Senescence is a developmental process in the life cycle of a plant or a plant organ which has an intrinsic link with longevity. In human and animal sciences, the question of what controls the length of life is a fundamental biological query which has been puzzling scientists for centuries. Plant senescence is an even more complicated topic, since plants have many life-forms which differ greatly in their maximal life-spans. On the applied side, plant senescence has a great impact on landscape, agriculture, and our daily lives, being tightly linked to crop yield and quality as well as biomass production and bio-energy development, which are of increasing concern in the current age of climate change and parallel population growth.
Leaf senescence occurs at the final stage of leaf development in a genetically well-controlled manner (Lim et al. 2007), and is considered the model for the study of plant senescence. Although substantial progress toward understanding leaf senescence has been achieved in the past, we still know very little about the regulation of leaf senescence. To add to this, despite the importance of leaf senescence, the senescence research community is relatively small. Bearing this in mind, the Journal of Integrative Plant Biology (JIPB) has published this special issue in an effort to highlight the latest research findings on leaf senescence, paying special attention to studies addressing model plants and their relevance to crops.
Leaf senescence can be regulated by many endogenous hormones in plants, and it is believed that many hormones interact with each other to regulate leaf senescence (Schippers et al. 2007). Ethylene and nitric oxide are important hormonal signals regulating leaf senescence (Jing et al. 2002; Guo and Crawford 2005). The relationship between these two hormonal pathways in regulating leaf senescence has, to this point, not been addressed. In this special issue, Fang-Qing Guo and co-workers show through an elegant double mutant study that nitric oxide-regulated, dark-induced leaf senescence completely relies on a functional EIN2 (Niu and Guo 2012). More importantly, the study shows that this dependence is specific for senescence, since the nitric oxide deficiency does not affect the ethylene signaling pathway.
The complexity of leaf senescence is evident in the fact that almost 10% of the total gene set from a genome is up-regulated during leaf senescence (Breeze et al. 2011). Many so-called senescence-associated genes (SAGs) have already been identified (http://www.eplantsenecence.org/). Hongwei Guo and co-workers took a computational approach by systematically analysing the SAG regulating network, and successfully identified several novel key regulatory genes, a finding illustrative of the importance of “dry experiments” in helping to dissect gene function (Li et al. 2012).
Reactive Oxygen Species (ROS) play an essential role in leaf senescence (Mittler et al. 2004; Jing et al. 2008). Zentgraf and co-workers used a smart system to monitor the levels of hydrogen peroxide in the cytoplasm as well as in the peroxisomes during Arabidopsis leaf senescence, and applied the same system to a study on oilseed rape, demonstrating that the knowledge gained from model plants can be translated to related crops (Bieker et al. 2012). However, most crops are not so closely related to the well-studied model plant Arabidopsis, and may have different regulatory mechanisms of leaf senescence. Surprisingly, few studies have focused on leaf senescence in crop plants. We tried to fill in these gaps by including three papers addressing leaf senescence in crops in this special issue. Derkx et al. (2012) used two hexaploid wheat senescence mutants – fast-senescence and stay-green mutants – to address the effect of the alteration of leaf senescence on wheat yield, biomass, and nitrogen partitioning, whereas Zhao et al. (2012b) used a detached wheat leaf assay to characterize the physiological and molecular changes associated with leaf senescence induced by various treatments. Cotton (Gossypium hirsutum L.), a crop exclusively used for fiber, constantly suffers from yield loss due to premature leaf senescence. However, the genetic basis of premature leaf senescence in cotton is not well defined. In this special issue, we have included for the first time a solid genetic study demonstrating that premature leaf senescence resistance is conferred by a single Mendelian gene, CPLSR1 (Zhao et al. 2012a).
In this special issue, we also address the broadness of the leaf senescence phenomenon by including papers on leaf senescence in a perennial and in a tree in field conditions. The authors of these papers showed that sugar signals and oxidative stresses may lead to leaf senescence in these wild species in a similar manner as in model plants and annual crops, but their effects may also be influenced by other environmental factors (Wingler et al. 2012; Juvany et al. 2012). Therefore, particular combinations of endogenous and exogenous factors may result in different outcomes for leaf senescence.
In conclusion, this special issue includes papers addressing leaf senescence in a wide range of plant species, thus reflecting the diversity of this fascinating topic in the plant sciences. As a matter of fact, while preparing this special issue, we realized that we as a community still know very little about leaf senescence, and that plant senescence is an incredibly broad and complicated topic to cover. Consequently, we had to be selective and thus only included papers addressing leaf senescence from currently active groups. Even then, the space we have is still limited, and it was not possible to address many important questions relevant to the regulation of leaf senescence. For instance, to name just a few: How many different signaling molecules exist to initiate leaf senescence? What is the nature of the age factors or age-related changes? How are signaling molecules perceived, or what is/are the senescence receptor(s)? How are senescence signals transduced? There is still a long way to go before we can fully address these important questions and novel approaches, and cutting-edge tools and technologies will be required.