Light- and hormone-mediated development in non-flowering plants: An overview

Light, hormones and their interaction regulate different aspects of development in non-flowering plants. They might have played a role in the evolution of different plant groups by conferring specific adaptive evolutionary changes. Plants are sessile organisms. Unlike animals, they lack the opportunity to abandon their habitat in unfavorable conditions. They respond to different environmental cues and adapt accordingly to control their growth and developmental pattern. While phytohormones are known to be internal regulators of plant development, light is a major environmental signal that shapes plant processes. It is plausible that light-hormone crosstalk might have played an important role in plant evolution. But how the crosstalk between light and phytohormone signaling pathways might have shaped the plant evolution is unclear. One of the possible reasons is that flowering plants have been studied extensively in context of plant development, which cannot serve the purpose of evolutionary comparisons. In order to elucidate the role of light, hormone and their crosstalk in the evolutionary adaptation in plant kingdom, one needs to understand various light- and hormone-mediated processes in diverse non-flowering plants. This review is an attempt to outline major light- and phytohormone-mediated responses in non-flowering plant groups such as algae, bryophytes, pteridophytes and gymnosperms.

Land plants drive photorespiration as higher electron-sink: comparative study of post-illumination transient O2 -uptake rates from liverworts to angiosperms through ferns and gymnosperms

In higher plants, the electron-sink capacity of photorespiration contributes to alleviation of photoinhibition by dissipating excess energy under conditions when photosynthesis is limited. We addressed the question at which point in the evolution of photosynthetic organisms photorespiration began to function as electron sink and replaced the flavodiiron proteins which catalyze the reduction of O₂ at photosystem I in cyanobacteria. Algae do not have a higher activity of photorespiration when CO₂ assimilation is limited, and it can therefore not act as an electron sink. Using land plants (liverworts, ferns, gymnosperms, and angiosperms) we compared photorespiration activity and estimated the electron flux driven by photorespiration to evaluate its electron-sink capacity at CO₂ -compensation point. In vivo photorespiration activity was estimated by the simultaneous measurement of O₂ -exchange rate and chlorophyll fluorescence yield. All C3-plants leaves showed transient O₂ -uptake after actinic light illumination (post-illumination transient O₂ -uptake), which reflects photorespiration activity. Post-illumination transient O₂ -uptake rates increased in the order from liverworts to angiosperms through ferns and gymnosperms. Furthermore, photorespiration-dependent electron flux in photosynthetic linear electron flow was estimated from post-illumination transient O₂ -uptake rate and compared with the electron flux in photosynthetic linear electron flow in order to evaluate the electron-sink capacity of photorespiration. The electron-sink capacity at the CO₂ -compensation point also increased in the above order. In gymnosperms photorespiration was determined to be the main electron-sink. C3-C4 intermediate species of Flaveria plants showed photorespiration activity, which intermediate between that of C3- and C4-flaveria species. These results indicate that in the first land plants, liverworts, photorespiration started to function as electron sink. According to our hypothesis, the dramatic increase in partial pressure of O₂ in the atmosphere about 0.4 billion years ago made it possible to drive photorespiration with higher activity in liverworts.

Alternative electron transport mediated by flavodiiron proteins is operational in organisms from cyanobacteria up to gymnosperms

Photo-reduction of O₂ to water mediated by flavodiiron proteins (FDPs) represents a safety valve for the photosynthetic electron transport chain in fluctuating light. So far, the FDP-mediated O₂ photo-reduction has been evidenced only in cyanobacteria and the moss Physcomitrella; however, a recent phylogenetic analysis of transcriptomes of photosynthetic organisms has also revealed the presence of FDP genes in several nonflowering plant groups. What remains to be clarified is whether the FDP-dependent O₂ photo-reduction is actually operational in these organisms. We have established a simple method for the monitoring of FDP-mediated O₂ photo-reduction, based on the measurement of redox kinetics of P700 (the electron donor of photosystem I) upon dark-to-light transition. The O₂ photo-reduction is manifested as a fast re-oxidation of P700. The validity of the method was verified by experiments with transgenic organisms, namely FDP knock-out mutants of Synechocystis and Physcomitrella and transgenic Arabidopsis plants expressing FDPs from Physcomitrella. We observed the fast P700 re-oxidation in representatives of all green plant groups excluding angiosperms. Our results provide strong evidence that the FDP-mediated O₂ photo-reduction is functional in all nonflowering green plant groups. This finding suggests a major change in the strategy of photosynthetic regulation during the evolution of angiosperms.

