PLANT PHYSIOLOGY , Vol 103, Issue 3 1001-1008, Copyright © 1993 by American Society of Plant Biologists
|
METABOLISM AND ENZYMOLOGY |
Effect of Different Carbon Sources on Relative Growth Rate, Internal Carbohydrates, and Mannitol 1-Oxidoreductase Activity in Celery Suspension Cultures
JMH. Stoop and D. M. Pharr
Department of Horticultural Science and Plant Physiology Program, North Carolina State University, Raleigh, North Carolina 27695-7609
Little information exists concerning the biochemical route of mannitol
catabolism in higher plant cells. In this study, the role of a recently
discovered mannitol 1-oxidoreductase (MDH) in mannitol catabolism was
investigated. Suspension cultures of celery (Apium graveolens L. var dulce
[Mill.] Pers.) were successfully grown on nutrient media with either
mannitol, mannose, or sucrose as the sole carbon source. Cell cultures
grown on any of the three carbon sources did not differ in relative growth
rate, as measured by packed cell volume, but differed drastically in
internal carbohydrate concentration. Mannitol-grown cells contained high
concentrations of mannitol and extremely low concentrations of sucrose,
fructose, glucose, and mannose. Sucrose-grown cells had high concentrations
of sucrose early in the growth cycle and contained a substantial hexose
pool. Mannose-grown cells had a high mannose concentration early in the
cycle, which decreased during the growth cycle, whereas their internal
sucrose concentrations remained relatively constant during the entire
growth cycle. Celery suspension cultures on all three carbon substrates
contained an NAD-dependent MDH. Throughout the growth cycle, MDH activity
was 2- to 4-fold higher in mannitol-grown cells compared with sucrose- or
mannose-grown cells, which did not contain detectable levels of mannitol,
indicating that MDH functions pre-dominantly in an oxidative capacity in
situ. The MDH activity observed in celery cells was 3-fold higher than the
minimum amount required to account for the observed rate of mannitol
utilization from the media. Cultures transferred from mannitol to mannose
underwent a decrease in MDH activity over a period of days, and transfer
from mannose to mannitol resulted in an increase in MDH activity. These
data provide strong evidence that MDH plays an important role in mannitol
utilization in celery suspension cultures.
