Plant Physiology and Biochemistry Questions and Answers

Answer: D

The statement that is not correct is Phytochrome are normally found in the chloroplast.

Phytochrome is a soluble protein made in the cytosol. In its inactive red-absorbing form (Pr) it stays in the cytoplasm; on conversion to the far-red absorbing Pfr form it moves into the nucleus, where it regulates gene expression. It is not a chloroplast protein.

The other statements are correct. The N-terminal photosensory region contains PAS, GAF and PHY domains (the chromophore binds in GAF), and the C-terminal region carries PAS-related (PRD) repeats and a histidine-kinase-related domain. Phytochrome has light-regulated serine/threonine kinase activity. In Arabidopsis it is encoded by a multigene family of five genes, PHYA to PHYE.

Answer: B

Auxin is made from Tryptophan.

The main natural auxin, indole-3-acetic acid (IAA), contains an indole ring that comes from tryptophan. The main pathway in plants converts tryptophan to indole-3-pyruvic acid (TAA enzymes) and then to IAA (YUCCA flavin monooxygenases); minor routes go through tryptamine or indole-3-acetaldoxime.

Serine contributes to many compounds but not the indole ring. Valine is a branched-chain amino acid, and tyrosine is aromatic but has a phenol ring; it is a precursor of other products, such as alkaloids and pigments, not of IAA.

Answer: C

The proteins are the late embryogenesis abundant (LEA) proteins.

LEA proteins were first found accumulating in seeds as they dry out, and they are also induced by drought, cold, heat and salt stress. They are highly hydrophilic and act as molecular shields and chaperone-like stabilisers, helping other proteins keep their folded structure and preventing aggregation when water is lost or temperatures are extreme.

Defensins and thionins are small antimicrobial peptides in plant defence. Osmotin is a pathogenesis-related protein linked to osmotic stress and fungal defence, not mainly a stabiliser of protein structure.

Answer: D

The oxygenase reaction yields 3-phosphoglycerate and 2-phosphoglycolate.

When rubisco adds O₂ instead of CO₂ to ribulose 1,5-bisphosphate, the five-carbon sugar is split into one molecule of 3-phosphoglycerate (three carbons) and one of 2-phosphoglycolate (two carbons). The phosphoglycolate is then salvaged through the photorespiratory pathway in the chloroplast, peroxisome and mitochondrion.

3-phosphoglycerate alone is the product of the carboxylase reaction (two molecules), and 2-phosphoglycolate is never the only product. Fructose 6-phosphate is formed later in the Calvin cycle, not by rubisco.

Answer: C

A mutation in ACC synthase blocks the synthesis of ethylene.

Ethylene is made from methionine through S-adenosylmethionine (SAM). ACC synthase converts SAM to 1-aminocyclopropane-1-carboxylic acid (ACC), usually the rate-limiting step, and ACC oxidase then converts ACC to ethylene. Tomatoes with suppressed ACC synthase ripen slowly for this reason.

Gibberellins come from the terpenoid pathway, salicylic acid (printed 'Salicyclic') from the phenylpropanoid or isochorismate pathway, and auxin mainly from tryptophan, so none depends on ACC synthase.

Answer: C

Auxin was first discovered in oats.

The decisive work used oat (Avena sativa) coleoptiles. F.W. Went in 1928 placed coleoptile tips on agar blocks, showed that the blocks alone could make decapitated coleoptiles bend, and named the diffusible growth substance auxin. The Avena curvature test became the classic bioassay for it.

Darwin's earlier phototropism experiments used canary grass. Mustard, pea and rice were not the plants in which auxin was discovered.

Answer: D

Cytochrome oxidase is not involved in the photosynthetic electron transport chain.

Cytochrome oxidase (complex IV) is the terminal enzyme of the mitochondrial respiratory chain, passing electrons to oxygen to form water.

The other three belong to the light reactions: electrons from photosystem II pass through plastoquinone to the cytochrome b₆f complex, then via plastocyanin to photosystem I, and from photosystem I through ferredoxin to NADP⁺.

Answer: C

Abscisic acid treatment is associated with closure of stomata.

Under water stress, ABA builds up in leaves and acts on guard cells: it triggers the efflux of K⁺ and anions, the guard cells lose turgor, and the stomata close, cutting water loss. This is why ABA is called the stress hormone.

ABA generally inhibits growth, so internode elongation (a gibberellin effect), leaf expansion (printed 'lead area') and root elongation are not effects it promotes.

Answer: D

Water splitting needs manganese and chlorine.

Photolysis of water takes place at the oxygen-evolving complex of photosystem II, whose catalytic core is a cluster of four manganese ions and one calcium ion. Chloride ions are an essential cofactor of the same complex. Deficiency of either impairs oxygen evolution.

Magnesium is the central atom of chlorophyll but is not part of the water-splitting reaction. Potassium is important for stomatal movement and enzyme activation, not for photolysis. So every option pairing them with the right element is only half correct.

Answer: C

ABREs are over-represented in genes for drought stress.

Abscisic acid accumulates when plants lose water and drives the drought and osmotic stress response. ABA-responsive elements, with a core of the form ACGTGG/TC, are bound by bZIP transcription factors (AREB/ABF) that switch on genes for dehydrins, LEA proteins and other protective proteins.

Biotic stress responses are mainly controlled by salicylic acid, jasmonic acid and ethylene. Nitrogen fixation and flowering are governed by other regulators, so ABREs are not characteristic of their promoters.