Characterization of light-dependent regulation of state transitions in gymnosperms

The goal of this study was to characterize the light-dependent regulation of state transitions in gymnosperms. Two species of conifer were examined: eastern white pine (Pinus strobus L.) and white spruce [Picea glauca (Moench) Voss], as well as the angiosperm pumpkin (Cucurbita pepo L. subsp. pepo). Both diurnal time courses in the field and manipulated light experiments in growth chambers were conducted. Results from chlorophyll fluorescence analysis indicated that pumpkin was able to use a larger fraction of absorbed light to drive photochemistry and retain a lower reduction state at a given light intensity relative to the conifers. Results from western blots using anti-phosphothreonine demonstrate that in field conditions, conifers maintained higher light-harvesting complex II (LHCII) phosphorylation than pumpkin; however, this was likely due to a more variable light environment. Manipulated light experiments showed that general patterns of light-dependent LHCII phosphorylation were similar in conifers and pumpkin, with low levels of LHCII phosphorylation occurring in darkness and maximal levels occurring in low light conditions. However, high light-dependent dephosphorylation of LHCIII appears to be regulated differently in conifers, with conifers maintaining phosphorylation of LHCII proteins at higher excitation pressure compared with pumpkin. Additionally, spruce needles maintained relatively high phosphorylation of LHCII even in very high light conditions. Our results suggest that this difference in dephosphorylation of LHCII may be due to differences in the stromal redox status in spruce relative to pine and pumpkin.

Inhibition of the light-independent synthesis of chlorophyll in pine cotyledons at low temperature

Cotyledons of Japanese black pine (Pinus thunbergii) were yellow when they developed in darkness at 8 degrees C since the light-independent synthesis of chlorophyll was almost completely inhibited in these cotyledons. The level of chlorophyll in dark-grown cotyledons was less than one-twentieth of that in light-grown cotyledons at the same temperature. In the yellow cotyledons, levels of transcripts of cab, rbcS, rbcL and psbA genes were quite high. The large and small subunits of ribulose-1,5-bisphosphate carboxylase/oxygenase were also detected at relatively high levels in yellow cotyledons. However, the accumulation of the two apoproteins of the light-harvesting chlorophyll a/b-binding protein of PSII was limited because of the limited supply of chlorophyll.

[Radiation-population monitoring of Pinus sylvestris L. in the zone of the Chernobyl power plant]

Cytogenetic and genetic effects in populations of Pinus sylvestris L. suffered wiak, average, strong and sublethal radiation damage after the Chernobyl accident in 1986 were studied. The absorbed dozes for trees in these plantings were from 0.1 up to 20 Gy. It was shown that the amount of cells with chromosome aberrations in sprouts of seeds of a crop of 1993, are comparable with effects marked at once after accident in 1986. In 1997 and in 1998 the amount of cells with chromosome aberrations in sprouts of seeds in majority inspected plantings decreased to control values. The effect of adaptation was detected, when seeds of Pinus sylvestris L., gathered in 1997 from inspected trees, were exposed to additional 4 Gy gamma-radiation.

Effects of shade and root confinement on the expression of plagiotropic growth in juvenile-origin Douglas-fir rooted cuttings

The purpose of this experiment was to determine why juvenile-origin Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) rooted cuttings, which remain plagiotropic (branchlike) when grown in containers in shaded glasshouses, become orthotropic (vertical) after they are transplanted to an outdoor environment. Plagiotropic rooted cuttings (mean angle from vertical = 45-50 degrees) from three full-sib families were transplanted into an outdoor nursery and subjected to four treatments consisting of a factorial of (1) shaded or unshaded and (2) bareroot or confined roots. After two growing seasons, treatments had significantly affected plant size and biomass in the order unshaded-bareroot > shaded-bareroot > unshaded-confined > shaded-confined, but plants in all treatments had become nearly orthotropic. It is concluded that neither shading nor root confinement is, but other glasshouse environmental conditions are, responsible for the persistence of plagiotropic growth.