Heterogeneity of plant mitochondrial responses underpinning respiratory acclimation to the cold in Arabidopsis thaliana leaves

In this study, we investigated whether changes in mitochondrial abundance, ultrastructure and activity are involved in the respiratory cold acclimation response in leaves of the cold-hardy plant Arabidopsis thaliana. Confocal microscopy [using plants with green fluorescence protein (GFP) targeted to the mitochondria] and transmission electron microscopy (TEM) were used to visualize changes in mitochondrial morphology, abundance and ultrastructure. Measurements of respiratory flux in isolated mitochondria and intact leaf tissue were also made. Warm-grown (WG, 25/20 °C day/night), 3-week cold-treated (CT) and cold-developed (CD) leaves were sampled. Although CT leaves exhibited some evidence of acclimation (as evidenced by higher rates of respiration at moderate measurement temperatures), it was only the CD leaves that were able to re-establish respiratory flux within the cold. Associated with the recovery of respiratory flux in the CD leaves were: (1) an increase in the total volume of mitochondria per unit volume of tissue in epidermal cells; (2) an increase in the ratio of cristae to matrix within mesophyll cell mitochondria; and (3) an increase in the capacity of the energy-producing cytochrome pathway in mitochondria isolated from whole leaf homogenates. Regardless of growth temperature, we found that contrasting cell types exhibited distinct differences in mitochondrial ultrastructure, morphology and abundance. Collectively, our data demonstrated the diversity and tissue-specific nature of mitochondrial responses that underpin respiratory acclimation to the cold, and revealed the heterogeneity of mitochondrial structure and abundance that exists within leaves

Adaptive variation in coral geometry and the optimization of internal colony light climates

1. The ability of photosynthetic organisms to adjust their light climate has high adaptive significance because irradiance can vary spatially by orders of magnitude. Using a plating (foliaceous) coral species (Turbinaria mesenterina), we tested the hypothesis that plasticity of colony geometry optimizes internal irradiance distributions. 2. We developed a two-dimensional model to predict the internal irradiance distribution of a foliaceous colony as a function of its geometry. Field tests showed that the model explained 85% of the variation in irradiance within colonies of T. mesenterina with minimal bias. 3. Colony plate angle, plate spacing and range of tissue distributions into the colony were exponential functions of water depth. In shallow water plates tended to be nearly vertical, narrowly spaced, and had living tissue only near the growing edge of the plate. In deep water plates grew more horizontally, were more widely spaced, and had living tissue extending well into the colony interior. This pattern of phenotypic plasticity effectively evens out differences in within-colony irradiances. 4. We compared within-colony irradiance distributions across light habitats (depth), based on the observed variation in colony geometry with water depth. Despite fourfold differences in environmental irradiance, within-colony irradiances had a common mode of 100-200 mu mol m(-2) s(-1). This is near the hypothesized photosynthetic optimum defined by the upper limit of the subsaturation parameter (E-k) of the photosynthesis-irradiance curve. 5. Our study demonstrates that phenotypic plasticity of colony geometry is an important mechanism for regulating light capture during growth in T. mesenterina, and facilitates near-optimal internal irradiances across a wide range of environmental light regimes