Extreme dehydration of plant tissues irreversibly converts the major and variable phyA' into the minor and conserved phyA''.

Effect of dehydration of plant tissues on the two native phenomenological phytochrome A (phyA) pools - major, variable and soluble phyA' and minor, relatively conserved and presumably membrane(protein)-associated phyA'' - was investigated on etiolated seedlings of barley and maize. With the use of in situ low-temperature fluorescence spectroscopy and photochemistry, it was found that even a considerable loss of water (up to 75-85% of the initial fresh weight) by coleoptiles does not bring about noticeable alterations of the spectroscopic and photochemical parameters of phytochrome pointing to a relative stability of the phyA'/phyA'' system in this regard. However, extreme dehydration (loss of weight 90%) of plant tissues including freeze-drying caused dramatic changes of the phytochrome properties - blue shift of the emission maximum and its widening and reduction in the extent of the Pr photoconversion into lumi-R at 85 K and into Pfr at 273 K. Rehydration of the dried tissues did not reverse the spectroscopic changes and did not recover the Pr-->lumi-R phototransformation at 85 K but restored the ability of Pr to photoconvert into Pfr at ambient temperatures. At the same time, the total phytochrome content was not affected by these treatments. These effects were interpreted as an irreversible transformation of phyA' into phyA'' upon extreme loss of water by plant tissues suggesting that water may play a role in stabilizing the conformation of the major and soluble phyA' species. The data also imply that phyA in dry and imbibing seeds is likely represented primarily by its phyA'' isoform.