The role of starch in the day/night re-programming of stomata in plants with Crassulacean Acid Metabolism

Abstract

Crassulacean acid metabolism (CAM) is a specialised type of photosynthesis characterised by the unique inverted stomatal rhythm, which increases water use efficiency (WUE) and enhances the potential for sustainable biomass production in warmer and drier conditions. Starch turnover in the mesophyll of CAM species supports nocturnal CO2 assimilation and CAM activity. In C3 plants, starch metabolism has been reported to play an important role in determining stomatal behaviour; in this case, guard cell starch degradation is triggered by blue light, producing osmolytes that promotes stomatal opening. Based on the importance of starch and the little knowledge regarding CAM stomatal behaviour, this study tested the hypothesis that starch metabolism has been re-programmed in CAM plants to enable nocturnal stomatal opening, by using biochemical and genetic characterisation of wild type and RNAi lines with curtailed starch metabolism in the constitutive CAM species Kalanchoë fedtschenkoi. Measurements of guard cell starch content over 24 hours in wild type plants of K. fedtschenkoi indicated a day/night shift in starch turnover compared to C3, evidenced by an increment over the first hours of the day and a diminution at the beginning of the night. The characterisation of the RNAi lines confirmed that curtailed starch metabolism affected nocturnal CO2 fixation, besides the phosphorolytic starch degradation has a pivotal role for driving nocturnal CAM activity evidenced, in the same way, in both mesophyll and epidermis proteome datasets. Furthermore, higher levels of soluble sugars in the epidermis of the RNAi lines appeared to curtail completely stomatal closure during the day, indicating that guard cell starch biosynthesis is an important sink for carbohydrates, ensuring day-time stomatal closure. Finally, this thesis constituted an approach for the understanding of starch metabolism in CAM stomata and together with further studies lead to the potential engineering of CAM into C3 to enhance WUE.