An oxygen-sensing mechanism for angiosperm adaptation to altitude.

Flowering plants (angiosperms) can grow at extreme altitudes, and have been observed growing as high as 6,400 metres above sea level1,2; however, the molecular mechanisms that enable plant adaptation specifically to altitude are unknown. One distinguishing feature of increasing altitude is a reduction in the partial pressure of oxygen (pO2). Here we investigated the relationship between altitude and oxygen sensing in relation to chlorophyll biosynthesis-which requires molecular oxygen3-and hypoxia-related gene expression. We show that in etiolated seedlings of angiosperm species, steady-state levels of the phototoxic chlorophyll precursor protochlorophyllide are influenced by sensing of atmospheric oxygen concentration. In Arabidopsis thaliana, this is mediated by the PLANT CYSTEINE OXIDASE (PCO) N-degron pathway substrates GROUP VII ETHYLENE RESPONSE FACTOR transcription factors (ERFVIIs). ERFVIIs positively regulate expression of FLUORESCENT IN BLUE LIGHT (FLU), which represses the first committed step of chlorophyll biosynthesis, forming an inactivation complex with tetrapyrrole synthesis enzymes that are negatively regulated by ERFVIIs, thereby suppressing protochlorophyllide. In natural populations representing diverse angiosperm clades, we find oxygen-dependent altitudinal clines for steady-state levels of protochlorophyllide, expression of inactivation complex components and hypoxia-related genes. Finally, A. thaliana accessions from contrasting altitudes display altitude-dependent ERFVII activity and accumulation. We thus identify a mechanism for genetic adaptation to absolute altitude through alteration of the sensitivity of the oxygen-sensing system.

Role of gibberellins in parthenocarpic fruit development induced by the genetic system pat-3/pat-4 in tomato.

The role of gibberellins (GAs) in the induction of parthenocarpic fruit-set and growth by the pat-3/pat-4 genetic system in tomato (Lycopersicon esculentum Mill.) was investigated using wild type (WT; Cuarenteno) and a near-isogenic line derived from the German line RP75/59 (the source of pat-3/pat-4 parthenocarpy). Unpollinated WT ovaries degenerated but GA3 application induced parthenocarpic fruit growth. On the contrary, parthenocarpic growth of pat-3/pat-4 fruits, which occurs in the absence of pollination and hormone treatment, was not affected by applied GA3. Unpollinated pat-3/pat-4 fruit growth was negated by paclobutrazol, an inhibitor of ent-kaurene oxidase, and this inhibitory effect was negated by GA3. The quantification of the main GAs of the early 13-hydroxylation pathway (GA1, GA8, GA19, GA20, GA29 and GA44) in unpollinated ovaries at 3 developmental stages (flower bud, FB; pre-anthesis, PR; and anthesis, AN), by gas chromatography-selected ion monitoring, showed that the concentration of most of them was higher in pat-3/pat-4 than in WT ovaries at PR and AN stages. The concentration of GA1, suggested previously to be the active GA in tomate, was 2-4 times higher. Unpollinated pat-3/pat-4 ovaries at FB, PR and AN stages also contained relatively high amounts (5-12 ng g-1) of GA3, a GA found at less than 0.5 ng g-1 in WT ovaries. It is concluded that the mutations pat-3/pat-4 may induce natural facultative parthenocarpy capacity in tomato by increasing the concentration of GA1 and GA3 in the ovaries before pollination.

Polyamine metabolism is altered in unpollinated parthenocarpic pat-2 tomato ovaries.

Facultative parthenocarpy induced by the recessive mutation pat-2 in tomato (Lycopersicon esculentum Mill.) depends on gibberellins (GAs) and is associated with changes in GA content in unpollinated ovaries. Polyamines (PAs) have also been proposed to play a role in early tomato fruit development. We therefore investigated whether PAs are able to induce parthenocarpy and whether the pat-2 mutation alters the content and metabolism of PAs in unpollinated ovaries. Application of putrescine, spermidine, and spermine to wild-type unpollinated tomato ovaries (cv Madrigal [MA/wt]) induced partial parthenocarpy. Parthenocarpic growth of MA/pat-2 (a parthenocarpic near-isogenic line to MA/wt) ovaries was negated by paclobutrazol (GA biosynthesis inhibitor), and this inhibition was counteracted by spermidine. Application of alpha-difluoromethyl-ornithine (-Orn) and/or alpha-difluoromethyl-arginine (-Arg), irreversible inhibitors of the putrescine biosynthesis enzymes Orn decarboxylase (ODC) and Arg decarboxylase, respectively, prevented growth of unpollinated MA/pat-2 ovaries. Alpha-difluoromethyl-Arg inhibition was counteracted by putrescine and GA(3), whereas that of alpha-difluoromethyl-Orn was counteracted by GA(3) but not by putrescine or spermidine. In unpollinated MA/pat-2 ovaries, the content of free spermine was significantly higher than in MA/wt ovaries. ODC activity was higher in pat-2 ovaries than in MA/wt. Transcript levels of genes encoding ODC and spermidine synthase were also higher in MA/pat-2. All together, these results strongly suggest that the parthenocarpic ability of pat-2 mutants depends on elevated PAs levels in unpollinated mutant ovaries, which correlate with an activation of the ODC pathway, probably as a consequence of elevated GA content in unpollinated pat-2 tomato ovaries.