Title : Regulatory circuits connect early leaf development and senescence in Arabidopsis thaliana
Abstract:
Plant senescence is a highly regulated processthat aims at breakdown and subsequent remobilization of macromolecule components of mature leaves, either for further growth or for deposition in developing fruits and seeds. Senescence is governed by endogenous cues like leaf or plant age; however, stress conditions can induce premature senescence. While developmental senescence tends to maximize seed quality and quantity, premature senescence describes an exit strategy, often resulting in severely decreased yields. Despite the agricultural importance of senescence, our knowledge on its regulatory mechanisms is still limited. In contrast to system biology approaches which characterize overall and genome-wide changes, we want to decipher the molecular mechanisms of this regulatory network starting from one point, namely the transcription factor WRKY53, which is tightly regulated by diverse mechanisms and is a convergence node between senescence and biotic and abiotic stress responses. WRKY53 is part of a complex regulatory network between many different WRKY factors but also many other proteins which built up several layers of regulatory feedback loops which fine-tune this network. Salicylic and jasmonic acid as well as reactive oxygen species (ROS)appear to be the most important signals that trigger this network. In contrast to calcium signaling, which is executed by storage and release of Ca2+, ROS signaling is controlled by production and scavenging.By scavenging H2 O2 specifically in different compartments, we could show that the cytoplasmic H2 O2 appears to be more effective in senescence signalling than the chloroplastic or peroxisomal ROS. Furthermore, we have already identified a connection between early leaf development and senescence. Class III homeodomain leucine zipper (HD-ZIPIII)transcription factors, which are involved in basicpattern formation, have an additional role in controlling the onset ofleaf senescence in Arabidopsis. REVOLUTA (REV) acts as a redox-sensitive transcription factor, and directly andpositively regulates the expression of WRKY53. HD-ZIPIII proteins are required for thefull induction of WRKY53 expressionin response to H2 O2 as signalling molecule.Consequently, mutations in HD-ZIPIII genes strongly delayed the onset of senescence.Thus, a crosstalk between early and late stages of leaf developmentappears to contribute to reproductive success.Moreover, the antagonist of REV in early leaf development, KANADI, appears to antagonize REV also in senescence. Here,KANADI does not antagonistically regulate WRKY53 expression but regulates the expression of the antagonist of WRKY53, namely WRKY18.
