Seeds are tough cookies — they can withstand hostile surroundings until conditions favour germination. Emergent seedlings, though, are very vulnerable — particularly when first exposed to sunlight. Seedlings accumulate a small amount of protochlorophyllide, the precursor of chlorophyll, before they reach the surface so that, on sensing the first rays of sun, they are ready to photosynthesize. But too much protochlorophyllide can induce oxidative damage (in the form of bleaching), so chlorophyll biosynthesis must be carefully controlled. The identification, by Huq et al., of PHYTOCHROME-INTERACTING FACTOR-1 (PIF1), provides a mechanism by which this can be achieved.

Phytochrome (phy) receptors perceive light signals, which are thought to be transduced through PIFs to impinge on gene expression. The authors' investigations focused on PIF1, a basic helix–loop–helix (bHLH) transcription factor. Whereas pif1-insertion-mutant-seedlings showed no adverse effects when grown in the light from germination, those that were germinated and grown in the dark before being transferred to the light became bleached. This was reminiscent of a mutant phenotype that is caused by excess protochlorophyllide and, sure enough, protochlorophyllide levels were higher in the pif1 mutants that grew in the dark after germination. And the longer the time spent in darkness before transfer to light, the more severe the bleaching phenotype. This hinted that PIF1 might prevent the build-up of excess protochlorophyllide in the dark. Huq et al. indeed found that PIF1 negatively regulated the chlorophyll biosynthetic pathway — if pif1-mutant seedlings were grown in the dark for only a short period and then exposed to light before they had accumulated lethal levels of protochlorophyllide, they subsequently accumulated chlorophyll much faster than wild-type seedlings did in response to light.

The authors next established that PIF1 could bind to the so-called G-box DNA-sequence motif that is present in the promoters of many light-regulated genes. So what was the effect of PIF1 on transcription? It induced a marked increase in the activity of a luciferase reporter gene when expressed in dark-incubated seedlings, and this transcriptional activity was suppressed in the presence of light treatment. PIF1 also interacted with the active (Pfr) form of phyA and phyB, the two main phytochromes that regulate the ability of seedlings to undergo 'greening' in response to light, and the light-induced suppression of PIF1 activity required these phy proteins. As PIF1 can't interact with DNA and phyA or phyB concurrently, it seems that the phytoreceptors, when active, might function to modulate the activity of PIF1 — perhaps by sequestering or degrading it — so that chlorophyll biosynthesis can occur in the presence of light. So PIF1 seems to function as “a critical modulator by which plants optimize chlorophyll biosynthesis in response to environmental light conditions and protect against accumulation of potentially toxic levels of intermediates.”