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Recent work has shown that changing the photoperiod induces stress in Arabidopsis thaliana. It has particularly dramatic consequences in cytokinin-deficient plants and clock mutants. Here, we argue that studying the impact of an altered photoperiod will provide novel insights into the circadian clock, factors regulating it, and pathways under its control., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

The circadian clock helps plants measure daylength and adapt to changes in the day-night rhythm. We found that changes in the light-dark regime triggered stress responses, eventually leading to cell death, in leaves of Arabidopsis thaliana plants with reduced cytokinin levels or defective cytokinin signaling. Prolonged light treatment followed by a dark period induced stress and cell death marker genes while reducing photosynthetic efficiency. This response, called circadian stress, is also characterized by altered expression of clock and clock output genes. In particular, this treatment strongly reduced the expression of CIRCADIAN CLOCK ASSOCIATED1 (CCA1) and LATE ELONGATED HYPOCOTYL (LHY). Intriguingly, similar changes in gene expression and cell death were observed in clock mutants lacking proper CCA1 and LHY function. Circadian stress caused strong changes in reactive oxygen species- and jasmonic acid (JA)-related gene expression. The activation of the JA pathway, involving the accumulation of JA metabolites, was crucial for the induction of cell death, since the cell death phenotype was strongly reduced in the jasmonate resistant1 mutant background. We propose that adaptation to circadian stress regimes requires a normal cytokinin status which, acting primarily through the AHK3 receptor, supports circadian clock function to guard against the detrimental effects of circadian stress., (© 2016 American Society of Plant Biologists. All rights reserved.)

Cytokinins are plant hormones that regulate diverse processes in plant development and responses to biotic and abiotic stresses. In this study, we show that Arabidopsis (Arabidopsis thaliana) plants with a reduced cytokinin status (i.e. cytokinin receptor mutants and transgenic cytokinin-deficient plants) are more susceptible to light stress compared with wild-type plants. This was reflected by a stronger photoinhibition after 24 h of high light (approximately 1,000 µmol m(-2) s(-1)), as shown by the decline in maximum quantum efficiency of photosystem II photochemistry. Photosystem II, especially the D1 protein, is highly sensitive to the detrimental impact of light. Therefore, photoinhibition is always observed when the rate of photodamage exceeds the rate of D1 repair. We demonstrate that in plants with a reduced cytokinin status, the D1 protein level was strongly decreased upon light stress. Inhibition of the D1 repair cycle by lincomycin treatment indicated that these plants experience stronger photodamage. The efficiency of photoprotective mechanisms, such as nonenzymatic and enzymatic scavenging systems, was decreased in plants with a reduced cytokinin status, which could be a cause for the increased photodamage and subsequent D1 degradation. Additionally, slow and incomplete recovery in these plants after light stress indicated insufficient D1 repair. Mutant analysis revealed that the protective function of cytokinin during light stress depends on the Arabidopsis histidine KINASE2 (AHK2) and AHK3 receptors and the type B Arabidopsis response regulator1 (ARR1) and ARR12. We conclude that proper cytokinin signaling and regulation of specific target genes are necessary to protect leaves efficiently from light stress.

Between the different types of Acyl-CoA dehydrogenases (ACADs), those specific for branched chain acyl-CoA derivatives are involved in the catabolism of amino acids. In mammals, isovaleryl-CoA dehydrogenase (IVD), an enzyme of the leucine catabolic pathway, is a mitochondrial protein, as other acyl-CoA dehydrogenases involved in fatty acid beta-oxidation. In plants, fatty acid beta-oxidation takes place mainly in peroxisomes, and the cellular location of the enzymes involved in the catabolism of branched-chain amino acids had not been definitely assigned. Here, we describe that highly purified potato mitochondria have important IVD activity. The enzyme was partially purified and cDNAs from two different genes were obtained. The partially purified enzyme has enzymatic constant values with respect to isovaleryl-CoA comparable to those of the mammalian enzyme. It is not active towards straight-chain acyl-CoA substrates tested, but significant activity was also found with isobutyryl-CoA, implying an additional role of the enzyme in the catabolism of valine. The present study confirms recent reports that in plants IVD activity resides in mitochondria and opens the way to a more detailed study of amino-acid catabolism in plant development.

The gene and cDNA of an Arabidopsis thaliana cytidine deaminase (CDA) were cloned and sequenced. The gene, At-cda1, is located on chromosome 2 and is expressed in all plant tissues tested, although with quantitative differences. Expression analysis suggest that At-cda1 probably codes for the housekeeping cytidine deaminase of Arabidopsis. The gene was functionally expressed in Escherichia coli and the protein, At-CDA1, shows similar enzymatic and substrate specificities as conventional cytidine deaminases: it deaminates cytidine and deoxycytidine and is competitively inhibited by cytosine-containing compounds. Because the protein shows no affinity to RNA, it is not likely to be involved in RNA-editing by C-to-U deamination. When compared to cytidine deaminases from other organisms, it becomes clear that At-CDA1 is related, both in sequence and structure, to the CDA of E. coli and other gram-negative bacteria. The eubacterial nature of the Arabidopsis CDA suggests that it is an additional example of a plant gene of endosymbiotic origin.