Fig. 4.

Signalling interactions between ABA and mitochondrial ROS. (A) Under salt stress conditions, proline (Pro) and mitochondrial ROS are involved in regulatory loops involving ABA pathway phospholipase Dα1 (PLDα1) (Pokotylo et al., 2012; Garcia de la Garma et al., 2015). Depending on stress conditions, proline dehydrogenase (ProDH) can be downregulated (Verbruggen and Hermans, 2008) or upregulated (Verslues et Sharma, 2010; Garcia de la Garma et al., 2015) by proline. Phospholipase Dα1 appears to be regulated by stress-related factors that remain to be identified (Garcia de la Garma et al., 2015). (B) The ABA-insensitive-4 (ABI4) transcription factor integrates mitochondrial retrograde signals, in particular ETC-derived ROS (Schwarzländer and Finkemeier, 2013; Waszczak et al., 2018), with chloroplast retrograde signals and ABA signalling (Koussevitzky et al., 2007; Giraud et al., 2009; Zhang et al., 2013). (C) According to in vitro studies, ABA may act on ATP/ADP and ROS dynamics through inhibition of mitochondrial adenine nucleotide translocator (ANT) (Kharenko et al., 2011; Berkowitz et al., 2016). Decrease in ADP availability in mitochondria is associated with the activation of ROS production (Basu et al., 2001; Møller, 2001; Dixit et al., 2002; Li et al., 2003; Turrens, 2003). P5CS, pyrroline-5-carboxylate synthetase; P5CDH, pyrroline-5-carboxylate dehydrogenase.