Flavonols Biosynthetic Genes are Activated during the Inclination Stress Response in Pine

Stem reorientation in response to inclination is a widely studied phenomenon in trees, but the molecular mechanism is still unknown. The most accepted theory that explains this differential response is auxin redistribution in stem tissues. Flavonols have been reported as potent inhibitors of polar auxin transport and therefore their participation in the inclination response has been addressed. Genes involved in the biosynthesis of flavonols and metabolite contents were analyzed in radiata pine seedlings exposed to 45º inclination through transcriptional and chemical approaches. Stems were cut in two different ways: sectioning stems into 3 segments (apical, medial and basal) or longitudinally dissected into upper and lower halves. Full-length sequences of genes involved in phenylpropanoids, and specifically, in flavonols biosynthesis pathway were isolated from radiata pine. The expression of chalcone synthase, flavanone 3–hydroxylase and flavonol synthase genes performed by qRT-PCR indicated an induction in response to stem inclination; higher expression levels were recorded at the basal zone and in the upper half of the stem. The reduction in abundance of auxin repressed protein transcripts at the lower half of inclined stems indicates auxin distribution towards it, supporting the role of auxins in the reorientation process. Quercetin accumulates in the upper half of stems avoiding auxin distribution. At the same time, the expression of flavonols biosynthesis genes increased in the upper side of responding stem. This indicates a concerted activation of genes that generates a misbalance in local auxin distribution that finally induces stem reorientation. Financial support: FONDECYT 11121170, 1120635, Anillo ACT-1110.