UV-B radiation affects plant growth inhibiting cell proliferation. This inhibition is in part controlled by the activity of transcription factors of the E2F family. In particular, the participation of E2Fc and E2Fe in responses to UV-B in Arabidopsis plants was previously reported. However, the contribution of E2Fa and E2Fb in these processes has not been yet investigated. Thus, in this work, we provide evidence that, in Arabidopsis, both E2Fa and E2Fb control leaf size under UV-B conditions without participating in DNA damage repair. On the contrary, in seedlings exposed to UV-B, E2Fa but not E2Fb regulates primary root elongation, cell proliferation and programmed cell death in the meristematic zone. Using e2fa mutants that overexpress E2Fb, we here demonstrate that the role of E2Fa in the roots cannot be replaced by E2Fb. Finally, the results presented show that E2Fa and E2Fb differentially regulate the expression of genes that activate the DNA damage response, both under conditions without UV-B and after exposure. Together, we here show that both E2Fa and E2Fb have different and non-redundant roles in developmental and DNA damage responses in Arabidopsis plants exposed to UV-B radiation.

In plants, salicylic acid (SA) hydroxylation regulates SA homoeostasis, playing an essential role during plant development and response to pathogens. This reaction is catalyzed by SA hydroxylase enzymes, which hydroxylate SA producing 2,3- dihydroxybenzoic acid (2,3-DHBA) and/or 2,5-dihydroxybenzoic acid (2,5-DHBA). Several SA hydroxylases have been recently identified and characterized from different plant species; however, no such activity has been previously reported in maize. In this work, we describe the identification and characterization of a new SA hydroxylase in maize plants. This enzyme, with high sequence similarity to previously described SA hydroxylases from Arabidopsis and rice, converts SA into 2,5-DHBA; however, it shows different kinetics properties to those from previously characterized enzymes, and it also catalyzes the conversion of the flavonoid dihydroquercetin into quercetin in in vitro activity assays, suggesting that the maize enzyme may have different roles in vivo as those previously reported from other species. Despite this, ZmS5H can complement the resistance to pathogen and early senescence phenotypes of Arabidopsis s3h mutant plants. Finally, we characterized a maize mutant in the S5H gene ( s5hMu) that has altered growth, senescence and increased resistance against Colletotrichum graminicola infection, showing not only changes in SA and 2,5-DHBA but also variations in flavonol levels. Together, the results presented here provide evidence that SA hydroxylases in different plant species have evolved to show differences in catalytic properties that may be important to fine tune SA levels and other phenolic compounds such as flavonols to regulate different aspects of plant development and defense against pathogens.