Thomas Schmulling’s research while affiliated with Freie Universität Berlin and other places

Periodic changes of light and dark regulate numerous processes in plants. Recently, a novel type of stress caused by an extended light period has been discovered in Arabidopsis and was named photoperiod stress. Photoperiod stress causes the induction of numerous stress response genes during the night following the extended light period of which many are indicators of oxidative stress. The next day, stress-sensitive genotypes display reduced photosynthetic efficiency and programmed cell death in leaves. Here, we have analysed further the consequences of photoperiod stress and report that it causes changes of the cellular redox status. A prolonged light period caused a strong reduction of the AsA redox during the following night indicating that it induces an oxidizing cellular environment. Further, photoperiod stress was associated with an increased activity of peroxidases and a decreased activity of catalases. Increased peroxidase activity was localized to the apoplast and might be causal for the oxidative stress induced by photoperiod stress.

Recently, a novel type of abiotic stress caused by a prolongation of the light period - coined photoperiod stress - has been described in Arabidopsis. During the night after the prolongation of the light period, stress and cell death marker genes are induced. The next day, strongly stressed plants display a reduced photosynthetic efficiency and leaf cells eventually enter programmed cell death. The phytohormone cytokinin (CK) acts as a negative regulator of this photoperiod stress syndrome. In this study, we show that Arabidopsis wild-type plants increase the CK concentration in response to photoperiod stress. Analysis of cytokinin synthesis and transport mutants revealed that root-derived trans-zeatin (tZ)-type CKs protect against photoperiod stress. The CK signaling proteins ARABIDOPSIS HISTIDINE PHOSPHOTRANSFER PROTEIN 2 (AHP2), AHP3 and AHP5 and transcription factors ARABIDOPSIS RESPONSE REGULATOR 2 (ARR2), ARR10 and ARR12 are required for the protective activity of CK. Analysis of higher order B-type arr mutants suggested that a complex regulatory circuit exists in which the loss of ARR10 or ARR12 can rescue the arr2 phenotype. Together the results revealed the role of root-derived CK acting in the shoot through the two-component signaling system to protect from the negative consequences of strong photoperiod stress.

Because stress experiences are often recurrent plants have developed strategies to remember a first so-called priming stress to eventually respond more effectively to a second triggering stress. Here, we have studied the impact of discontinuous or sustained cold stress (4 C) on in vitro grown Arabidopsis thaliana seedlings of different age and their ability to get primed and respond differently to a later triggering stress. Cold treatment of 7-d-old seedlings induced the expression of cold response genes but did not cause a significantly enhanced freezing resistance. The competence to increase the freezing resistance in response to cold was associated with the formation of true leaves. Discontinuous exposure to cold only during the night led to a stepwise modest increase in freezing tolerance provided that the intermittent phase at ambient temperature was less than 32 h. Seedlings exposed to sustained cold treatment developed a higher freezing tolerance which was further increased in response to a triggering stress during three days after the priming treatment had ended indicating cold memory. Interestingly, in all scenarios the primed state was lost as soon as the freezing tolerance had reached the level of naive plants indicating that an effective memory was associated with an altered physiological state. Known mutants of the cold stress response (cbfs, erf105) and heat stress memory (fgt1) did not show an altered behaviour indicating that their roles do not extend to memory of cold stress.

One of the classical functions of the plant hormone cytokinin is the regulation of plastid development but the underlying molecular mechanisms remain elusive. In this study, we employed a genetic approach to evaluate the role of cytokinin and its signalling pathway in the light-induced development of chloroplasts from etioplasts in Arabidopsis thaliana. Cytokinin increases the rate of greening and stimulates ultrastructural changes characteristic for the etioplast-to-chloroplast transition. The steady-state levels of metabolites of the tetrapyrrole biosynthesis pathway leading to the production of chlorophyll are enhanced by cytokinin. This effect of cytokinin on metabolite levels arises due to the modulation of expression for chlorophyll biosynthesis genes such as HEMA1, GUN4, GUN5 and CHLM. Increased expression of HEMA1 is reflected in an enhanced level of the encoded glutamyl-tRNA reductase, which catalyses one of the rate-limiting steps of chlorophyll biosynthesis. Mutant analysis indicates that the cytokinin receptors ARABIDOPSIS HISTIDINE KINASE2 (AHK2) and AHK3 play a central role in this process. Furthermore, the B-type response regulators ARR1, ARR10, and ARR12 play an important role in mediating the transcriptional output during etioplast-chloroplast transition. B-type ARRs bind to the promotors of HEMA1 and LHCB6 genes indicating that cytokinin-dependent transcription factors directly regulate genes of chlorophyll biosynthesis and the light harvesting complex. Together, these results demonstrate an important role for the cytokinin signalling pathway in chloroplast development, with the direct transcriptional regulation of chlorophyll biosynthesis genes as a key aspect for this hormonal control.

