Osmotic stress in roots drives lipoxygenase-dependent plastid remodeling through singlet oxygen production

Osmotic stress, caused by the lack of water or by high salinity, is a common environmental problem in roots. Osmotic stress can be reproducibly simulated with the application of solutions of the high-molecular-weight and impermeable polyethylene glycol. Different reactive oxygen species such as singlet oxygen, superoxide and hydrogen peroxide accompany this stress. Among them, singlet oxygen, produced as a byproduct of lipoxygenase activity, was shown to be associated with limiting root growth. To better understand the source and effect of singlet oxygen, its production was followed at the cellular level. Osmotic stress initiated profound changes in plastid morphology and vacuole structure. By confocal and electron microscopy the plastids were shown to be a source of singlet oxygen accompanied by the appearance of multiple small extraplastidic bodies that were also an intense source of singlet oxygen. A marker protein, CRUMPLED LEAF, indicated that these small bodies originated from the plastid outer membrane. Remarkably a type 9 lipoxygenase, LOX5, was shown to change its distribution from uniformly cytoplasmic to a more clumped distribution together with plastids and the small bodies. In addition, oxylipin products of type 9 lipoxygenase increased while products of type 13 lipoxygenases decreased. Inhibition of lipoxygenase by SHAM inhibitor or in down-regulated lipoxygenase lines prevented cells from initiating the cellular responses leading to cell death. In contrast, singlet oxygen scavenging halted terminal cell death. These findings underscore the reversible nature of osmotic stress-induced changes, emphasizing the pivotal roles of lipoxygenases and singlet oxygen in root stress physiology.