Please explain the statement:
The hormone auxin induces cell wall loosening or relaxation in plants although auxin promotes growth in plants by cell elongation.
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Answers
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Plant cells are surrounded by cell walls, which are dynamic structures displaying a strictly regulated balance between rigidity and flexibility. Walls are fairly rigid to provide support and protection, but also extensible, to allow cell growth, which is triggered by a high intracellular turgor pressure. Wall properties regulate the differential growth of the cell, resulting in a diversity of cell sizes and shapes. The plant hormone auxin is well known to stimulate cell elongation via increasing wall extensibility. Auxin participates in the regulation of cell wall properties by inducing wall loosening. Here, we review what is known on cell wall property regulation by auxin. We focus particularly on the auxin role during cell expansion linked directly to cell wall modifications. We also analyze downstream targets of transcriptional auxin signaling, which are related to the cell wall and could be linked to acid growth and the action of wall-loosening proteins. All together, this update elucidates the connection between hormonal signaling and cell wall synthesis and deposition.
Plant cells exhibit a great diversity in size and shape. Meristematic cells are usually isodiametric and then differentiate by developing distinct forms to acquire specific functions. This is easily noticeable in cells such as tip-growing root hairs or multi-lobed pavement cells. In contrast with animal cells, plant cells have the particularity of being tightly connected to each other by their surrounding walls located outside of the plasma membrane. Cell walls are dynamic structures that act as an exoskeleton by participating in the establishment and maintenance of cell shape and by protecting the cell content from biological, chemical and biophysical sources of aggression [1,2]. Large plants such as trees are able to resist external forces due to the strength given by their cell walls [1]. Moreover, cell walls play a significant role in processes such as cell adhesion, intercellular communication and water movement [1,3]. Plant cell walls are classified into two groups; primary and secondary walls. The latter are usually present in specialized, non-growing cells and are beyond the scope of this review [1,2,3,4].
Primary cell walls (around 100–1000 nm thick in young growing cells) are essentially made of glucan-based cellulose microfibrils (CMFs) embedded in a highly hydrated matrix composed of pectins, hemicelluloses, structural proteins and proteoglycans [1,2,3,5]. The cell wall has to be fairly rigid, to provide support and protection, but also extensible, to allow cell expansion, which is driven by a strong intracellular turgor pressure [6,7,8,9,10,11]. A strictly regulated balance between wall rigidity and flexibility is therefore imperative to regulate the differential growth that results in such a diversity of cell sizes and shapes [2,7,12,13]. The plant hormone auxin is identified as a stimulator of cell elongation, as it increases cell wall extensibility [14,15,16]. Specifically, auxin regulates cell wall properties by initiating wall loosening [17,18]. The close link between hormonal action and cell wall synthesis and deposition has been investigated for many years, but much of the details still need to be clarified. Here we summarize what is currently known about the regulation of cell wall properties and the role of auxin in this process.
2. Plant Cell Walls
Plant cell walls are highly heterogeneous and cell wall composites vary among different species and cell types. Walls are very dynamic and their composition changes even within the same cell over time [1,19,20,21,22,23,24]. Nonetheless, the key polysaccharides are usually present and their structure, biosynthesis and interaction are summarized in this chapter.
2.1. Cellulose Microfibrils (CMFs)
Cell wall consists of different polymers including CMFs, which are embedded in components such as non-cellulosic polysaccharides and structural proteins. CMFs are the largest cell wall polysaccharides, composed of (1,4)-β-d-glucan parallel arrays assembled into long cylinders [25,26]. Due to their stiff and load-bearing properties, CMFs are resistant to tensional forces [1,2,3]. CMFs determine the direction of cell expansion. Indeed, their deposition and alignment define cell growth anisotropy [2,27,28], as shown by the characterization of cellulose-deficient Arabidopsis mutants, in which cell elongation is drastically reduced [29]. Cellulose synthesis takes place beneath the cell wall at the plasma membrane via large rosette complexes made of CELLULOSE SYNTHASEs (CESAs), and certainly other components such as KORRIGAN1 (KOR1), the function of which remains elusive [25,26,30,31]. The CMF patterning of the wall is mediated via cortical microtubules (cMT) and CESAs at the plasma membrane, with the orientation of CMFs within the wall following the pattern given by the cMTs