Oxygen shortage--hypoxia is a common phenomenon in the environment. Plants response to such stress conditions by developing a number of morphological and metabolic strategies. These changes are usually preceded or accompanied by activation or repression of specific genes. DNA microarray technology showed that differentially expressed genes include the known anaerobic proteins as well transcriptions factors, signal transductions components, and genes that encode enzymes of pathways not known previously to be involved in low-oxygen metabolism. Selection and characterization of various mutants with altering tolerance to hypoxia provide information that help in elucidating possible signal transduction pathways that regulate responses to oxygen deficiency. Recently, many studies have been focused on the role of Rop proteins, H2O2 and Ca2+ as second messengers in hypoxia responses. Stress-induced hemoglobins may help maintaining the energy status of cells under low oxygen stress or function as dioxygenases, detoxifying NO produced during hypoxia.