Study of Hydrogen Peroxide as a Senescence-Inducing Signal.

In many plant species, leaf senescence correlates with an increase in intracellular levels of reactive oxygen species (ROS) as well as differential regulation of anti-oxidative systems. Due to their reactive nature, reactive oxygen species (ROS) were considered to have only detrimental effects for long time. However, ROS turned out to be more than just toxic by-products of aerobic metabolism but rather major components in different signaling pathways. Considering its relatively long half-life, comparably low reactivity, and its ability to cross membranes, especially hydrogen peroxide, has gained attention as a signaling molecule. In this article, a set of tools to study hydrogen peroxide contents and the activity of its scavenging enzymes in correlation with leaf senescence parameters is presented.

REVOLUTA and WRKY53 connect early and late leaf development in Arabidopsis.

As sessile organisms, plants have to continuously adjust growth and development to ever-changing environmental conditions. At the end of the growing season, annual plants induce leaf senescence to reallocate nutrients and energy-rich substances from the leaves to the maturing seeds. Thus, leaf senescence is a means with which to increase reproductive success and is therefore tightly coupled to the developmental age of the plant. However, senescence can also be induced in response to sub-optimal growth conditions as an exit strategy, which is accompanied by severely reduced yield. Here, we show that class III homeodomain leucine zipper (HD-ZIPIII) transcription factors, which are known to be involved in basic pattern formation, have an additional role in controlling the onset of leaf senescence in Arabidopsis. Several potential direct downstream genes of the HD-ZIPIII protein REVOLUTA (REV) have known roles in environment-controlled physiological processes. We report that REV acts as a redox-sensitive transcription factor, and directly and positively regulates the expression of WRKY53, a master regulator of age-induced leaf senescence. HD-ZIPIII proteins are required for the full induction of WRKY53 in response to oxidative stress, and mutations in HD-ZIPIII genes strongly delay the onset of senescence. Thus, a crosstalk between early and late stages of leaf development appears to contribute to reproductive success.

bZIPs and WRKYs: two large transcription factor families executing two different functional strategies.

bZIPs and WRKYs are two important plant transcription factor (TF) families regulating diverse developmental and stress-related processes. Since a partial overlap in these biological processes is obvious, it can be speculated that they fulfill non-redundant functions in a complex regulatory network. Here, we focus on the regulatory mechanisms that are so far described for bZIPs and WRKYs. bZIP factors need to heterodimerize for DNA-binding and regulation of transcription, and based on a bioinformatics approach, bZIPs can build up more than the double of protein interactions than WRKYs. In contrast, an enrichment of the WRKY DNA-binding motifs can be found in WRKY promoters, a phenomenon which is not observed for the bZIP family. Thus, the two TF families follow two different functional strategies in which WRKYs regulate each other's transcription in a transcriptional network whereas bZIP action relies on intensive heterodimerization.