Arabidopsis FIBRILLIN6 influences carotenoid biosynthesis by directly promoting phytoene synthase activity

Carotenoids are health-promoting plastidial isoprenoids with essential functions in plants as photoprotectants and photosynthetic pigments in chloroplasts. They also accumulate in specialized plastids named chromoplasts, providing color to non-photosynthetic tissues such as flower petals and ripe fruit. Carotenoid accumulation in chromoplasts requires specialized structures and proteins such as fibrillins (FBNs). The FBN family includes structural components of carotenoid sequestering structures in chromoplasts and members with metabolic roles in chloroplasts and other plastid types. However, the association of FBNs with carotenoids in plastids other than chromoplasts has remained unexplored. Here, we show that Arabidopsis (Arabidopsis thaliana) FBN6 interacts with phytoene synthase (PSY), the first enzyme of the carotenoid pathway. FBN6, but not FBN4 (a FBN that does not interact with PSY), enhances the activity of plant PSY (but not of the bacterial PSY crtB) in Escherichia coli cells. Overexpression of FBN6 in Nicotiana benthamiana leaves results in a higher production of phytoene, the product of PSY activity, whereas loss of FBN6 activity in Arabidopsis mutants dramatically reduces the production of carotenoids during seedling de-etiolation and after exposure to high light. Our work hence demonstrates that FBNs promote not only the accumulation of carotenoids in chromoplasts but also their biosynthesis in chloroplasts.

Environmentally driven transcriptomic and metabolic changes leading to color differences in "Golden Reinders" apples

Apple is characterized by its high adaptation to diverse growing environments. However, little is still known about how different environments can regulate at the metabolic or molecular level specific apple quality traits such as the yellow fruit peel color. In this study, changes in carotenoids and chlorophylls, antioxidants as well as differences in the transcriptome were investigated by comparing the peel of "Golden Reinders" apples grown at different valley and mountain orchards. Mountain environment favored the development of yellow color, which was not caused by an enhanced accumulation of carotenoids but rather by a decrease in the chlorophyll content. The yellow phenotype was also associated to higher expression of genes related to chloroplast functions and oxidative stress. Time-course analysis over the last stages of apple development and ripening, in fruit from both locations, further revealed that the environment differentially modulated isoprenoids and phenylpropanoid metabolism and pointed out a key role for H₂O₂ in triggering apple peel degreening. Overall, the results presented herein provide new insights into how different environmental conditions regulate pigment and antioxidant metabolism in apple leading to noticeable differences in the apple peel color.

The Lack of Alternative Oxidase 1a Restricts in vivo Respiratory Activity and Stress-Related Metabolism for Leaf Osmoprotection and Redox Balancing Under Sudden Acute Water and Salt Stress in Arabidopsis thaliana

In plants salt and water stress result in an induction of respiration and accumulation of stress-related metabolites (SRMs) with osmoregulation and osmoprotection functions that benefit photosynthesis. The synthesis of SRMs may depend on an active respiratory metabolism, which can be restricted under stress by the inhibition of the cytochrome oxidase pathway (COP), thus causing an increase in the reduction level of the ubiquinone pool. However, the activity of the alternative oxidase pathway (AOP) is thought to prevent this from occurring while at the same time, dissipates excess of reducing power from the chloroplast and thereby improves photosynthetic performance. The present research is based on the hypothesis that the accumulation of SRMs under osmotic stress will be affected by changes in folial AOP activity. To test this, the oxygen isotope-fractionation technique was used to study the in vivo respiratory activities of COP and AOP in leaves of wild-type Arabidopsis thaliana plants and of aox1a mutants under sudden acute stress conditions induced by mannitol and salt treatments. Levels of leaf primary metabolites and transcripts of respiratory-related proteins were also determined in parallel to photosynthetic analyses. The lack of in vivo AOP response in the aox1a mutants coincided with a lower leaf relative water content and a decreased accumulation of crucial osmoregulators. Additionally, levels of oxidative stress-related metabolites and transcripts encoding alternative respiratory components were increased. Coordinated changes in metabolite levels, respiratory activities and photosynthetic performance highlight the contribution of the AOP in providing flexibility to carbon metabolism for the accumulation of SRMs.

Light signals generated by vegetation shade facilitate acclimation to low light in shade-avoider plants

When growing in search for light, plants can experience continuous or occasional shading by other plants. Plant proximity causes a decrease in the ratio of R to far-red light (low R:FR) due to the preferential absorbance of R light and reflection of FR light by photosynthetic tissues of neighboring plants. This signal is often perceived before actual shading causes a reduction in photosynthetically active radiation (low PAR). Here, we investigated how several Brassicaceae species from different habitats respond to low R:FR and low PAR in terms of elongation, photosynthesis, and photoacclimation. Shade-tolerant plants such as hairy bittercress (Cardamine hirsuta) displayed a good adaptation to low PAR but a poor or null response to low R:FR exposure. In contrast, shade-avoider species, such as Arabidopsis (Arabidopsis thaliana), showed a weak photosynthetic performance under low PAR but they strongly elongated when exposed to low R:FR. These responses could be genetically uncoupled. Most interestingly, exposure to low R:FR of shade-avoider (but not shade-tolerant) plants improved their photoacclimation to low PAR by triggering changes in photosynthesis-related gene expression, pigment accumulation, and chloroplast ultrastructure. These results indicate that low R:FR signaling unleashes molecular, metabolic, and developmental responses that allow shade-avoider plants (including most crops) to adjust their photosynthetic capacity in anticipation of eventual shading by nearby plants.