Plant growth is driven by photosynthetic carbon fixation during the day. Some photosynthate is accumulated, often as starch, to support nocturnal metabolism and growth at night. The rate of starch degradation in Arabidopsis leaves at night is essentially linear, and is such that almost all of the starch is used by dawn. We have investigated the timer that matches starch utilization to the duration of the night. The rate of degradation adjusted immediately and appropriately to an unexpected early onset of night. Starch was still degraded in an appropriate manner when the preceding light period was interrupted by a period of darkness. However, when Arabidopsis was grown in abnormal day lengths (28 h or 17 h) starch was exhausted ∼24 h after the last dawn, irrespective of the actual dawn. A mutant lacking the LHY and CCA1 clock components exhausted its starch at the dawn anticipated by its fast-running circadian clock, rather than the actual dawn. Reduced growth of wild-type plants in 28-h days and lhy/cca1 mutants in 24-h days was attributable to the inappropriate rate of starch degradation and the consequent carbon starvation at the end of night. Thus, starch degradation is under circadian control to ensure that carbohydrate availability is maintained until the next anticipated dawn, and this control is necessary for maintaining plant productivity.circadian clock | starch degradation | carbohydrate metabolism P lants are exposed to a daily alternation between light and dark. Although growth in the light can be directly fueled by photosynthesis, in the dark it depends on stored carbohydrate. In many species, starch accumulates in the light and is broken down to provide sugars for metabolism and growth at night. The rate of starch degradation in Arabidopsis leaves at night is essentially linear under a wide range of day/night lengths, and is such that about 95% of the starch is used by dawn (1, 2). This match between the length of time taken to degrade the starch reserves and the length of the night is vitally important for the normal growth of the plant. If the night is artificially extended beyond the normal dawn, the growth rate of the plant drops abruptly (3). Mutant plants that cannot accumulate starch, or degrade it only very slowly, have much lower growth rates than wild-type plants during normal nights and a reduced overall growth rate, except in continuous light or very long days (2-4). These reductions in growth rate are accompanied by large transcriptional changes indicative of carbon starvation.Remarkably, the rate of starch degradation in Arabidopsis plants adjusts immediately to an unexpected early or late onset of night. When plants grown in 16-h light/8-h dark were subjected to darkness after only 8 h of light, the rate of starch degradation was much slower than on previous nights. Conversely, when plants grown in 8-h light/16-h dark were given a 16-h light period, the subsequent rate of starch degradation was much faster than on previous nights (5). These observations imply that a timing mechanism ...

The timing of the induction of flowering determines to a large extent the reproductive success of plants. Plants integrate diverse environmental and endogenous signals to ensure the timely transition from vegetative growth to flowering. Carbohydrates are thought to play a crucial role in the regulation of flowering, and trehalose-6-phosphate (T6P) has been suggested to function as a proxy for carbohydrate status in plants. The loss of TREHALOSE-6-PHOSPHATE SYNTHASE 1 (TPS1) causes Arabidopsis thaliana to flower extremely late, even under otherwise inductive environmental conditions. This suggests that TPS1 is required for the timely initiation of flowering. We show that the T6P pathway affects flowering both in the leaves and at the shoot meristem, and integrate TPS1 into the existing genetic framework of flowering-time control.

Phosphorus (P) is an essential element for plant growth. Crop production of common bean (Phaseolus vulgaris), the most important legume for human consumption, is often limited by low P in the soil. Functional genomics were used to investigate global gene expression and metabolic responses of bean plants grown under P-deficient and P-sufficient conditions. P-deficient plants showed enhanced root to shoot ratio accompanied by reduced leaf area and net photosynthesis rates. Transcript profiling was performed through hybridization of nylon filter arrays spotted with cDNAs of 2,212 unigenes from a P deficiency root cDNA library. A total of 126 genes, representing different functional categories, showed significant differential expression in response to P: 62% of these were induced in P-deficient roots. A set of 372 bean transcription factor (TF) genes, coding for proteins with Inter-Pro domains characteristic or diagnostic for TF, were identified from The Institute of Genomic Research/Dana Farber Cancer Institute Common Bean Gene Index. Using real-time reverse transcription-polymerase chain reaction analysis, 17 TF genes were differentially expressed in P-deficient roots; four TF genes, including MYB TFs, were induced. Nonbiased metabolite profiling was used to assess the degree to which changes in gene expression in P-deficient roots affect overall metabolism. Stress-related metabolites such as polyols accumulated in P-deficient roots as well as sugars, which are known to be essential for P stress gene induction. Candidate genes have been identified that may contribute to root adaptation to P deficiency and be useful for improvement of common bean.