Figure 8.

A proposed model of the biological processes and genes potentially involved in hypoxia inducible aerenchyma formation in tomato roots under short- and long-term hypoxia, modified from former studies on rice⁶². Low oxygen conditions result in the induction of genes encoding ethylene biosynthesis enzymes ACS9 and ACO1. Moreover, expression of RBOHB, which is involved in the production of O₂⁻ radicals from oxygen in the apoplast; increases under low oxygen. On the other hand, hypoxia may stimulate Ca²⁺ influx from the apoplast to the cytosol resulting in direct stimulation of RBOHB via its EF-hand motif and higher activity of group I CDPK encoding genes such as CPK9, CPK7 and CPK21. RBOHB phosphorylation may enhance its activity and may increase the level of ROS in the apoplast as well as cytosol. Moreover, the expression of the genes encoding ROS scavengers such as MT2B decreases, potentially leading to ROS accumulation and PCD induction for lysigenous aerenchyma formation in root cortical cells. Heat maps display the up-regulated (red bars) or down-regulated (green bars) tomato genes in response to 6 h and 48 h hypoxia and their Arabidopsis thaliana homologs. Genes with expression ratios ≥2 and Padj <0.05 (n = 3) are depicted. Red and green boxes with arrows refer to the formerly reported categories of aerenchyma formation genes related to ethylene biosynthesis, RBOH, metallothionein, protein kinase and cell wall, based on^{62,63,87–89}. SAM, S-adenosyl-l-methionine; ACS, 1-aminocyclopropane-1-carboxylic acid (ACC) synthase; ACC, 1-aminocyclopropane-1-carboxylic acid; ACO, and ACC oxidase; SOD, Superoxide dismutase; RBOH, respiratory burst oxidase homolog (RBOH) isoform B; O₂⁻, superoxide anion; Ca²⁺, Calcium; CDPK, Ca²⁺-dependent protein kinases; MT2B, Metallothionein-like protein 2B; ROS, reactive oxygen species; XTH, xyloglucan endotransglucosylase/hydrolase.