The Arabidopsis transcription factor PHR1 is essential for adaptation to high light and retaining functional photosynthesis during phosphate starvation.

The transcription factor PHR1 (PHOSPHATE STARVATION RESPONSE 1; encoded by gene At4g28610) is central for adaptation to phosphate deficiency in Arabidopsis (Arabidopsis thaliana). A rapid turnover of phosphate pools in the leaves is essential for energy transfer and metabolism within photosynthesis, and consequently, we hypothesized that PHR1 is needed for adaptation to high-light stress during P deficiency. We analyzed three Arabidopsis plant lines: wild-type, a transgenic PHR1 overexpressor line and a knockout mutant, phr1. The plants were grown under phosphate-limiting and sufficient conditions and exposed to different light conditions. Photosynthetic activity and light stress of the leaves were characterized by analyzing accumulation of carbohydrates, chlorophyll fluorescence, immunoblot detection of photosystem subunits and anthocyanin accumulation. Compared to the wild-type and the overexpressor line, the phr1 mutant has decreased levels of phosphate, anthocyanins and carbohydrates during combined P deficiency and light stress. The stressed mutant also has strongly decreased photosystem II (PSII) quantum efficiency, and shows degradation of the core units of PSII demonstrating extensive irreversible photodamage. We conclude that PHR1 is needed for the metabolic balance, for retaining P(i) levels and for inducing anthocyanin production, and during P deficiency PHR1 is vital for adaptations to avoid permanent damage to photosystems during high-light conditions.

Global analysis of microRNA in Arabidopsis in response to phosphate starvation as studied by locked nucleic acid-based microarrays.

MicroRNAs (miRNAs) are short RNA chains (20-24 bp) which are emerging as important regulators of gene expression. miRNAs are encoded by specific genes, and in Arabidopsis, 190 genes have presently been identified. It has been shown that miR399 is essential for the phosphate starvation response, and recent studies have shown transcriptional changes in a number of additional miRNAs in response to a shortage of phosphate. In this study, global profiles of the miRNA in shoots of Arabidopsis plants grown on limited phosphate or full nutrient in combination with sucrose feed were analysed using the miRCURY LNA microRNA Array system. Furthermore, changes in miRNA transcript were compared between a mutant lacking the transcription factor phosphate starvation responses 1 (PHR1) and wild-type plants. The global analysis identified miRNAs belonging to nine families to respond to P deprivation, sucrose or PHR1. Among these, miR399d, miR827, miR866, miR391 and miR163 were most prominently induced upon P starvation, whereas miR169b/c was strongly induced in previously starved plants when provided with sufficient P and more so when combined with an addition of sucrose. This study shows that array analysis is in general agreement with data obtained by other high-throughput technologies. The array data were confirmed by real-time reverse transcriptase-polymerase chain reaction analyses of selected pri-miRNAs. Our data corroborate the implication that several miRNAs are involved in the P-starvation response and further identify miR866 and miR163 as new candidates of miRNAs associated with the regulation of the P-starvation response.

Low temperature maximizes growth of Crocus vernus (L.) Hill via changes in carbon partitioning and corm development.

In Crocus vernus, a spring bulbous species, prolonged growth at low temperatures results in the development of larger perennial organs and delayed foliar senescence. Because corm growth is known to stop before the first visual sign of leaf senescence, it is clear that factors other than leaf duration alone determine final corm size. The aim of this study was to determine whether reduced growth at higher temperatures was due to decreased carbon import to the corm or to changes in the partitioning of this carbon once it had reached the corm. Plants were grown under two temperature regimes and the amount of carbon fixed, transported, and converted into a storable form in the corm, as well as the partitioning into soluble carbohydrates, starch, and the cell wall, were monitored during the growth cycle. The reduced growth at higher temperature could not be explained by a restriction in carbon supply or by a reduced ability to convert the carbon into starch. However, under the higher temperature regime, the plant allocated more carbon to cell wall material, and the amount of glucose within the corm declined earlier in the season. Hexose to sucrose ratios might control the duration of corm growth in C. vernus by influencing the timing of the cell division, elongation, and maturation phases. It is suggested that it is this shift in carbon partitioning, not limited carbon supply or leaf duration, which is responsible for the smaller final biomass of the corm at higher temperatures.

Levanase from Bacillus subtilis hydrolyses ß-2,6 fructosyl bonds in bacterial levans and in grass fructans.

