Synthesis, processing and export of cytoplasmic endo‐β‐1,4‐xylanase from barley aleurone during germination

Abstract

SummaryWe have identi®ed the major endo-b-1,4-xylanase (XYN-1) in the aleurone of germinating barley grain, and show that it is expressed as a precursor of M r 61 500 with both N-and C-terminal propeptides. XYN-1 is synthesized as an inactive enzyme in the cytoplasm, and only becomes active at a late stage of germination when the aleurone ceases to secrete hydrolases. A series of processing steps, mediated in part by aleurone cysteine endoproteases, yields a mature active enzyme of M r 34 000. Processing and extracellular release of the mature enzyme coincide with the programmed cell death (PCD)-regulated disintegration of aleurone cells. We discuss the signi®cance of delayed aleurone cell-wall degradation by endoxylanases in relation to the secretory capacity of the aleurone, and propose a novel role for aleurone PCD in facilitating the export of hydrolases.

Bifunctional Family 3 Glycoside Hydrolases from Barley with α-l-Arabinofuranosidase and β-d-Xylosidase Activity

“…4) that presumably directs secretion from cells in which they are synthesized. This is a significant observation, given recent indications that the (1 3 4)-␤-D-xylan endohydrolase involved in arabinoxylan depolymerization in germinated barley grain is not located in the endomembrane secretory compartment of aleurone layers but is found instead in the cytosol and is likely to be released from aleurone layers only after programmed cell death (56,57). In isolated aleurone layers, ␣-L-arabinofuranosidases and ␤-D-xylosidases are secreted and can be detected in the surrounding medium much earlier than the (1 3 4)-␤-xylan endohydrolases (10).…”

“…No biological rationale for COOH-terminal processing of the barley ARA-I and XYL enzymes can be provided at this stage. In the case of the barley (1 3 4)-␤-D-xylan endohydrolase, both NH 2 -and COOH-terminal processing of the primary translation product occurs (56,57).…”

Mutational Analysis of Endoxylanases XylA and XylB from the Phytopathogen Fusarium graminearum Reveals Comprehensive Insights into Their Inhibitor Insensitivity

“…They are produced by a large variety of organisms including bacteria, fungi, and plants and are of significant importance in many physiological, pathological, and even biotechnological processes which involve degradation or remodeling of the plant cell wall. The physiological role of plant endogenous endoxylanases is linked to their involvement in processes such as secondary cell wall biogenesis and metabolism (1,36), germination (9,41), and the initiation of sexual reproduction by facilitating pollen tube penetration (35). Endoxylanases secreted by phytopathogenic microorganisms are generally considered (4) and occasionally proven (7) to be essential components of their offensive arsenal to penetrate and colonize plant tissues.…”

Maize Tapetum Xylanase Is Synthesized as a Precursor, Processed and Activated by a Serine Protease, and Deposited on the Pollen

“…DNA fragment markers are indicated on the right. rone layer, the processing produced many intermediates (16), and active enzymes of molecular masses of 55 and 30 kDa in wheat (27), and 29 (28), 34 (29), and 41 kDa (30) in barley have been reported. In addition, the tapetum xylanase is apparently encoded by a single gene in maize (15) and barley (this report), whereas the aleurone-layer xylanase is likely to be encoded by more than one gene (this report).…”

“…A recent report (16) showed that during germination of barley seed, the aleurone layer contains a long xylanase-mRNA similar to that in the maize tapetum reported earlier (15). This barley mRNA produces a large precursor, which is processed to become an active xylanase of about 30 -34 kDa.…”