Response of sucrose phosphate synthase activity to cool temperatures in cotton fiber

The aim of this work was to study the response of sucrose phosphate synthase to cool temperatures in order to better understand its possible role in providing substrate for cellulose biosynthesis. Cotton {Gossypium hirsutwn L.) ovules were cultured in vitro at 34°C constant temperature until 18 DPA, and later some of the cultures were exposed to cycUng temperatures (12 h 34°C/12 h 15°C). SPS enzyme was extracted from fibers and assayed on 12, 18, 21, and 24 DPA. Spectrophotometric analysis of SPS showed that SPS activity increased under constant 34°C in cultivars such as Paymaster HS-200 (HS-200), Coker 312, and Acala SJ-1 (SJ-1). These cultivars represent a range of cool tolerance from most to least in the order listed. In SJ-1 and Coker 312, SPS activity lowered under cychng temperatures, whereas in HS-200 activity it remained unaltered. In vivo fiber SPS activity also increased at constant 28°C. However, in vivo SPS activity for SJ-1 was higher than in vitro, whereas the reduction in vitro was not observed for HS-200. The SPS activity pattern was maintained in both SJ-1 and HS-200 even under variable conditions such as transferring ovules to cycling temperature on different DPA, assaying at different time points in a day, and assaying at different temperatures. The effect of cool temperatures on SPS activity was not related to whether or not fibers had previously entered a high-rate stage of cellulose synthesis. SPS activity in fiber was not regulated by an internal biological rhythm, because SPS activity remained the same throughout the day despite alternating warm and cool temperatures. Assay temperatures of 15°C and 22°C were sub-optimal, whereas 30°C, 34°C, and 40°C were optimal for SPS enzyme activity. Sucrose pools analyzed using HPLC showed a tendency to decrease under cycling temperatures compared to 34°C constant in both SJ-1 and HS-200, with only a slightly greater decrease in SJ-1. Therefore, sucrose pool size is highly regulated and does not decrease commensurately with the large decrease in SPS activity under cycling temperatures observed for SJ-1. However, since these data say nothing about flux through the pool, it remains possible that the cool-temperature insensitivity of SPS activity in HS-200 is related to the increased cool tolerance of this cultivar.