A Levanase, LevB, from Bacillus subtilis 168, was expressed as a His6-tagged protein in Escherichia coli. The enzyme was purified and characterised for its activity and substrate specificity. LevB has a pH optimum of 6.0-6.5 and a maximum observed specific activity of 3 U mg(-1) using levan from Erwinia herbicola as substrate. Hydrolysis products were analysed by HPAEC, TLC, and NMR using chicory root inulin, mixed linkage fructans purified from ryegrass (Lolium perenne) and levan from E. herbicola as substrates. This revealed that LevB is an endolevanase that selectively cleaves the (ß-2,6) fructosyl bonds and does not hydrolyse inulin. Ryegrass fructans and bacterial levan was hydrolysed partially releasing oligosaccharides, but together with exoinulinase, LevB hydrolysed both ryegrass fructans and bacterial levan to near completion. We suggest that LevB can be used as a tool to achieve more structural information on complex fructans and to achieve complete degradation and quantification of mixed linkage fructans.

Overexpression of the MYB-related transcription factor GCC7 in Arabidopsis thaliana leads to increased levels of Pi and changed P-dependent gene regulation.

A proper concentration and turnover of inorganic phosphate (Pi) is essential to maintain cellular processes. Consequently, plants have mechanisms to control Pi homeostasis and to alleviate Pi limitation. The MYB-related transcription factor, PHR1, is important for gene induction during Pi starvation. PHR1 belongs to a family, characterised by the presence of a GARP- and a coiled coil domain. We propose that this family, with 15 members in Arabidopsis thaliana (L.) Heynh., be termed the GCC-family. In this study, transgenic plants overexpressing one member, GCC7, and a T-DNA knockout mutant, gcc7, are characterised. We find overexpressor plants to accumulate more Pi in shoots, irrespective of the Pi supply. Therefore, GCC7 was characterised in relation to Pi starvation. We conclude that GCC7 is not strictly required for a P-starvation response since the gcc7 mutant responds to Pi limitation. However, overexpression of GCC7 strongly interferes with the P-starvation response, abolishing induction of several P-responsive genes including AT4, IPS1 and several P-transporter genes, whereas GCC7 does not directly interfere with the PHR1 (GCC1) dependent regulation of miR399d. Thus GCC7 influences P-accumulation and P-dependent gene regulation, but GCC7 has a function entirely different from PHR1.

Acclimation of photosynthesis and respiration is asynchronous in response to changes in temperature regardless of plant functional group.

Gas exchange, fluorescence, western blot and chemical composition analyses were combined to assess if three functional groups (forbs, grasses and evergreen trees/shrubs) differed in acclimation of leaf respiration (R) and photosynthesis (A) to a range of growth temperatures (7, 14, 21 and 28 degrees C). When measured at a common temperature, acclimation was greater for R than for A and differed between leaves experiencing a 10-d change in growth temperature (PE) and leaves newly developed at each temperature (ND). As a result, the R : A ratio was temperature dependent, increasing in cold-acclimated plants. The balance was largely restored in ND leaves. Acclimation responses were similar among functional groups. Across the functional groups, cold acclimation was associated with increases in nonstructural carbohydrates and nitrogen. Cold acclimation of R was associated with an increase in abundance of alternative and/or cytochrome oxidases in a species-dependent manner. Cold acclimation of A was consistent with an initial decrease and subsequent recovery of thylakoid membrane proteins and increased abundance of proteins involved in the Calvin cycle. Overall, the results point to striking similarities in the extent and the biochemical underpinning of acclimation of R and A among contrasting functional groups differing in overall rates of metabolism, chemical composition and leaf structure.

Carbon partitioning and export in transgenic Arabidopsis thaliana with altered capacity for sucrose synthesis grown at low temperature: a role for metabolite transporters.

We investigated the role of metabolite transporters in cold acclimation by comparing the responses of wild-type (WT) Arabidopsis thaliana (Heynh.) with that of transgenic plants over-expressing sucrose-phosphate synthase (SPSox) or with that of antisense repression of cytosolic fructose-1,6-bisphosphatase (FBPas). Plants were grown at 23 degrees C and then shifted to 5 degrees C. We compared the leaves shifted to 5 degrees C for 3 and 10 d with new leaves that developed at 5 degrees C with control leaves on plants at 23 degrees C. At 23 degrees C, ectopic expression of SPS resulted in 30% more carbon being fixed per day and an increase in sucrose export from source leaves. This increase in fixation and export was supported by increased expression of the plastidic triose-phosphate transporter AtTPT and, to a lesser extent, the high-affinity Suc transporter AtSUC1. The improved photosynthetic performance of the SPSox plants was maintained after they were shifted to 5 degrees C and this was associated with further increases in AtSUC1 expression but with a strong repression of AtTPT mRNA abundance. Similar responses were shown by WT plants during acclimation to low temperature and this response was attenuated in the low sucrose producing FBPas plants. These data suggest that a key element in recovering flux through carbohydrate metabolism in the cold is to control the partitioning of metabolites between the chloroplast and the cytosol, and Arabidopsis modulates the expression of AtTPT to maintain balanced carbon flow. Arabidopsis also up-regulates the expression of AtSUC1, and to lesser extent AtSUC2, as down-stream components facilitate sucrose transport in leaves that develop at low temperatures.