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Main conclusion In accordance with a key role of WHIRLY1 in light-acclimation mechanisms, typical features of acclimation to high light, including photosynthesis and leaf morphology, are compromised in WHIRLY1 deficient plants. Abstract Acclimation to the environment requires efficient communication between chloroplasts and the nucleus. Previous studies indicated that the plastid-nucleus located WHIRLY1 protein is required for the communication between plastids and the nucleus in situations of high light exposure. To investigate the consequences of WHIRLY1 deficiency on the light acclimation of photosynthesis and leaf anatomy, transgenic barley plants with an RNAi-mediated knockdown of HvWHIRLY1 were compared to wild-type plants when growing at low and high irradiance. While wild-type plants showed the typical light acclimation responses, i.e. higher photosynthetic capacity and thicker leaves, the WHIRLY1 deficient plants were not able to respond to differences in irradiance. The results revealed a systemic role of WHIRLY1 in light acclimation by coordinating responses at the level of the chloroplast and the level of leaf morphology.

WHIRLY1 is a chloroplast-nucleus located DNA/RNA-binding protein with development and stress tolerance functions. By overexpression of HvWHIRLY1 in barley, lines with 10- and two lines with a 50-fold accumulation of the protein were obtained. In these lines, the relative abundance of the nuclear form exceeded that of the chloroplast form, indicating that over-accumulating WHIRLY1 exceeded the amount that chloroplasts can sequester. Growth of the plants was shown to be compromised in a WHIRLY1 abundance-dependent manner. Over-accumulation of WHIRLY1 in chloroplasts had neither an evident impact on nucleoid morphology nor the composition of the photosynthetic apparatus. Nevertheless, oeW1 plants were found to be compromised in the efficiency of photosynthesis. The reduction in growth and photosynthesis was shown to be accompanied by a decrease in the levels of cytokinins and an increase in the level of jasmonic acid. Gene expression analyses revealed that in non-stress conditions, the oeW1 plants had enhanced levels of pathogen response (PR) gene expression indicating activation of constitutive defense. During growth in continuous light of high irradiance, PR1 expression increased in addition to an increase in the expression of PR10 and the gene encoding phenylalanine lyase (PAL), the key enzyme of salicylic acid biosynthesis in barley. The activation of defense gene expression in oeW1 plants coincided with an enhanced resistance towards powdery mildew, which in barley is independent of salicylic acid. The results show that over-accumulation of WHIRLY1 in barley to levels of 10 or more amplified the tradeoff between growth and stress resistance.