Meta Analysis of Photosynthetic Pathway in Phalaenopsis aphrodite Combining Physiological Approach and Gene Expression Profiling Studies
Date Issued
2012
Date
2012
Author(s)
Hung, Ting-Chen
Abstract
Among higher plants, about 7% of vascular plants perform crassulacean acid metabolism (CAM). Those plants separate two carboxylation reactions by time. This strategy not only elevates intercellular CO2 concentration but also increases water usage efficiency, enabling CAM plants to survive under stresses. Current knowledge defines Phalaenopsis exhibiting CAM photosynthesis. Using Phalaenopsis aphrodite Rchb. f. as plant material, combining physiological and microarray techniques, we are expecting to reveal the strategy of CAM from the two points of view.
Photosynthesis measurement was done with a 12-h photoperiod from 0700 HR to1900 HR, a photosynthetic flux density of 300 μmol•m-2•s-1, and with 50%-70% relative humidity. Malate concentration in Phalaenopsis leaves exhibited diurnal fluctuation. Net CO2 uptake rate performed typical four CAM phases of diurnal changes. Expression levels of malic enzyme and malate dehydrogenase (MDH) decreased during day and increased during night. Both malic enzyme and MDH are circadian genes.
The activity of phosphoenolpyruvate carboxylase (PEPC) increased after noon and decreased at night. It reached its highest value, 278.2 μmol•mg Chl-1•hr-1, at 1800 HR. The expression level of PEPC changed similarily to the trend of enzyme activities. Among all PEPC genes, PATC152779 is the most likely one as a functional phosphoenolpyruvate carboxylase. Another carboxylase of CAM plants is Rubisco. Initial activity of Rubisco in Phalaenopsis increased from 2.5 μmol•mg Chl-1•hr-1 to 6.0 μmol•mg Chl-1•hr-1 during daytime. However, the expression level of Rubisco did not change much during a day, although it increased slightly at 2200 HR.
Some diurnal changes in CAM were considered to be related to circadian rhythm. In spite of elongation of photoperiod, these physiological changes remain its rhythm for several days. In our study, we treated Phalaenopsis plants with a 12-h extended dark and a 12-h extended light after 24-h normal photoperiod, respectively. We selected ‘circadian genes’ by using GeneSpring® software. Among the genes identified, some genes were directly involved in carbon fixation in photosynthesis, such as PEPC, PEPC kinase, Rubisco, and Rubisco activase; others included the genes which may regulate malate translocation and distribution between vacuole and cytoplasm, such as transporters of potassium, calcium, and carbohydrates. It is speculated that circadian rhythm is regulated by enzymes of carbon reaction and several transporters.
Phalaenopsis showed similar responses to drought as other plants. Abscisic acid was accumulated, leaf relative water content was decreased, and net CO2 uptake was inhibited after drought treatment. The difference between Phalaenopsis and C3 plants is the slow rate of those responses. Short-term drought induced PEPC activities in young leaf. Among all PEPC genes of Phalaenopsis, expression levels of PATC224556 and PATC 235744 were induced by drought. Expression levels of other PEPC genes were inhibited by drought. However, the initial activity and the level of gene expression of Rubisco were inhibited under drought stress. There are 1758 and 1654 genes which were induced or inhibited by short-term drought, respectively. We speculate that lots of these genes are related to the ability of drought tolerance in CAM plants. The information could provide references for future research.
Subjects
Phalaenopsis
photosynthesis
crassulacean acid metabolism
microarray
drought
Type
thesis
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