Cultures of Gossypium barbadense cotton ovules offer insights into the microtubule-mediated control of fiber cell expansion

Efficient imaging and computer vision detection of two cell shapes in young cotton fibers

PREMISE

The shape of young cotton (Gossypium) fibers varies within and between commercial cotton species, as revealed by previous detailed analyses of one cultivar of G. hirsutum and one of G. barbadense. Both narrow and wide fibers exist in G. hirsutum cv. Deltapine 90, which may impact the quality of our most abundant renewable textile material. More efficient cellular phenotyping methods are needed to empower future research efforts.

METHODS

We developed semi-automated imaging methods for young cotton fibers and a novel machine learning algorithm for the rapid detection of tapered (narrow) or hemisphere (wide) fibers in homogeneous or mixed populations.

RESULTS

The new methods were accurate for diverse accessions of G. hirsutum and G. barbadense and at least eight times more efficient than manual methods. Narrow fibers dominated in the three G. barbadense accessions analyzed, whereas the three G. hirsutum accessions showed a mixture of tapered and hemisphere fibers in varying proportions.

DISCUSSION

The use or adaptation of these improved methods will facilitate experiments with higher throughput to understand the biological factors controlling the variable shapes of young cotton fibers or other elongating single cells. This research also enables the exploration of links between early cell shape and mature cotton fiber quality in diverse field-grown cotton accessions.

Microtubules play a crucial role in regulating actin organization and cell initiation in cotton fibers

Dynamic organization of actin and microtubule cytoskeletons directs a distinct expansion behavior of cotton fiber initiation from cell elongation. Cotton fibers are highly elongated single cells derived from the ovule epidermis. Although actin and microtubule (MT) cytoskeletons have been implicated in cell elongation and secondary wall deposition, their roles in fiber initiation is poorly understood. Here, we used fluorescent probes and pharmacological approaches to study the roles of these cytoskeletal components during cotton fiber initiation. Both cytoskeletons align along the growth axis in initiating fibers. The dorsal view of ovules shows that unlike the fine actin filaments (AFs) in nonfiber cells, the AFs in fiber cells are dense and bundled. MTs are randomized in fiber cells and well-ordered in nonfiber cells. The pharmacological experiments revealed that the depolymerization of AFs and MTs assisted fiber initiation. Both AF stabilization and depolymerization inhibited fiber elongation. In contrast, the proper depolymerization of MTs promoted cell elongation, although the MT-stabilizing drug consistently resulted in a negative effect. Notably, we found that the organization of AFs was correlated with MT dynamics. Stabilizing the MTs by taxol treatment promoted the formation of AF bundles (in fiber initials) and transversely aligned AFs (in elongating fibers), whereas depolymerizing the MTs by oryzalin treatment promoted the fragmentation of AFs. Collectively, our data indicates that MTs plays a crucial role in regulating AF organization and early development of cotton fibers.

Microtubules exert early, partial, and variable control of cotton fiber diameter

Variable cotton fiber diameter is set early in anisotropic elongation by cell-type-specific processes involving the temporal and spatial regulation of microtubules in the apical region. Cotton fibers are single cells that originate from the seed epidermis of Gossypium species. Then, they undergo extreme anisotropic elongation and limited diametric expansion. The details of cellular morphogenesis determine the quality traits that affect fiber uses and value, such as length, strength, and diameter. Lower and more consistent diameter would increase the competitiveness of cotton fiber with synthetic fiber, but we do not know how this trait is controlled. The complexity of the question is indicated by the existence of fibers in two major width classes in the major commercial species: broad and narrow fibers exist in commonly grown G. hirsutum, whereas G. barbadense produces only narrow fiber. To further understand how fiber diameter is controlled, we used ovule cultures, morphology measurements, and microtubule immunofluorescence to observe the effects of microtubule antagonists on fiber morphology, including shape and diameter within 80 µm of the apex. The treatments were applied at either one or two days post-anthesis during different stages of fiber morphogenesis. The results showed that inhibiting the presence and/or dynamic activity of microtubules caused larger diameter tips to form, with greater effects often observed with earlier treatment. The presence and geometry of a microtubule-depleted-zone below the apex were transiently correlated with the apical diameter of the narrow tip types. Similarly, the microtubule antagonists had somewhat different effects between tip types. Overall, the results demonstrate cell-type-specific mechanisms regulating fiber expansion within 80 µm of the apex, with variation in the impact of microtubules between tip types and over developmental time.

The Cytoskeleton and Its Role in Determining Cellulose Microfibril Angle in Secondary Cell Walls of Woody Tree Species

Recent advances in our understanding of the molecular control of secondary cell wall (SCW) formation have shed light on molecular mechanisms that underpin domestication traits related to wood formation. One such trait is the cellulose microfibril angle (MFA), an important wood quality determinant that varies along tree developmental phases and in response to gravitational stimulus. The cytoskeleton, mainly composed of microtubules and actin filaments, collectively contribute to plant growth and development by participating in several cellular processes, including cellulose deposition. Studies in Arabidopsis have significantly aided our understanding of the roles of microtubules in xylem cell development during which correct SCW deposition and patterning are essential to provide structural support and allow for water transport. In contrast, studies relating to SCW formation in xylary elements performed in woody trees remain elusive. In combination, the data reviewed here suggest that the cytoskeleton plays important roles in determining the exact sites of cellulose deposition, overall SCW patterning and more specifically, the alignment and orientation of cellulose microfibrils. By relating the reviewed evidence to the process of wood formation, we present a model of microtubule participation in determining MFA in woody trees forming reaction wood (RW).