Cell wall-targeted APases up-regulated by P-deficient plants were hypothesized to recycle Pi from this leaked Po (Barrett-Lennard et al., 1993; Robinson et al., 2012a). Senescence involves the regulated catabolism of macromolecules, including conversion of membrane lipids to sugars; the progressive disintegration of the plasma membrane would also permit significant levels of Po (e.g. P-esters and RNA oligonucleotides) to escape from the cytoplasm (Fig. 5) (Lim et al., 2007).