Phytochrome B1/B2 and auxin transport are involved in the regulation of shoot: root ratio by far-red radiation in tomato

Abstract

Plants possess a set of photoreceptors to perceive changes in the light spectrum. Phytochromes (PHY) B1/B2 sense changes in red: far red ratios and are involved in mediating the shade-avoidance responses (SAR) in tomato. Far red (FR) increases the fraction of dry mass partitioning to the shoot at the expense of the root in tomato, but the control of this response has not yet been explained. We studied the role of phytochromes and auxin transport in the regulation of shoot: root ratio. We hypothesized that a loss-of-function mutation in phyB1/B2 leads to a strong increase in shoot: root ratio, similar to the effect of reduced R: FR ratio in wildtype plants. We also hypothesized that the phyB1/B2 double mutation suppresses shoot: root ratio responses when exposed to a reduced R: FR ratio. Furthermore, we hypothesized that the increased shoot: root ratio is linked to the changes in auxin transport between shoot and root. To test these hypotheses, we conducted an experiment in a climate chamber where both wildtype and phyB1/B2 double mutant tomato plants (Solanum lycopersicum cv. Moneymaker) were grown for 21 days with 0, 55 or 85 μmol m−2 s−1 FR in a background of white + red LED light at 150 μmol m−2 s−1. On the 14th day, auxin polar transport inhibitor 1-N-naphthylphthalamic acid (NPA) was applied at the shoot-root junction. PhyB1/B2 mutants showed a higher shoot: root ratio than the wildtype plants. Both wildtype and phyB1/B2 mutants responded to FR with an increase in shoot: root ratio. Blocking auxin transport from shoot to root led to an increase in shoot: root ratio for both genotypes under all light conditions. These results suggest that, similar to other SAR responses like stem elongation, the response of shoot: root ratio to additional FR also involves the regulation by phytochromes, possibly via affecting auxin transport between shoot and root.