Cotton fibers are unicellular, unbranched, simple trichomes (or seed hairs) (Figure 2) differentiate from ~25% of the epidermal cells in the outer integument of a developing seed [2, 8, 14]. Visible signs of fiber development are first evident on the day of flower opening (anthesis) (Figure 2A), and subsequent development is described temporally by days post-anthesis (DPA). The fiber cells (Figure 2B) elongate to 3 to ~6 cm in G. hirsutum and G. barbadense, respectively, placing them among the largest plant cells. Elongation occurs via synthesis of a thin (0.1 – 0.2 µm) primary wall with a composition typical of many plant cells . Facilitated by the cell wall-mediated coordination of the packing of elongating fibers , half to 3/4 million fibers differentiate on 24-30 seeds within one cotton fruit (or boll) (Figure 2C). The percentage of cellulose in cotton fibers is far higher than most plant cells partly because the thick secondary wall in the fibers consists of nearly pure cellulose . G. barbadense produces longer, stronger, and finer fiber than G. hirsutum. (Finer means less mass per length, which can arise from a thinner cell wall and/or a smaller diameter fiber.) Typically G. barbadense has a longer phase of fiber elongation compared to G. hirsutum [6, 11], but it is unknown how elongation is sustained in G. barbadense even though secondary wall deposition has begun. The differences in fiber traits may reflect convergent evolution of useful fiber morphology with a different genetic basis. For example, A-genome ESTs are enriched during early stages of fiber development , but the D-genome expression was enhanced during domestication, and different sets of genes showed this bias in the two separate domestication events . The ability to introduce key fiber quality genes from G. barbadense into agronomically more robust G. hirsutum depends on identifying all key cellular differences and regulatory genes. Several groups have compared the fiber transcriptomes and/or analyzed recombinant inbred lines in the two species [1, 10, 13], as well as identified the QTLs related to fiber elongation, fineness, and length using a backcross population between G. hirsutum L. and G. barbadense L. [3-5].
Figure 2.Fiber cell initiation from epidermal layer of an ovule on the day of anthesis (0 DPA) (A), elongated cotton fibers under electron microscope (B), and mature cotton bolls (C).
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