The phytohormone cytokinin was originally discovered as a regulator of cell division. Later, it was described to be involved in regulating numerous processes in plant growth and development including meristem activity, tissue patterning, and organ size. More recently, diverse functions for cytokinin in the response to abiotic and biotic stresses have been reported. Cytokinin is required for the defence against high light stress and to protect plants from a novel type of abiotic stress caused by an altered photoperiod. Additionally, cytokinin has a role in the response to temperature, drought, osmotic, salt, and nutrient stress. Similarly, the full response to certain plant pathogens and herbivores requires a functional cytokinin signalling pathway.Conversely, different types of stress impact cytokinin homeostasis. The diverse functions of cytokinin in responses to stress and crosstalk with other hormones are described. Its emerging roles as a priming agent and as a regulator of growthdefence trade-offs are discussed.

Vitamin B6 represents a highly important group of compounds ubiquitous in all living organisms. It has been demonstrated to alleviate oxidative stress and in its phosphorylated form participates as a cofactor in >100 biochemical reactions. By means of a genetic approach, we have identified a novel mutant, rsr4-1 (for reduced sugar response), with aberrant root and leaf growth that requires supplementation of vitamin B6 for normal development. Cloning of the mutated gene revealed that rsr4-1 carries a point mutation in a member of the PDX1/SOR1/SNZ (for Pyridoxine biosynthesis protein 1/Singlet oxygen resistant 1/Snooze) family that leads to reduced vitamin B6 content. Consequently, metabolism is broadly altered, mainly affecting amino acid, raffinose, and shikimate contents and trichloroacetic acid cycle constituents. Yeast two-hybrid and pull-down analyses showed that Arabidopsis thaliana PDX1 proteins can form oligomers. Interestingly, the mutant form of PDX1 has severely reduced capability to oligomerize, potentially suggesting that oligomerization is important for function. In summary, our results demonstrate the critical function of the PDX1 protein family for metabolism, whole-plant development, and vitamin B6 biosynthesis in higher plants.

We have tested in our system the behaviour of several mutants of genes known to be involved in cold acclimation including cbfs 38 , erf105 23 and ahk2 ahk3 27. Cbfs showed a slightly weaker response to priming while erf105 and ahk2 ahk3 responded similar to wild type. This indicated a limited function of CBFs in response to short cold stress in seedlings and suggested that ERF105 is required only later in development 23. This result supports the notion that the CBF-dependent signalling pathway is not exclusively responsible for the cold stress response in Arabidopsis but that additional pathways must exist 20,21,23,38,49. Neither these mutants nor the heat shock memory gene mutant fgt1 showed an altered duration of the cold memory under our experimental conditions. We conclude therefore that there must be other factors that are important to respond to low temperature and establish and/or maintain a memory to cold stress. We propose that the protocol established here will be helpful to systematically test additional candidate genes of cold stress memory and/or establish genetic screens that will lead to the discovery of factors involved in cold stress memory. For example, such screens may involve the search for altered expression characteristics of cold response genes or survivors of temporal harsh cold treatments, similar to the successful approaches taken to study the memory of heat stress 36,50. Methods Plant material. Arabidopsis thaliana accession Col-0 was used as wild type. Seeds of the forgetter1-1 (fgt1-1) 36 and the cfb triple mutant (cfbs) 38 were kindly provided by Isabel Bäurle and Shuhua Yang. The ahk2 ahk3 51 and erf105 23 mutants were published previously.

Background: The concept of metabolite profiling has been around for decades and technical innovations are now enabling it to be carried out on a large scale with respect to the number of both metabolites measured and experiments carried out. However, studies are generally confined to polar compounds alone. Here we describe a simple method for lipophilic compounds analysis in various plant tissues.

In plants, vitamin B₆ biosynthesis requires the activity of PDX1 and PDX2 proteins. Arabidopsis thaliana encodes for three PDX1 proteins, named PDX1.1, 1.2, and 1.3, but only one PDX2. Here, we show in planta complex assembly of PDX proteins, based on split-YFP and FPLC assays, and can demonstrate their presence in higher complexes of around 750 kDa. Metabolic profiling of plants ectopically expressing the different PDX proteins indicates a negative influence of PDX1.2 on vitamin B₆ biosynthesis and a correlation between aberrant vitamin B6 content, PDX1 gene expression, and light sensitivity specifically for PDX1.3. These findings provide first insights into in planta vitamin B₆ synthase complex assembly and new information on how the different PDX proteins affect plant metabolism.