The root systems of plants have developed adaptive architectures to exploit soil resources. The formation of lateral roots (LRs) contributes to root system architecture. Roots of plants with a lower cytokinin status form LR primordia (LRP) in unusually close proximity, indicating a role for the hormone in regulating the positioning of LRs along the main root axis. Data obtained from cytokinin-synthesis mutants of Arabidopsis thaliana combined with gene expression analysis indicate that cytokinin synthesis by IPT5 and LOG4 occurring early during LRP initiation generates a local cytokinin signal abbreviating LRP formation in neighbouring pericycle cells. In addition, IPT3, IPT5, and IPT7 contribute to cytokinin synthesis in the vicinity of existing LRP, thus suppressing initiation of new LRs. Interestingly, mutation of CYP735A genes required for trans-zeatin biosynthesis caused strong defects in LR positioning, indicating an important role for this cytokinin metabolite in regulating LR spacing. Further it is shown that cytokinin and a known regulator of LR spacing, the receptor-like kinase ARABIDOPSIS CRINKLY4 (ACR4), operate in a non-hierarchical manner but might exert reciprocal control at the transcript level. Taken together, the results suggest that cytokinin acts as a paracrine hormonal signal in regulating root system architecture.

A role of the plant hormone cytokinin in regulating the development and activity of chloroplasts was described soon after its discovery as a plant growth regulator more than 50 years ago. Its promoting action on chloroplast ultrastructure and chlorophyll synthesis has been reported repeatedly, especially during etioplast-to-chloroplast transition. Recently, a protective role of the hormone for the photosynthetic apparatus during high light stress was shown. Details about the molecular mechanisms of cytokinin action on plastids are accumulating from genetic and transcriptomic studies. The cytokinin receptors AHK2 and AHK3 are mainly responsible for the transduction of the cytokinin signal to B-type response regulators, in particular ARR1, ARR10, and ARR12, which are transcription factors of the two-component system mediating cytokinin functions. Additional transcription factors linking cytokinin and chloroplast development include CGA1, GNC, HY5, GLK2, and CRF2. In this review, we summarize early and more recent findings of the long-known relationship between the hormone and the organelle and describe crosstalk between cytokinin, light, and other hormones during chloroplast development.

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 thaliana plants with a reduced cytokinin status, i.e. cytokinin receptor mutants and transgenic cytokinin-deficient plants, are more susceptible to light stress compared to wild-type plants. This was reflected by a stronger photoinhibition after 24 hours high light (~1000 µmol m-2 s-1), as shown by the decline in maximum quantum efficiency of photosystem II photochemistry (Fv/Fm). 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. Additionally, slow and incomplete recovery in these plants after light stress indicated insufficient D1 repair. Inhibition of the D1 repair cycle by lincomycin treatment indicated that these plants experience more photodamage. The efficiency of photoprotective mechanisms, such as non-enzymatic 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. Mutant analysis revealed that the protective function of cytokinin during light stress depends on the AHK2 and AHK3 receptors and the type-B response regulators ARR1 and ARR12. We conclude that proper cytokinin signaling and regulation of specific target genes is necessary to protect plants efficiently from light stress.

Background The plant hormone cytokinin regulates growth and development of roots and shoots in opposite ways. In shoots it is a positive growth regulator whereas it inhibits growth in roots. It may be assumed that organ-specific regulation of gene expression is involved in these differential activities, but little is known about it. To get more insight into the transcriptional events triggered by cytokinin in roots and shoots, we studied genome-wide gene expression in cytokinin-treated and cytokinin-deficient roots and shoots. Results It was found by principal component analysis of the transcriptomic data that the immediate-early response to a cytokinin stimulus differs from the later response, and that the transcriptome of cytokinin-deficient plants is different from both the early and the late cytokinin induction response. A higher cytokinin status in the roots activated the expression of numerous genes normally expressed predominantly in the shoot, while a lower cytokinin status in the shoot reduced the expression of genes normally more active in the shoot to a more root-like level. This shift predominantly affected nuclear genes encoding plastid proteins. An organ-specific regulation was assigned to a number of genes previously known to react to a cytokinin signal, including root-specificity for the cytokinin hydroxylase gene CYP735A2 and shoot specificity for the cell cycle regulator gene CDKA;1. Numerous cytokinin-regulated genes were newly discovered or confirmed, including the meristem regulator genes SHEPHERD and CLAVATA1, auxin-related genes (IAA7, IAA13, AXR1, PIN2, PID), several genes involved in brassinosteroid (CYP710A1, CYP710A2, DIM/DWF) and flavonol (MYB12, CHS, FLS1) synthesis, various transporter genes (e.g. HKT1), numerous members of the AP2/ERF transcription factor gene family, genes involved in light signalling (PhyA, COP1, SPA1), and more than 80 ribosomal genes. However, contrasting with the fundamental difference of the growth response of roots and shoots to the hormone, the vast majority of the cytokinin-regulated transcriptome showed similar response patterns in roots and shoots. Conclusions The shift of the root and shoot transcriptomes towards the respective other organ depending on the cytokinin status indicated that the hormone determines part of the organ-specific transcriptome pattern independent of morphological organ identity. Numerous novel cytokinin-regulated genes were discovered which had escaped earlier discovery, most probably due to unspecific sampling. These offer novel insights into the diverse activities of cytokinin, including crosstalk with other hormones and different environmental cues, identify the AP2/ERF class of transcriptions factors as particularly cytokinin sensitive, and also suggest translational control of cytokinin-induced changes.