Lukas M. Müller - The sucrose storage metabolism regulates carbohydrate supply for growth by a concentration-dependent kinetics but not the circadian clock in barley

The circadian clock controlscarbohydrate metabolism and growth in Arabidopsis, suggesting it as key targetfor crop improvement. However, my recent work shows that the circadian clockthrough EARLY FLOWERING 3 (ELF3) does not determine carbohydrate supply forgrowth in barley at night, in contrast to Arabidopsis. I find that carbohydratesupply from the leaves at night is dominated by sucrose in barley but notstarch as in Arabidopsis. Depletion of transitory sucrose from the leaf wasexponential and almost exhausted at the end of the night, even under unexpectedextension of photoperiod. This depletion pattern depended on the sucrosecontent at the end of the day, was independent from circadian control andpresumably determined by SUCROSE TRANSPORTER 2 (SUT2) through catalyzingsucrose export from the vacuole by a kinetics of first order. On the otherhand, degradation of the little amounts of transitory starch in barley wastemporally controlled by the clock component ELF3 in both barley and Arabidopsis.Therefore, barley and Arabidopsis apply two different forms of nocturnalcarbohydrate supply: The sucrose storage and the starch storage metabolism.Consistent with regulation by a clock-driven program or a transporter-catalyzedkinetic, carbohydrate supply from starch was compensated against lowtemperature while reduced for sucrose so that barley, but not Arabidopsis,reduced biomass after growth in cool nights. Growth of wheat, rice and wildbarley but not Brachypodium also depended on the sucrose storage metabolism. Insummary, my findings explain how cool nights and the circadian clock determinegrowth in different species despite the conservation of the circadian clock.