Successional changes in bacterial phyllosphere communities are plant-host species dependent

Phyllosphere microbial communities are increasingly experiencing intense pulse disturbance events such as drought. It is currently unknown how phyllosphere communities respond to such disturbances and if they are able to recover. We explored the stability of phyllosphere communities over time, in response to drought stress, and under recovery from drought on temperate forage grasses. Compositional or functional changes were observed during the disturbance period and whether communities returned to non-stressed levels following recovery. Here, we found that phyllosphere community composition shifts as a result of simulated drought but does not fully recover after irrigation is resumed and that the degree of community response to drought is host species dependent. However, while community composition had changed, we found a high level of functional stability (resistance) over time and in the water deficit treatment. Ecological modeling enabled us to understand community assembly processes over a growing season and to determine if they were disrupted during a disturbance event. Phyllosphere community succession was characterized by a strong level of ecological drift, but drought disturbance resulted in variable selection, or, in other words, communities were diverging due to differences in selective pressures. This successional divergence of communities with drought was unique for each host species. Understanding phyllosphere responses to environmental stresses is important as climate change-induced stresses are expected to reduce crop productivity and phyllosphere functioning.

IMPORTANCE

Leaf surface microbiomes have the potential to influence agricultural and ecosystem productivity. We assessed their stability by determining composition, functional resistance, and resilience. Resistance is the degree to which communities remain unchanged as a result of disturbance, and resilience is the ability of a community to recover to pre-disturbance conditions. By understanding the mechanisms of community assembly and how they relate to the resistance and resilience of microbial communities under common environmental stresses such as drought, we can better understand how communities will adapt to a changing environment and how we can promote healthy agricultural microbiomes. In this study, phyllosphere compositional stability was highly related to plant host species phylogeny and, to a lesser extent, known stress tolerances. Phyllosphere community assembly and stability are a result of complex interactions of ecological processes that are differentially imposed by host species.

Circadian changes of brown adipose tissue thermogenesis in juvenile rats

The aim of this study was to determine whether the circadian cycle of core temperature (Tc) and oxygen consumption (VO2) in juvenile rats is correlated with changes in the thermogenic activity of brown adipose tissue (BAT). Nine-day-old rat pups derived from two colonies with 12:12 h light-dark cycles 180 degrees out of phase were artificially reared in continuous light for 12-48 h while Tc and VO2 were recorded. For measurement of GDP binding half the pups from each colony were killed during the maximum phase of their circadian Tc cycle and the other half during the minimum phase. During the maximum phase (0.5-2 h after lights-off in the colony of origin), when Tc was 36.1 +/- 0.1 degrees C, GDP binding (pmol/mg protein) was 575 +/- 31 (n = 19). During the minimum phase (0.5-2 h after lights-on), when Tc was 32.9 +/- 0.5 degrees C, GDP binding was 361 +/- 47 (n = 19). Moreover, the correlations between Tc, VO2, and GDP binding determined during the maximum plateau of the circadian cycle differ substantially from those determined during the minimum phase. Consequently, developmental studies of thermoregulation and BAT activity in suckling pups are seriously compromised if circadian phase is neglected.

Development of circadian cycle of core temperature in juvenile rats

To find out whether or not juvenile rats can sustain circadian rhythms of core temperature (Tc) in the absence of diurnal cues, we continuously recorded Tc for 12 days in isolated pups artificially reared in continuous dim light. Possible effects of uncontrolled environmental 24-h cycles were excluded by simultaneously studying pups born on the same day to two mothers entrained to 12:12 h light-dark cycles 180 degrees out of phase. Age-dependent changes were discriminated from the effects of sustained isolation by comparing Tc records between pups isolated from 10 or from 4 days of age (late- or early-isolated pups). Both groups of pups showed Tc cycles in phase with the light cycle in the colony of origin and with highest amplitudes from 10 to 13 days of age. Tc cycles with a period length close to 24 h could be seen through the 12th day of artificial rearing for early-isolated pups but only until the 6th day for late-isolated pups. In the third postnatal week the rhythm faded gradually. Self-sustained circadian Tc rhythms observed in isolated 1- to 2-wk-old pups demonstrate a functional endogenous clock in the juvenile rat.