The function of the WHIRLY1 protein in photosynthetic light acclimation of barley

WHIRLY1 is a chloroplast-nucleus located DNA-binding protein. Its association to nucleoids makes this protein an ideal candidate to be involved in responses to environmental changes, which are signalled to the nucleus by retrograde signalling. The latter consequently leads to orchestrating gene expression in the plastid and nucleus in response to environmental changes.

In this study, light acclimation was compared between transgenic barley plants with an RNAi-mediated knockdown of HvWHIRLY1 (W1) and wild-type plants (WT). Whereas WT plants showed the typical light acclimation responses, e.g. higher photosynthetic capacity and thicker leaves, W1 did not respond to high irradiance at any developmental stage. The results revealed a systemic role of WHIRLY1 in light acclimation by coordinating responses at both levels of photosynthesis and leaf morphology.

Intriguingly, W1 plants did not show symptoms of photoinhibition after chloroplast development. Therefore, their survival under light energy exceeding their photosynthetic capacity with compromised HL acclimation must have been owed to different photoprotective strategies. Among many possible mechanisms, this study revealed some. An extremely large xanthophyll cycle pool, mostly present as zeaxanthin, was in accordance with enhanced NPQ which compensated their low photosynthetic efficiency. Beyond its role in NPQ, enhanced zeaxanthin content together with a high lutein content was assumed to function as an antioxidant. Besides, the dihydroxyflavone lutonarin was highly induced in W1 plants. Due to its antioxidant properties, lutonarin was proposed to be involved in the photoprotection of W1 plants under HL conditions in addition to the carotenoids.

Taken together, our results suggest that WHIRLY1 might be a scaffold protein whose ‘optimal abundance’ rather than ‘high or low abundance’ is required for the light acclimation of barley plants.


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