Section 2 Germination

**75**

**Chapter 4**

for Life

**Abstract**

*Angel J. Matilla*

a puzzle far from being solved.

vacuolar processing enzymes

**1. Introduction**

Programmed Cell Death in Seeds:

An Adaptive Mechanism Required

The regeneration of the mother plant through germinative process is the main reason that evolutionarily justifies the existence of a viable seed. Current knowledge indicates that the control of germination is a sophisticated process mainly controlled by hormones and reactive oxygen species (ROS), among other endogenous factors. One of the events that directly participate in the germination is the degradation of storage proteins (SPs). Thus, vacuolar processing enzymes (VPEs) contribute to SPs' degradation and mobilization due to direct proteolysis or through the activation of other peptidases. In parallel, the relationship between VPEs and programmed cell death (PCD) is beyond doubt. As an alternative to VPEs, the formation of vesicles called ricinosomes containing papain-like Cys-proteases (PLCPs) and located in the reserve tissues of some germinating seeds also collaborates to protein degradation. Finally, there are increasing evidences linking nucleases to PCD in different tissues of seed. However, its state of the art is still little developed. Together, this current overview illustrates a part of the complexity of PCD in seeds,

**Keywords:** Cys-endoproteases, endosperm, lytic vacuoles, nucleases, papain-like Cys-proteases, ricinosomes, seeds, seed storage proteins,

The life cycle of organisms requires targeted cell types to be removed in a predictable and genetically organized way. This process of cellular suicide, named programmed cell death (PCD), occurs from embryogenesis to senescence and is an essential part of development and cell homeostasis of any multicellular organism [1–3]. Thus, PCD has been observed from the onset of zygotic embryogenesis until the germinative process ends [4–6]. The mechanism through which specific cells are targeted for PCD without affecting neighboring cells has not yet been resolved. Notable cellular compartments (i.e., mitochondria, chloroplasts, Golgi complex, endoplasmic reticulum (ER), and vacuoles) have been shown to be involved in PCD [7]. Plant PCD exhibits several hallmarks: (i) DNA laddering and strong chromatin condensation [8]; (ii) sometimes, release of cytochrome-c from the mitochondria to the cytosol, and its subsequent degradation, which is dependent on reactive oxygen species (ROS) and caspase-like activity [9]; (iii) generation of autophagic vacuoles due to the absence of an active phagocytosis system [10, 11];

#### **Chapter 4**
