Plant lecture 7 - Leaf metabolism in the dark

Question 1

Overview of assimilation in dark

Answer 1

• Starch accumulates in the light + degraded in the dark
• In dark, mobilisation occurs at same rate so decline is linear
• Ensures starch reserves are almost completely gone by end of night

Question 2

How do enzymes access insoluble starch granules

Answer 2

• Glucosyl units within glucan chains are phosphorylated by glucan water dikinase (GWD) using ATP
• GWD phosphorylates around 1 in every 30 glucosyl units in a-glycan polymer C6
• Steric effects + -ve charge of phosphorylation disrupts crystalline structure of granule + allows more E to enter
• Phosphoglucan water dikinase (PWD) adds additional P onto C3. Further disrupts structure
• Allows access to hydrolytic E that cleave a1,4 + a1,6 that link glucosyl units
• Phosphate removed by phosphatases + oligosaccharides are further cleaved by B-amalases that release maltose

Question 3

Pathway of C export at night

Answer 3

• Maltose itself is exported directly from cytoplasm → cytosol
• Maltose is condensed to longer chain glucans
• Accumulate + = substrate for a-glucan phosphorylase.
• Cleaves a1,4 glucan bonds + releases glucose1-P
• Glucose1P → other hexose-P ultimately forming sucrose that can be exported from the leaf even at night
• Thought maltose degradation involving resynthesis of cytosolic oligosaccharides could help buffer against provision of substrates

Evidence

• Characterised critical mutants defective in starch degradation
• Mex1 mutant = defective in maltose transport. Low state of starch mobilisation in dark

Question 4

Experiments to show control of starch degradation

Answer 4

Experiment 1

• Genetically identical Arabidopsis plants are grown in 2 different conditions
  1. = 12h light, 12h dark (normal). Starch accumulates in day + is degraded linearly + completely in dark
  2. = 6h light, 18h dark. Starch accumulates in day + completely degraded at night
• Rate of starch degradation is slower than 12h dark plants to ensure starch synthesis supplies x exhausted
• Amount of starch accumulates in 6h light is > than 1st 6h of 12light/12 dark plants
• Recognise growing in limited photoperiod + adjust rate of starch synthesis

Experiment 2

• Test if plants can test length of time passed since ‘dawn’ at start of day
• Introduce skeleton photoperiod of where plants grown in normal 12h dark/light cycle + are subjected to start of the day to few hrs of illumination
• Light switched off for photoperiod + switched on for last few hours for ‘normal’ photoperiod then off
• Accumulates less starch compared to normal
• Know don’t count time since light was last switched on as would only recognise few short hours + would degrade slowly
• So, plants determine passage of time based on time since dawn

Experiment 3

• Plants grown in 14hr light + 14hr dark (28 hr total)
• Compromises growth as rather than recognising 28hr day, think 24hr
• After 14hr light, think night =10hr so adjust starch degradation so used by end of 24th hour
• Starvation = last 4 hrs