Does singlet oxygen activate cell death in Arabidopsis cell suspension cultures? Analysis of the early transcriptional defence responses to high light stress.

Can Arabidopsis cell suspension cultures (ACSC) provide a useful working model to investigate genetically-controlled defence responses with signalling cascades starting in chloroplasts? In order to provide a convincing answer, we analysed the early transcriptional profile of Arabidopsis cells at high light (HL). The results showed that ACSC respond to HL in a manner that resembles the singlet oxygen ( ( 1) O 2)-mediated defence responses described for the conditional fluorescent (flu) mutant of Arabidopsis thaliana. The flu mutant is characterized by the accumulation of free protochlorophyllide (Pchlide) in plastids when put into darkness and the subsequent production of ( 1) O 2 when the light is on. In ACSC, ( 1) O 2 is produced in chloroplasts at HL when excess excitation energy flows into photosystem II (PSII). Other reactive oxygen species are also produced in ACSC at HL, but to a lesser extent. When the HL stress ceases, ACSC recovers the initial rate of oxygen evolution and cell growth continues. We can conclude that chloroplasts of ACSC are both photosynthetically active and capable of initiating ( 1) O 2-mediated signalling cascades that activate a broad range of genetically-controlled defence responses. The up-regulation of transcripts associated with the biosynthesis and signalling pathways of OPDA (12-oxophytodienoic acid) and ethylene (ET) suggests that the activated defence responses at HL are governed by these two hormones. In contrast to the flu mutant, the ( 1) O 2-mediated defence responses were independent of the up-regulation of EDS1 (enhanced disease susceptibility) required for the accumulation of salicylic acid (SA) and genetically-controlled cell death.