Chloroplasts in C3 grasses move in response to blue-light

Abstract

Brachypodium distachyon is a good model for studying chloropla st movements in the crop plants, wheat, rye and barley. The movements are activated only by blue light, similar to Arabidopsis. Chloroplast translocations are ubiquitous in photosynthetic organisms. On the one hand, they serve to optimize energy capture under limiting light, on the other hand, they minimize potential photodamage to the photosynthetic apparatus in excess light. In higher plants chloroplast movements are mediated by phototropins (phots), blue light receptors that also control other light acclimation responses. So far, Arabidopsis thaliana has been the main model for studying the mechanism of blue light signaling to chloroplast translocations in terrestrial plants. Here, we propose Brachypodium distachyon as a model in research into chloroplast movements in C3 cereals. Brachypodium chloroplasts respond to light in a similar way to those in Arabidopsis. The amino acid sequence of Brachypodium PHOT1 is 79.3% identical, and that of PHOT2 is 73.6% identical to the sequence of the corresponding phototropin in Arabidopsis. Both phototropin1 and 2 are expressed in Brachypodium, as shown using quantitative real-time PCR. Intriguingly, the light-expression pattern of BradiPHOT1 and BradiPHOT2 is the opposite of that for Arabidopsis phototropins, suggesting potential unique light signaling in C3 grasses. To investigate if Brachypodium is a good model for studying grass chloroplast movements we analyzed these movements in the leaves of three C3 crop grasses, namely wheat, rye and barley. Similarly to Brachypodium, chloroplasts only respond to blue light in all these species.

Keywords: Blue light; Brachypodium distachyon; Cereals; Phototropins expression.

Fig. 1

Parameters of transmittance changes reflecting chloroplast relocations induced by continuous blue light in four species, wheat (Triticum aestivum), rye (Secale cereal), barley (Hordeum vulgare) and Brachypodium distachyona). a Amplitudes of transmittance changes reflecting chloroplast positions after 45 min of continuous blue light; b velocities of transmittance changes reflecting chloroplast relocations in continuous blue light. The results are the means of 5–8 experiments; number of replicates is given in brackets. Error bars represent standard deviations. c Light microscopy images of tissues after 1 h of blue light irradiation (108 μmol m⁻² s⁻¹—upper row, or 1.4 μmol m⁻² s⁻¹—lower row). Scale bars 20 μm

Fig. 2

Fluence rate response curves and mean transmittance changes. Responses of chloroplasts in leaves of wheat, rye, barley and Brachypodium distachyon to low and high intensities of continuous blue light measured as a percentage transmission at 660 nm as a function of time. a Representative fluence rate response curves; b mean leaf transmittance changes measured after consecutive steps in fluence rates. Number of replicates is given in brackets

Fig. 3

Phylogenetic analysis of PHOT1 and PHOT2 sequences in a selection of monocot and dicot species. Red color denotes species for which phototropin has the highest degree of homology to that of the model plant Brachypodium and crop species, barley and wheat. Bootstrap values (1–100) are given at each branch (colour figure online)

Fig. 4

The expression of BradiPHOT1 and BradiPHOT2 in Brachypodium leaves irradiated with blue or red light of different fluence rates. The relative mRNA levels found in leaves of dark-adapted plants and plants irradiated for 3 h. Strong red (SR), strong blue (SB), weak red (wR) or weak blue (wB) light treatments were used. Strong light was equal to 36 µmol m⁻² s⁻¹ and weak light had a fluence rate of 2 μmol m⁻² s⁻¹. Each column represents the mean of three biological replicates for mRNA isolated from a pool of three leaves detached from three plants. Error bars indicate the standard error. Asterisks indicate the statistical significance of the difference between dark- and light-treated samples