Regulation of carbonic anhydrase gene expression in cotyledons of cotton (Gossypium hirsutum L.) seedlings during post-germinative growth

Recently, plastidial carbonic anhydrase (CA) cDNA clones encoding functional carbonic anhydrase enzymes were isolated from a 48 h dark-grown cotton seedling (cotyledons) cDNA library (Hoang et al., Plant Cell Physiol. 40: 1999). Here we examined the levels of relative transcript abundance and enzyme activities in cotyledons at different developmental stages and under different environmental conditions (i.e. altering CO2 and light conditions), during post-germinative seedling growth. Relative CA transcript levels and total CA enzyme activity in cotyledons of cotton seedlings increased from 18 h to 72 h of post-germinative growth in the dark, although somewhat later than the glyoxylate cycle enzyme, MS. When 24 h old seedlings were exposed to light for an additional 24 h, CA activity in greening cotyledons increased about 2-fold (compared with controls kept in the dark), whereas relative CA transcript levels were essentially the same. Removal of seed coats from cotyledons of 24 h old seedlings dramatically increased relative CA transcript abundance (measured 24 h later) in the dark, but did not influence CA enzyme activity. Manipulation of external CO2 environments (zero, ambient, or high) modulated coordinately the relative transcript abundance of CA (and rbcS) in cotyledons, but did not affect enzyme activity. On the other hand, regardless of the external CO2 conditions, cotyledons of seedlings exposed to light exhibited increased CA activity, concomitant with increased Rubisco activity and increased chlorophyll content. Collectively, our data suggest that steady-state levels of CA and rbcS transcripts are increased in response to environmental CO2 conditions, while CA (and Rubisco) enzyme activities are likely modulated at the post-transcriptional level following exposure of seedlings to light, and in parallel with development of functional chloroplasts.