New light shed on life and death: the role of staygreen in the hypersensitive response
Leaf senescence is one of the most striking and beautiful visual seasonal changes that we experience in temperate regions. A key component of leaf senescence is the breakdown of chlorophyll, which results in loss of the green colour and reveals the underlying yellow carotenoids. It has been argued that this is the core ancestral senescence event and that it triggers regulatory steps which contribute to the establishment of the rest of the senescence programme (Thomas et al., 2009). Senescence ultimately leads to a form of programmed cell death (PCD), which is common also to other plant processes, such as the development of specialized organs, and for protection from pathogens through the hypersensitive response (HR). Therefore, one question that arises is whether there are common regulators in HR-linked, developmental and senescence-associated PCD. Another interesting cross-over between different types of PCD is the involvement of light. The role of light in the progression of leaf senescence has been well established, and is related to photoperiodic signals in perennial plants (Keskitalo et al., 2005) and to nutrient status in shaded leaves (Wingler & Roitsch, 2008), but could light also contribute to the downstream PCD events? Increasing evidence shows that light is important in modulating HR-linked PCD. In this issue of New Phytologist, Mur et al. (pp. 161–174) identify a key link between these processes in the role of the staygreen gene (sgr). A compelling case is exposed of a requirement for chlorophyll degradation in HR-linked PCD and conversely an activation of PCD by promoting chlorophyll degradation.
‘This is an interesting finding as the accepted relationship between senescence and PCD seems here to be reversed.’
A new role for an old gene?
The sgr gene has a long and illustrious history, from the time of Mendel to the present day, being the gene responsible for the green/yellow pea polymorphism used by Mendel to elucidate his inheritance laws (Armstead et al., 2007). Later identified in Festuca pratensis (Thomas & Stoddart, 1975), as the name implies, mutants in the sgr gene remain green during senescence while in wild-type plants the leaves turn yellow. Known as a ‘cosmetic’ stay green phenotype, photosynthesis declines normally in sgr mutants, but chlorophyll is not broken down. It seems that the SGR protein has a role in the disassembly of chlorophyll–protein complexes in photosystem II (PSII) and interacts physically with PSII proteins (Park et al., 2007; Aubry et al., 2008), but enigmatically we still do not know the exact function of the protein that sgr encodes.
Mur et al., during their investigations of the role of SGR during pathogen attack, revealed an important function for the sgr gene in mediating the effect of light on pathogenesis. Zeier et al. (2004) and others have already shown the importance of light in the HR. Thus, in the dark, the HR was inhibited, resulting in an increased spread of pathogen. So why would light affect the HR? The production of reactive oxygen species (ROS) is a key feature of the HR, but where are they produced? Most attention has been focussed on the apoplast, where NADPH oxidases are major sources of ROS (Torres et al., 2002), but more recently it has shifted to organelles: critically, the mitochondrion and the chloroplast. The chloroplast is particularly vulnerable to the over-production of ROS as a result of the requirement for dissipation of the vast amounts of energy that it absorbs through its light-harvesting complexes. The electron-transfer chain in chloroplasts is the main site of ROS generation in photosynthetic cells, and disruption of the photosynthetic electron transport system rapidly results in ROS photoproduction (Baier & Dietz, 2005). Therefore, the chloroplast could be the key link between ROS production and the effects of light on the HR. Mur et al. show that the premature disassembly of PSII in SGR over-expressing lines results in localized cell death linked to the over-production of ROS, and, significantly, in the over-expressing plants the induction of cell death by pathogen attack is accelerated. Conversely, it is delayed in sgr RNA interference lines and the expression of sgr itself is induced by pathogen attack. Thus, it would seem that SGR-mediated PSII disassembly is a core component of the pathogen-induced HR. From the data of Mur et al. it also seems that chlorophyll catabolite accumulation is required for an efficient HR. Thus, SGR-mediated chlorophyll breakdown may be important both in light-dependent hydrogen peroxide (H2O2) production through the disassembly of the light-harvesting complex, and also in releasing chlorophyll, which is converted to the catabolic intermediate pheide, which in turn becomes a source of singlet oxygen. As a result of the timing of the process it seems unlikely that this accumulation is a trigger for the HR but may be important in mediating the cellular collapse occurring during HR-associated PCD.
So how does the role of SGR fit with other players in PCD and senescence?
Evidence indicates that the HR is a complex process involving both light-mediated and light-independent processes (Zeier et al., 2004). Activation of phenylalanine ammonia-lyase, and the production of salicylic acid and the pathogenesis-related protein, PR-1, during the HR, appear to be light-dependent events; however, the induction of other pathogen-induced defence genes, such as glutathione-S-transferase, were not light induced. How these two processes are coordinated, and how the SGR function is integrated with them, remains to be fully elucidated. Other forms of PCD also have light-dependent components: for example, both light and chloroplast content were important factors in modulating heat-induced PCD in Arabidopsis cell cultures (Doyle et al., 2010), and it will be interesting to investigate the role of sgr here. sgr was first identified as a gene whose mutation resulted in a disruption of chlorophyll degradation during senescence. So, does sgr provide a regulatory link between senescence and HR-induced PCD? Significantly, Mur et al. showed that at low light intensities, sag12, an established marker for senescence, was switched on at the margins of the HR lesion. This is an interesting finding because the accepted relationship between senescence and PCD seems here to be reversed. We normally think of PCD as the final stage, or the stage following, leaf senescence, but here senescence would appear to occur as a consequence of the HR-induced PCD. Clearly, more remains to be done to clarify the role of SGR in both processes. So, like Mendel, we still may not know exactly what the SGR protein does, but we are getting closer to understanding its important role in the life and death of a plant.