Overexpression of wound-responsive RNA-binding proteins induces leaf senescence and hypersensitive-like cell death

Leaf senescence is a form of programmed cell death, and involves regulated expression of a specific set of senescence-associated genes (SAGs). In Arabidopsis, three UBA2 genes, UBA2a, UBA2b, and UBA2c, encode heterogeneous nuclear ribonucleoprotein (hnRNP)-type RNA-binding proteins. Previously, it has been demonstrated that expression of UBA2 genes is induced by mechanical wounding in a splice variant-dependent manner. Constitutive overexpression of the UBA2 genes proved lethal. Accordingly, a conditional gain-of-function system was used here to assess phenotypes related to UBA2 overexpression. Overexpression of each of the three UBA2 genes leads to a leaf yellowing/cell death-like phenotype in Arabidopsis plants. Expression levels of a number of SAGs, such as SAG13, SAG14, SAG15, SAG101, WRKY6, WRKY53, WRKY70, ACS2, ACS6, CML38 and SIRK, were elevated upon induction of UBA2 overexpression, as were transcripts of multiple wounding- and defense-related genes, including EDS1, CK1, JR1, WR3 and MPK3. Elevated ethylene biosynthesis and hypersensitive-like patterns of cell death and callose deposition, shown by Trypan blue and aniline blue staining, respectively, were also observed following induced overexpression of UBA2a, UBA2b, and UBA2c. These results indicate that induction of UBA2 gene expression stimulates leaf yellowing and cell death phenotypes through senescence and defense response pathways.