Phytochrom Interacting Factor 4 and 5 (PIF4 and PIF5) Play a Less Important Role in Weed Perception and Response Than Previous Hypothesized for Arabidopsis.

The common conception is that weeds reduce crop yield because they compete with crops for resources such as light, nutrients or water, or because they produce allelopathic chemicals. However, recent evidence suggests the primary mechanism of yield loss is due to alteration of crop developmental programs when they perceive the presence of weeds through altered light quality (red:far-red light ratio) – often long before there is any actual competition for resources. These changes include altering root and shoot growth rates, stem elongation, and early flowering- all changes designed to give a plant a selective advantage in anticipation of resource competition. However, in high input agricultural settings where resources are rarely limiting, these developmental responses needlessly reduce yield. Here, we begin investigating the signals involved in this response using the model plant Arabidopsis thaliana as a tool to test the role of the phytochrome signaling pathway in regulating developmental changes resulting from interspecies competition. We used well defined mutations in two PHYTOCHROM INTERACTING FACTOR (PIF) genes known as pif4-2 and pif5-3 that had previously been shown to alter the ability of Arabidopsis to respond to differences in the red:far red light ratios. Arabidopsis double mutants (pif4/5) and wild type plants displayed no significant differences when growing with or without competition with grassy weeds. Variance in response to weeds was not substantially different in the pif4/5 double mutant. Separation of the grassy weeds from the Arabidopsis using horticultural fabric that had or had not been permeated with activated charcoal also failed to allow penetration of the phenotypic differences resulting from the pif4/5 double mutants although preliminary results suggest that some soil borne signal that is blocked by activated charcoal rather than a light response may be responsible for the early flowering that is often observed when Arabidopsis is grown with weeds.