The cell cycle

A major difference between plants and animals is that plants contain clearly defined regions, termed meristems, which remain embryonic throughout the life of the plant and are responsible for continued growth. Root and shoot meristems generate axial growth: the cambium is an example of a lateral meristem; the pericycle is a potential meristem for the generation of lateral roots; and grasses possess an additional intercalary meristem at the base of nodes. Cells in a meristematic region undergo a precise sequence of events, known as the cell cycle, which is divided into four discrete periods, each lasting for a time specific to a particular species. In G1 (first gap phase) each nuclear chromosome is a single chromatid containing one DNA molecule and S phase (DNA synthesis) is characterised by a doubling of the DNA content of the nucleus, so that in G2 (second gap phase) each chromosome now consists of two identical chromatids with identical DNA molecules. The

Herbicides and Plant Physiology, Second Edition Andrew H. Cobb and John P.H. Reade © 2010 A.H. Cobb and J.P.H. Reade. ISBN: 978-1-405-12935-0

prophase metaphase anaphase telophase

- Interphase

Figure 10.1 The mitotic cell cycle.

gap phases allow the operation of controls that ensure that the previous phase has been accurately completed. The main regulatory steps operate at the Gj/S and G2/M boundaries, and both are sensitive to environmental conditions. Gj, S and G2 are collectively termed interphase, to be followed by cell division (mitosis, or M), which is itself subdivided into prophase, metaphase, anaphase and telophase (Figure 10.1). During prophase the nuclear envelope disintegrates, and pairs of distinctly shorter and fatter chromosomes are observed. In metaphase the chromosome pairs move to the centre of the cell along a structure known as the spindle which consists of proteinaceous microtubules extending between the poles of the cell. At anaphase the chromosome pairs separate and migrate to opposite ends of the cell, so that two identical sets of chromosomes are evident at each pole of the spindle. Mitosis is completed in telophase by the formation of a nuclear envelope around each chromosome set, and the creation of a new cell wall by the fusion of the Golgi apparatus at the equatorial region of the cell. A completed cell cycle may take between 12 and 18 hours, and the full metabolic machinery of the cell is required to ensure progression from one stage to the next. It therefore follows that any chemical interference with cell cycle progression will cause growth inhibition and have herbicidal potential. Indeed, herbicides are known that either prevent mitotic entry or disrupt the mitotic sequence.

Substantial progress has been made in recent years in our understanding of the molecular mechanisms and control processes of cell division in plants. To ensure that each daughter cell receives the full complement of DNA requires close regulation of the S and M phases. It seems that the basic control mechanisms are highly conserved in all eukaryotes, the main drivers being a class of serine/threonine kinases, known as cyclin-dependent kinases (CDKs). Many regulatory features have evolved in plants that influence CDK activity, including metabolic, hormonal and environmental signals.

To be active, the kinase requires the binding of regulatory proteins, termed cyclins. In plants there are several CDKs, each having a role at different points in the cell cycle. Five types of cyclins exist in plants: the A-type is observed at the beginning of S phase and is destroyed at the G2/M transition; the B-type appears during G2 control of the

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