The function and regulatory networks of Ca2+ binding proteins in fiber development

Functional Genomics

oral

Tu, Lili; Tang, Wenxin; He, Yonghui

Cotton fiber is a single cell that differentiates from the ovule epidermis and undergoes synchronous elongation with high secretion and growth rate. Apart from economic importance, cotton fiber provides an excellent single-celled model for studying mechanisms of cell growth. Annexins are Ca2+- and phospholipid-binding proteins that have been reported to be localized in multiple cellular compartments and involved in control of vesicle secretions. Although several annexins have been found to be highly expressed in elongating cotton fibers, their functional roles in fiber development remain unknown. Here, 14 annexin family members were identified from the fully sequenced diploid G. raimondii (D5 genome), and half of which were expressed in fibers of the cultivated tetraploid species G. hirsutum (cv YZ1). Among them, GhAnn2 (DT genome) displayed high expression level in elongating fiber. The expression of GhAnn2 could be induced by some phytohormones that played important roles in fiber elongation, such as IAA and GA3. RNAi-mediated down-regulation of GhAnn2 inhibited fiber elongation and secondary cell wall synthesis, resulting in shorter and thinner mature fibers in the transgenic plants. Measurement with non-invasive scanning ion-selective electrode revealed that the rate of Ca2+ influx from extracellular to intracellular was decreased at the fiber cell apex of GhAnn2 silencing lines, in comparison to that in the wild type. These results indicate that GhAnn2 may regulate fiber development through modulating Ca2+ fluxes and signaling. A gene encoding a calcium sensor, GhCaM7, was also isolated based on its high expression level relative to other GhCaMs in fiber cells at the fast elongation stage. The level of expression of GhCaM7 in wild-type and the fuzzless/lintless mutant corresponds to the presence and absence respectively of fiber initials. Overexpressing GhCaM7 promotes early fiber elongation, whereas GhCaM7 suppression by RNAi delays fiber initiation and inhibits fiber elongation. Reactive oxygen species (ROS) play important roles in fiber early development. ROS induced by exogenous H2O2 and Ca2+ starvation promotes early fiber elongation. GhCaM7 overexpression (OE) fiber cells show increased ROS levels compared to wildtype, while GhCaM7 RNAi fiber cells have reduced levels. Furthermore, we show that H2O2 enhances Ca2+ influx into the fiber and feedback regulates the expression of GhCaM7.