In a germinating maize seed the enzyme a-amylase is produced by:

Question

Accepted Answer

The correct option is 3:

The correct option is 3: **Aleurone layer**. This layer is the outermost, living cell layer of the endosperm in cereal grains like maize and barley, situated just beneath the seed coat. It is distinguished from the inner starchy endosperm by its rich content of proteins, lipids, and minerals, rather than starch, and it remains metabolically active in the mature seed. During the crucial process of germination, the embryo (specifically the scutellum) senses the appropriate environmental conditions, such as sufficient water and temperature, and releases the plant hormone **gibberellin (GA)**. This gibberellin hormone then diffuses to the adjacent aleurone layer, acting as a critical biochemical signal. In response to this GA signal, the aleurone cells become highly active, undergoing a rapid synthesis and secretion of various hydrolytic enzymes, with $\alpha$-amylase being the most prominent. This $\alpha$-amylase then diffuses from the aleurone layer into the massive starchy endosperm, where it breaks down the large, stored starch reserves into simple, soluble sugars (primarily maltose and glucose), providing the essential chemical energy and carbon skeletons required for the immediate growth of the young embryo's root (radicle) and shoot (plumule). Therefore, the aleurone layer functions as the fundamental digestive engine for the germinating seed.

The incorrect option is 1:

The incorrect option is **1: Seed coat**. The seed coat, or testa, is the protective outer covering of the maize seed, a non-living layer derived entirely from the maternal tissues (integuments) of the ovule. Its primary function is entirely structural and protective: to shield the internal embryo and endosperm from mechanical damage, desiccation, and pathogen invasion. Because it is a non-living tissue at maturity, it fundamentally lacks the cellular machinery, such as ribosomes and endoplasmic reticulum, necessary to synthesize and secrete complex proteins like $\alpha$-amylase. Furthermore, in monocots like maize, the seed coat is often fused with the fruit wall (pericarp) to form a single protective layer, but its functional role remains purely defensive and structural, not enzymatic.

The incorrect option is 2:

The incorrect option is **2: Coleorhiza**. The coleorhiza is a sheath-like, undifferentiated structure that belongs to the embryo of a monocot seed. Its specific and limited biological purpose is to provide a protective covering for the radicle, which is the embryonic root tip, prior to its emergence. Upon the initiation of germination, the primary root (radicle) grows and breaks through this sheath. The coleorhiza itself does not undergo significant cell elongation or differentiation and remains underground, having a function that is solely structural and protective for the emerging root. It is not designed to be the enzyme-secreting tissue responsible for the large-scale hydrolysis of the endosperm's stored food reserves.

The incorrect option is 4:

The incorrect option is **4: Coleoptile**. The coleoptile is another protective sheath originating from the monocot embryo, but it is the covering that encloses the young shoot, or plumule. Its biological design is to form a pointed, sturdy cap that actively pushes through the soil surface during seedling emergence, thereby protecting the delicate shoot apical meristem from abrasion. Once it emerges into the light, its growth stops, and the plumule breaks through its apex. While it is an active embryonic structure, its cellular specialization is geared towards physical protection and vertical emergence. It does not possess the unique hormonal receptors or the metabolic capacity to synthesize and secrete $\alpha$-amylase for starch degradation; that highly specialized digestive task is reserved for the aleurone layer.