The involvement of hydrogen peroxide in the differentiation of secondary walls in cotton fibers

H2O2 is a widespread molecule in many biological systems. It is created enz ymatically in living cells during various oxidation reactions and by leakag e of electrons from the electron transport chains. Depending on the concent ration H2O2 can induce cell protective responses, programmed cell death, or necrosis. Here we provide evidence that H2O2 may function as a development al signal in the differentiation of secondary walls in cotton (Gossypium hi rsutum) fibers. Three lines of evidence support this conclusion: (a) the pe riod of H2O2 generation coincided with the onset of secondary wall depositi on, (b) inhibition of H2O2 production or scavenging the available H2O2 from the system prevented the wall differentiation process, and (c) exogenous a ddition of H2O2 prematurely promoted secondary wall formation in young fibe rs. Furthermore, we provide support for the concept that H2O2 generation co uld be mediated by the expression of the small GTPase Rac, the accumulation of which was shown previously to be strongly induced during the onset of s econdary wall differentiation. In support of Rac's role in the activation o f NADPH oxidase and the generation of reactive oxygen species, we transform ed soybean (Glycine max) and Arabidopsis cells with mutated Rac genes. Tran sformation with a dominantly activated cotton Rac13 gene resulted in consti tutively higher levels of H2O2, whereas transformation with the antisense a nd especially with dominant-negative Rac constructs decreased the levels of H2O2.