A mechanistic understanding of the controls on carbon storage and losses is essential for our capacity to predict and mitigate human impacts on the global carbon cycle. Plant litter decomposition is an important first step for carbon and nutrient turnover, and litter inputs and losses are essential in determining soil organic matter pools and the carbon balance in terrestrial ecosystems. Photodegradation, the photochemical mineralization of organic matter, has been recently identified as a mechanism for previously unexplained high rates of litter mass loss in arid lands; however, the global significance of this process as a control on carbon cycling in terrestrial ecosystems is not known. Here we show that, across a wide range of plant species, photodegradation enhanced subsequent biotic degradation of leaf litter. Moreover, we demonstrate that the mechanism for this enhancement involves increased accessibility to plant litter carbohydrates for microbial enzymes. Photodegradation of plant litter, driven by UV radiation, and especially visible (blue-green) light, reduced the structural and chemical bottleneck imposed by lignin in secondary cell walls. In leaf litter from woody species, specific interactions with UV radiation obscured facilitative effects of solar radiation on biotic decomposition. The generalized effect of sunlight exposure on subsequent microbial activity, mediated by increased accessibility to cell wall polysaccharides, suggests that photodegradation is quantitatively important in determining rates of mass loss, nutrient release, and the carbon balance in a broad range of terrestrial ecosystems.carbon cycle | plant litter decomposition | photodegradation | lignin |

Spore development and stress resistance in Bacillus subtilis are governed by the master transcription factors Spo0A and B , respectively. Here we show that the coding genes for both regulatory proteins are dramatically induced, during logarithmic growth, after a temperature downshift from 37 to 20°C. The loss of B reduces the stationary-phase viability of cold-adapted cells 10-to 50-fold. Furthermore, we show that B activity is required at a late stage of development for efficient sporulation at a low temperature. On the other hand, Spo0A loss dramatically reduces the stationary-phase viability of cold-adapted cells 10,000-fold. We show that the requirement of Spo0A for cellular survival during the cold is independent of the activity of the key transition state regulator AbrB and of the simple loss of sporulation ability. Furthermore, Spo0A, and not proficiency in sporulation, is required for the development of complete stress resistance of cold-adapted cells to heat shock (54°C, 1 h), since a loss of Spo0A, but not a loss of the essential sporulation transcription factor F , reduced the cellular survival in response to heat by more than 1,000-fold. The overall results argue for new and important roles for Spo0A in the development of full stress resistance by nonsporulating cells and for B in sporulation proficiency at a low temperature.The exposure of bacteria to diverse growth-limiting conditions induces the synthesis of a large set of proteins (called general stress proteins) that protect the cell against internal (metabolic) or external (environmental) stresses (22,23,29,32,33). In the gram-positive, endospore-forming bacterium Bacillus subtilis, the general stress response is controlled mainly by B , the alternative transcription factor of the RNA polymerase that brings about a special physiological state which significantly enhances bacterial survival (11,20,22,23,29,32,33,37). It is estimated that over 200 genes (5% of the coding capacity of the genome) are directly or indirectly under B control, and the loss of B function leads to multiple-stress sensitivity, compromising the survival of the B null mutant strain (23,29,32). Besides having this very important, rapid, reversible, and plastic adaptive response (22,29), B. subtilis is also able to differentiate into dormant spores when nutritional conditions become so extreme that the B -dependent response would not be adequate to guarantee the survival of the cell (19,21,24,30,31). While B is the key regulatory protein involved in the reversible adaptive stress response of vegetative cells, the master transcription factor Spo0A is the key regulator responsible for the decision of a vegetative cell to differentiate into a dormant and highly resistant new cell, i.e., the spore (31). It is accepted that these responses, general stress adaptation and sporulation, are important for the survival of B. subtilis in its natural environment, i.e., soil (29-33). Furthermore, high levels of expression of general stress proteins provide stressed or starved cells with mu...