| Drought stress is one of the major abiotic factors limiting fiber yield and lint quality. As drought has the greatest effect during boll formation, it is important to understand the changes that occur during the critical stages of boll formation that make the mature bolls (secondary cell wall formation stage) more tolerant to drought stress. Four cDNA libraries were constructed at two different stages of boll formation, 8--10 days post-anthesis (dpa) and 15--20 dpa from drought-stressed and control bolls. These stages encompass fiber elongation and secondary cell wall formation stage, respectively. The 5' ends of 1033, 1029, 1150 and 1430 randomly selected clones from 8 dpa stressed, 8 dpa control, 15 dpa stressed, and 15 dpa control boll libraries were sequenced and annotated. Sequence homology search indicated that 470 (45.5%), 542 (53.2%), 364 (31.7%) and 699 (48.9%) from 8 dpa stressed, 8 dpa control, 15 dpa stressed, and 15 dpa control boll libraries exhibited significant similarities to characterized database entries. These ESTs were grouped into 14 categories depending upon gene functions. There was two fold increase in the apparent expression of abiotic stress response genes in 8--10 dpa stressed bolls relative to 8--10 dpa control bolls. ESTs encoding late embryogenesis abundant (lea) protein 5-D, heat shock protein and prolyl 4-hydroxylase alpha subunit-like protein were expressed in the stressed bolls at elongation stage of boll formation.; cDNA macroarray analysis showed that 176 drought stress responsive genes were up-regulated in stressed bolls at the elongation stage and constitutively expressed in control and stressed bolls at the secondary cell wall formation stage. cDNAs encoding lea, prolyl 4-hydroxylase, betaine aldehyde, glutathione s-transferase, superoxide dismutase were up-regulated in stressed bolls at the elongation stage and constitutively expressed at the secondary cell wall formation stage. This is the first systematic throughput analysis of genes involved in drought stress tolerance during cotton boll formation. This study revealed that the stress responsive genes are constitutively expressed in mature bolls and are induced in immature bolls and this makes the mature bolls (at secondary cell wall formation stage) more resistant to drought stress. |
