Systems biology or the biology of systems: routes to reducing hunger

Abstract

ReferencesThe chloroplast as a regulator of Ca 2+ signallingMany of the attributes associated with multicellular plant life, including a sedentary habit, a decentralized organization, signalling in the absence of a nervous system and a plastic developmental programme, can be attributed to the autotrophism facilitated by the chloroplast. In this issue of New Phytologist, ) identify a new role for the chloroplast in Ca 2+ signalling, which suggests that the plastid can exert control over signalling events in the cytosol. (Han et al., 2003)). In Arabidopsis, CAS is expressed in the shoots and is found in guard cells (Han et al., 2003). CAS binds Ca 2+ at the Nterminus with low affinity and high capacity (Han et al., 2003). CAS was first proposed to be a plasma membrane receptor that senses [Ca 2+ ] ext (Han et al., 2003 (Allen et al., 1999; Staxén et al., 1999; Weinl et al.).The sensing, by CAS, of changes in [Ca 2+ ] stroma could potentially affect the release or uptake of Ca 2+ by the chloroplast. It is possible that the plastids act either as Ca 2+ stores that release Ca 2+ into the cytosol or as a Ca 2+ buffer that removes Ca 2+ from the cytosol following stimulation. The plastids could have a similar role to mitochondria in Ca 2+ signalling. In mammals, mitochondria act as Ca 2+ buffers that take up Ca 2+ from the cytosol following release from the endoplasmic reticulum (ER) and the sarcoplasmic reticulum (SR). The tight coupling between ER/SR release and mitochondrial uptake has profound effects on localized [Ca 2+ ] cyt dynamics, and mitochondria also contain a pool of releasable Ca 2+ (Hetherington & Brownlee, 2004 (Miller & Sanders, 1987; Sai & Johnson, 2002; Johnson et al., 2006). Chloroplasts take up Ca 2+ in the light (Miller & Sanders, 1987; Xiong et al., 2006), and CASTOR and POLLUX are required for nodulation (NOD) factor-induced Ca 2+ oscillations in root hairs of Lotus japonicus and are predicted to encode plastid-localized ion channels of unknown selectivity (Imaizumi-Anraku et al., 2005). Similarly, the pea PPF1 protein localizes to the chloroplast, delays flowering when expressed in Arabidopsis and is capable of carrying Ca 2+ currents (Wang et al., 2003).In addition to plastid regulation of [Ca 2+ ] cyt , there are dark-induced increases in [Ca 2+ ] stroma that can persist with a circadian rhythm in constant dark (Sai & Johnson, 2002 ( Johnson et al., 1995). Sai & Johnson (2002) proposed that the thylakoid is a dark-dischargeable Ca 2+ store that releases into the stroma. The thylakoid is suggested to be filled with Ca 2+ from the cytosol via the stroma as a result of the action of a Ca 2+ /H + antiporter acting in the light (Ettinger et al., 1999). Lengthening the light period appears to increase the amount of Ca 2+ stored in the thylakoid because dark-induced discharge is increased with longer periods of light (Sai & Johnson, 2002).It is not known if CAS affects the daily dark-induced increase in [Ca 2+ ] stroma but CAS antisense reduces the amplitude of daily oscillations of [...

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