Effect of Red:Far-Red ratios on partitioning to root and shoot in young tomato plants: the role of auxin

MSc-thesis abstract (submitted 20 March 2017): The objective of this research was to determine whether the effect of Phytochrome Photostationary State (PSS) defined by R:FR ratios on partitioning to the roots and shoots can be explained by auxin biosynthesis and signalling.
To investigate this, four experiments were executed in a fully-controlled climate chamber. These experiments included light treatments with PSS values of 0.73, 0.77 and 0.88, auxin (IAA) applications and auxin transport blocker (NPA) applications on Moneymaker wildtype and a phyB1phyB2 double mutant genotype, all executed in 4 repeats. On day 0, 7, 14 and 21 from the start of the experiment plant growth parameters and gene expression levels were determined. Explanatory plant growth parameters were mainly dry mass partitioning to root, stem and leaf and plant height, because of its big interest in the shade avoidance response.

This research confirmed that tomato (cv. Moneymaker) seedlings obtain a higher Shoot:Root ratio under additional Far-Red lighting. The role of auxin in the response is still scattered. NPA application on 21 days old Moneymaker wildtype plants was not effective, however effects of a disturbed auxin transport were observed after NPA application on 14 days old Moneymaker phyB1phyB2 mutant plants.
These observations indicate that auxin transport to the root system is requisite for healthy plant growth. Disturbed auxin transport to the roots under high PSS gave a similar shading response as plants under low PSS, with undisturbed auxin transport. Unfortunately, it was not possible to investigate the effect of increased auxin levels in the root, since the IAA application was not effective on 21 days old wildtype plants and neither on 14 days old mutants.
Genes analysed on interest in auxin regulated shade avoidance were phytochrome interacting factors (PIF) 4 and 7, auxin efflux carrier PIN 3 and 4 and the ToFZY involved in auxin biosynthesis. However, gene expression analyses did not clearly support the plant morphological data, due to incoherent results within the gene expression analyses

Keywords: Shade avoidance, auxin, phytochromes, physiology, molecular biology, tomato (Solanum lycopersicum)