Abscisic acid signalling determines susceptibility of bundle sheath cells to photoinhibition in high light-exposed <i>Arabidopsis</i> leaves

<jats:p> The rapid induction of the bundle sheath cell (BSC)-specific expression of <jats:italic>ASCORBATE PEROXIDASE2</jats:italic> ( <jats:italic>APX2</jats:italic> ) in high light (HL)-exposed leaves of <jats:italic>Arabidopsis thaliana</jats:italic> is, in part, regulated by the hormone abscisic acid (ABA) produced by vascular parenchyma cells. In this study, we provide more details of the ABA signalling that regulates <jats:italic>APX2</jats:italic> expression and consider its importance in the photosynthetic responses of BSCs and whole leaves. This was done using a combination of analyses of gene expression and chlorophyll <jats:italic>a</jats:italic> fluorescence of both leaves and individual BSCs and mesophyll cells. The regulation of <jats:italic>APX2</jats:italic> expression occurs by the combination of the protein kinase SnRK2.6 (OST1):protein phosphatase 2C ABI2 and a Gα (GPA1)-regulated signalling pathway. The use of an <jats:italic>ost1-1/gpa1-4</jats:italic> mutant established that these signalling pathways are distinct but interact to regulate <jats:italic>APX2</jats:italic> . In HL-exposed leaves, BSC chloroplasts were more susceptible to photoinhibition than those of mesophyll cells. The activity of the ABA-signalling network determined the degree of susceptibility of BSCs to photoinhibition by influencing non-photochemical quenching. By contrast, in HL-exposed whole leaves, ABA signalling did not have any major influence on their transcriptomes nor on their susceptibility to photoinhibition, except where guard cell responses were observed. </jats:p>