Increased phytochrome B alleviates density effects on tuber yield of field potato crops

Abstract

The possibility that reduced photomorphogenic responses could increase field crop yield has been suggested often, but experimental support is still lacking. Here, we report that ectopic expression of the Arabidopsis PHYB (phytochrome B) gene, a photoreceptor involved in detecting red to far-red light ratio associated with plant density, can increase tuber yield in field-grown transgenic potato (Solanum tuberosum) crops. Surprisingly, this effect was larger at very high densities, despite the intense reduction in the red to far-red light ratios and the concomitant narrowed differences in active phytochrome B levels between wild type and transgenics at these densities. Increased PHYB expression not only altered the ability of plants to respond to light signals, but they also modified the light environment itself. This combination resulted in larger effects of enhanced PHYB expression on tuber number and crop photosynthesis at high planting densities. The PHYB transgenics showed higher maximum photosynthesis in leaves of all strata of the canopy, and this effect was largely due to increased leaf stomatal conductance. We propose that enhanced PHYB expression could be used in breeding programs to shift optimum planting densities to higher levels.

Figure 1.

Expression of the Arabidopsis PHYB transgene reduces stem growth responses to plant density. A, PHYB protein levels detected in leaf extracts. The ratio between PHYB-associated alkaline phosphatase band intensity and total protein Ponceau S staining is expressed relative to the WT ratio. B, Stem length plotted against plant density. Plants were grown outdoors at the indicated plant density. Data are means and se of four blocks.

Figure 2.

Field performance of potato plants either of the WT (Desiree) or of lines expressing the Arabidopsis PHYB transgene (Dara-5 and Dara-12). A, Number of branches, number of tubers, and tuber yield. Plant density was 10 or 20 plants m^-2 (70 or 35 cm between rows, respectively). B, Tuber yield response to plant density. Note that the se overestimates the error used in ANOVA because blocking reduced the error.

Figure 3.

Transgenic expression of the Arabidopsis PHYB transgene modifies R to FR ratio signals. The R to FR ratio of the light propagating horizontally was measured at midday. The sensor was placed at the base of a given plant facing neighbor plants in different rows (facing west) or within the same row (facing north). Data are means and se of four independent replicate measurements.

Figure 4.

PPFD interception by potato crops either of the WT or of lines expressing the Arabidopsis PHYB transgene (Dara-5 and Dara-12). A, Light interception (10 d after planting). B, Transgenic crops fail to cover the soil when planted at 10 plants m^-2, but this effect is diluted at 20 plants m^-2 (48 d after planting) C, Irradiance reaching different strata (20 plants m^-2; 48 d after planting). Data are means and se of four replicates.

Figure 5.

Response of net CO₂ uptake per unit area to PPFD (A) and actual net CO₂ uptake per unit area (B) in leaves of different strata of potato plants of the WT or expressing the Arabidopsis PHYB transgene (Dara-5 and Dara-12). Plant density was 10 (A) or 20 (B) plants m^-2. Data are means and se of four replicates.

Figure 6.

Time course of maximum net CO₂ uptake per unit area (A), CO₂ conductance (B), and transpiration (C) in upper leaves of WT or transgenic potato expressing Arabidopsis PHYB (Dara-5 and Dara-12).