The seedlings of higher plants must orchestrate an ordered degradation of their cotyledon cells, which store carbohydrates and proteins, in order to germinate. Toyooka et al. (page 973) sought to characterize the breakdown of starch by α-amylase in these cells, and managed to identify two distinct autophagic pathways in Vigna mungo seedlings. These two types of autophagy may be general components of programmed cell death in senescing plant cells.
After determining that α-amylase localizes to protein storage vacuoles and lytic vacuoles, the authors found that starch granules enter the lytic vacuoles through autophagy. The starch granules are wrapped with an acidic cell compartment in a process similar to micropexophagy observed in yeast cells. In addition, cotyledon cells degrade mitochondria and cytoplasm in a process mediated by autophagosomes, similar to yeast macroautophagy. In contrast to yeast autophagy, however, the degradation of cotyledon cells is part of a normal developmental process rather than a response to nutritional stress. The two V. mungo autophagic pathways can be uncoupled by removing the embryo axis, indicating that they are controlled by separate cellular mechanisms.
The degradation of cotyledon cells is structurally and biochemically distinct from the apoptotic cell death seen in plants defending themselves against pathogens, presumably reflecting the need to extract nutrients from the senescing cotyledons. In recent work, the authors have also observed autophagy in the cells of senescing leaves, in which chloroplasts are engulfed by vacuoles before degradation. ▪