting in increased total tuber weight per plant (Table III). Bar = 5 cm.