Early investigations had alluded to effects of light, temperature and air humidity on circadian clocks10,11. Yet, most studies of plant and animal circadian clocks have been conducted using only light or temperature as a clock entraining signal even though correlations between humidity changes and potential clock outputs, including metabolism12,13,14,15, animal behaviours (e.g., the biting activity of mosquitos16,17,18,19) and incidence of plant diseases20,21 have been reported. The extent to which daily humidity oscillation directly regulates the circadian clock has not been investigated. Our study reveals that in the absence of light and temperature cues, air humidity entrains the plant circadian clock probably through high humidity-mediated induction of CCA1; whereas, in the presence of light-dark cycles, oscillating air humidity increases the amplitude of clock genes, further improves plant fitness-related traits and enhances resistance against evening-inoculated pathogens. Therefore, this study not only demonstrates humidity oscillation as a regulator of the circadian clock but also shows its specific impacts on plant physiology.
Next, we investigated the influence of simulated natural conditions on several physiological traits previously shown to be affected by the clock32,33,34,35. We compared biomass accumulation, flowering time and seed production in plants grown under light-dark cycles with either simulated natural conditions or constant 70% RH. We found that simulated natural conditions increased plant biomass accumulation and shortened flowering time in WT (Fig. 4a, b). More importantly, despite having no detectable influence on seed quantity, humidity oscillation improved seed quality as determined by seed mass36 (Fig. 4c, d). These influences were not observed in the cca1 lhy toc1 triple mutant (Fig. 4a, b, d), which is known to have near WT growth phenotype in LD37. These results indicate that simulated natural conditions influence plant physiology in a core clock gene-dependent manner.
Plant Physiology.pdf
In this study, we show that the daily oscillation of air humidity regulates the plant circadian clock and influences plant physiology in a core clock gene-dependent manner. In the absence of the light cycle, humidity oscillation entrains the circadian clock to its antiphase (Fig. 2, Supplementary Figs. 3 and 4), whereas under simulated natural conditions, it increases the amplitude of the clock and further improves fitness-related traits (Figs. 3 and 4). Our entrainment results support the existence of a humidity-sensitive oscillator within the circadian clock, but with CCA1 and LHY, the two parallel morning genes, responding to humidity differently; CCA1 responded rapidly to humidity, whereas LHY did not (Fig. 2g). Rhythmic expression of LHY and TOC1 seen in the oscillating humidity could conceivably be driven by the altered expression of CCA1. This suggests that the humidity regulation of the circadian clock might be distinct from the regulation by light and temperature, which both involve phytochromes as sensors and have similar effects on CCA1 and LHY46,47,48,49. In addition, we detected a novel effect on ETI (Fig. 5, Supplementary Fig. 6), indicating that the ability to sense oscillating humidity might allow the clock to influence distinct physiological processes that are not controlled by the light/temperature-entrained clock.
It is important to determine how plants perceive humidity oscillation and how this signal is transduced to the clock. Since it is likely that the humidity perception involves leaf transpiration due to changes in VPD, we investigated whether humidity entrainment was dependent on leaf temperature, a consequence of leaf transpiration. Temperature is a known entraining signal. If a correlated oscillation in leaf temperature caused by the known effect of humidity on transpiration was sufficient to entrain the clock, it would be expected to cause rhythms in the same phase as observed in our experiments and the prr7 prr9 double mutant, which is unresponsive to changes in air temperature, may also be unresponsive to changes in humidity. However, our data showed that humidity is still able to entrain rhythms in the temperature-insensitive prr7 prr9 mutant (Supplementary Fig. 4). We, therefore, suggest that humidity may act via a temperature-independent mechanism. However, a pleiotropic effect of the mutant background on the clock cannot be completely ruled out. Even if humidity does not regulate the clock through leaf temperature, it is still possible that the humidity perception involves other effects of leaf transpiration or sensors shared with the temperature signalling similar to what was recently reported in Drosophilae50,51. It also remains unknown how, under simulated natural conditions, oscillating humidity increases the amplitude of the light-driven clock without perturbing its phase. Elucidating this mechanism may require identification of all the humidity targets in the clock, besides CCA1, and mathematical modelling of the clock signalling network. Such information may also shed light on the question whether humidity is a novel environmental zeitgeber distinct from light and temperature.
Our study supports previous findings that the circadian clock not only allows plants to anticipate infection at the time when it is most likely to occur44,52,53 but also gates the immune response when infection does occur to minimize conflicts with growth and development54. Previously, we found that untimely induction of defence by the immune signal salicylic acid (SA) at night could lead to a severe loss of plant fresh weight. This was likely due to SA-mediated repression of genes in water transport, such as 9 of the 26 Arabidopsis aquaporin genes54. Water availability is not only essential for plant growth but also critical for pathogen virulence as recently demonstrated for Pst whose proliferation in the apoplast requires both establishment of aqueous conditions by its effectors and high humidity in the environment45. Therefore, humidity may function as a cue to allow scheduling of defence and growth activities with minimal fitness cost.
The daily oscillation of humidity is a universal natural phenomenon experienced by all terrestrial organisms. A thorough investigation of humidity as a regulator of circadian clock in other species may further advance the field of chronobiology by uncovering new clock components, or even organisms to which humidity might be a primary zeitgeber. In addition to our finding that high humidity can activate T3SS in bacteria (Supplementary Fig. 6a), air humidity is known to greatly influence organisms such as insects with high surface to body volume ratios, which make them vulnerable to rapid desiccation55. This is in line with the early observations that insect activity cycles are influenced by the diurnal humidity fluctuations reported by Colin Pittendrigh, a founder of chronobiology, in his work on malaria-transmitting mosquitoes19. We also anticipate that our study in plants may lead to more in-depth investigation of humidity influences on mammalian physiology, as multiple studies have reported a correlation between humidity and disease susceptibility in humans56,57,58.
The book by Taiz and Zieger is a worldwide used and well-established textbook in the field of life science. Plant Physiology by Taiz and Zieger 6th edition book targets to guide the life science students in the field of plant physiology. This 6th edition of the book is not just informative but has eye-catchy images as well.
The second unit focuses on Biochemistry and metabolism which is the core part of any plant physiology book, which includes photosynthesis ( light reaction, dark reaction, C3, C4, CAM Pathway), respiration, Phloem translocation, and nutrient assimilation.
This second unit of the book is the most valuable and highlighted point in CSIR NET Exam. Because this unit contains a weightage of 2 questions from part C itself and 2 from part B in the exam. Unit third is about Growth and Development which has been divided into 11 distinct chapters. This is a praiseworthy thing that authors have managed to keep the length of a book only 761 pages although the book covers nearly all aspects of plant physiology.
The salient characteristics of this book are ample numbers of vivid and sharp coloured figures. The figures in the books are expressive and eye-catchy, no figure is overloaded with the information or detail or unbalanced colour which defiantly enhances the reader experience. Plant Physiology by Taiz and Zieger 6th edition is amongst the modern Life science books with well-managed book size and quality content, it is brilliantly produced, the fun book which has been acting as an aid to all students to understand the field of plant physiology.
This title includes a number of Open Access chapters.The field of plant physiology includes the study of all chemical and physical processes of plants, from the molecular-level interactions of photosynthesis and the diffusion of water, minerals, and nutrients within the plant, to the larger-scale processes of plant growth, dormancy and reproduction 2ff7e9595c
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