PIN-formed protein, a door to reveal the mechanism for auxin-triggered initiation of cotton fiber

ABSTRACT

Cotton fibers are differentiated ovule epidermal cells that provide an ideal model to study cell differentiation and elongation. Establishment of auxin maximum in fiber cells is crucial for cotton-fiber protrusion from ovule surface. However, it is unclear where the auxin originates from and how the auxin accumulates in fiber cells. Our recent results indicate that the auxin is mainly imported from the outside of ovules, and transported to fiber cells through GhPIN (homolog of PIN-formed proteins in cotton) -mediated polar auxin transport, rather than in situ synthesis. Based on our finding in GhPINs, we discuss here briefly how auxin flow to fiber cells and auxin gradient in ovule epidermis is established mainly by GhPIN3a protein.

Cotton fibers, the largest natural fiber source in the world, are differentiated from ovule epidermis.¹ Plant hormone auxin plays a crucial role in fiber production.^2,3 In earlier study, it has been demonstrated that indole-3-acetic acid (IAA), the major auxin in plants, determines cotton fiber initiation (development of first stage) through an establishment of concentration maxima in fiber cells.⁴ Thus, where the auxin originates from becomes an important question for revealing the mechanism of cotton fiber differentiation and initiation. In this issue, our latest research further exhibits dynamic pattern of auxin distribution in ovules during cotton fiber initiation.⁵ When fibers start protruding from ovule surface on the day of anthesis, auxin predominantly accumulates in the ovule nucellus and fiber cells (Fig. 1A). In contrast to some other flowering plants, the auxin in ovules is mainly imported from outside of ovules rather than de novo synthesized in the accumulation site.^5-8 This agrees with previous microarray data that auxin synthesis is not upregulated corresponding to fiber initiation.^9,10 Moreover, these 2 auxin sinks (fiber cells and the ovule nucellus) are relatively independent, because the auxin that has accumulated in the ovule nucellus at -1 DPA (days post anthesis) cannot make up for the auxin shortage in fiber initials when the ovule was cultured in absence of IAA.⁵

Figure 1.

Possible model for GhPIN3a-mediated auxin accumulation in cotton fibers. (A) Auxin distribution in 0-DPA ovules. (B) Expression of GhPIN3a in 0-DPA ovules. (C) Speculative auxin routes to the ovule nucellus and outer integument. (D) Possible localization of GhPIN3a in ovule epidermal and sub-epidermal layers. F, fiber cells; NF, non-fiber cells; OI, ovule outer integument; II, ovule inner integument; N, ovule nucellus.

PIN-formed proteins, especially long-type ones, are well documented to play key roles in establishing auxin gradient,¹¹ Among 11 PIN homologs in cotton, GhPIN3a transcript showed an impressive localization in the ovule outer integument, including fiber cells, in addition to the nucellus (Fig. 1B), suggesting a key role of GhPIN3a in regulating auxin transport in ovule epidermis,⁵ Similar localization is also detected for PIN3 of Arabidopsis during ovule development by genechip analysis,¹² PIN3 should be required for the further differentiation of ovule epidermal cells (e.g. mucilage cells). This can give reference to the study in cotton ovules. Indeed, ovule epidermis of Arabidopsis has a potential to develop trichome cells.¹³ In addition, the concerted action of PIN family members in cotton ovule epidermis should not be ignored.^14,15

According to the auxin distribution and GhPIN3a expression in ovules (Fig. 1A,B), the possible flow route for auxin in cotton ovules is predicted: auxin enters ovules from the funiculus to the chalaza through the vascular strand, then is distributed to fiber cells and the nucellus separately (Fig. 1C). At least, the auxin route to fiber cells is supported by the observation that fiber initiation occurs first at the crest of funiculus and final at the micropylar end.¹⁶ A key question raised at present is how does GhPIN3a establish the auxin maxima in fiber cells.

PIN polar localization at the plasma membrane (PM), which mediates the movement of auxin into and out of individual cells is extraordinarily important for establishment of auxin gradient. In the ovular outer integument from the chalaza to the micropyle, auxin should generally flow to the epidermal layer and other integument cells. According to the localization of GhPIN3a transcript, the whole outer integument of ovules may be involved in the auxin transport. Here, we propose 2 possible models for GhPIN3a to establish the auxin maxima in fiber cells (Fig. 1D). The Model 1 is based on the studies in shoot morphogenesis of Arabidopsis.^17,18 The auxin maxima in fiber cells may be caused by GhPIN3a localization in the adjacent non-fiber cells, where GhPIN3a may localize at the PM toward the fiber cell. Consequently, auxin in the surrounding cells would be directed to fiber cells, and the cellular auxin level would increase in the fiber. Another model is hypothesized in concern of generality of fiber-specific regulation during cotton fiber development (Model 2 in Fig. 1D). Relocalization of GhPIN3a may take place in fiber cells and result in auxin accumulation in these cells. In this hypothesis, auxin should be transported to all epidermal cells through non-fiber cells. Localization of GhPIN3a at the side PM toward the micropyle might be more abundant than that toward the chalaza according to auxin flow direction in the outer integument (Fig. 1C). During this process, GhPIN3a in epidermal non-fiber cells may additionally localize at the inner PM toward the ovule nucellus to reduce cellular auxin level or recycle excess auxin. Whereas in fiber cell, we proposed a possibility that the PM localization of GhPIN3a may be lost due to a fiber-specific regulation. Thus the net efflux of auxin would thereby be decreased and the cellular auxin level would increase subsequently to promote fiber cell initiation.

Previous work has indicated that GhPIN3a expression is not different between fiber and non-fiber cells of cotton ovule epidermis.⁹ That is to say, GhPIN3a localization, rather than the transcription, between the 2 cell types determines the specific accumulation of auxin in fiber cells. It is widely reported that the regulation of PIN localization, such as PIN1/2 in lateral organ development,^19,20 PIN3 relocalization²¹ and PIN phosphorylation.²² is tightly associated with gravitropism. However, within cotton ovule epidermis, it is impossible to define morphological apical and basal ends regarding to the gravity. How to determine the specific localization of GhPIN3a and further to establish auxin maxima in cotton fiber cells is of particular interest. In the future study, recycling and polar targeting²³ of GhPIN3a in fiber cells would be the key issue to reveal the regulation of cotton fiber initiation. Besides, some other factors, including calcium ion,²⁴ reactive oxygen species (ROS)²⁵ and plant hormone cytokinins^26,27 should also be concerned because of their possible link with auxin or cotton fiber initiation.

Disclosure of potential conflicts of interest

No potential conflicts of interest were disclosed.

Funding

This work was supported by the National Natural Science Foundation of China [31130039 to Y.P., 31201248 to M.Z.], the Natural Science Foundation of Chongqing, China [cstc2012jjjq80001 to M.Z.], the Program for New Century Excellent Talents in University [to M.Z.], and the Graduate Student Research Innovation Project of Chongqing [CYB2015068 to J.Z.].