Q.63 Identify the CORRECT combination of statements with respect to chemical defense in
plants.
P. Pisatin, a phytoalexin produced by Ricinus communis is a constitutive defense
compound
Q. Phaseolus vulgaris produces Phaseolus agglutinin I, which is toxic to the cowpea weevil
R. A single step non–enzymatic hydrolysis of cyanogenic glycoside releases the toxic
hydrocyanic acid (HCN) to protect plant against herbivores and pathogens
S. Avenacin, a triterpenoid saponin from oat prevents infection by Gaeumannomyces
graminis, a major pathogen of cereal roots
(A) P, Q (B) Q, S (C) R, S (D) P, S

The correct answer is (B) Q, S.

Statement Analysis

Pisatin serves as a phytoalexin primarily produced by pea (Pisum sativum), not Ricinus communis, which synthesizes casbene as its phytoalexin; phytoalexins represent inducible defenses, not constitutive ones. Phaseolus agglutinin I (PHA I), a lectin from Phaseolus vulgaris, deters the cowpea weevil (Callosobruchus maculatus) through toxicity, though some studies note contaminants like α-amylase inhibitors contributing to effects. Cyanogenic glycoside hydrolysis to release HCN involves two enzymatic steps—β-glucosidase followed by hydroxynitrile lyase—not a single non-enzymatic step. Avenacin, a triterpenoid saponin in oat roots, confers resistance against Gaeumannomyces graminis var. tritici by inhibiting the pathogen.

Option Evaluation

• (A) P, Q: Incorrect, as P misattributes pisatin to Ricinus communis and labels it constitutive.

• (B) Q, S: Correct, since Q accurately describes PHA I’s toxicity to cowpea weevil and S correctly identifies avenacin’s role.

• (C) R, S: Incorrect, because R wrongly claims single-step non-enzymatic HCN release.

• (D) P, S: Incorrect due to P’s errors on pisatin source and type.

Plants deploy sophisticated chemical defenses, including phytoalexins, lectins, cyanogenic glycosides, and saponins, to combat herbivores and pathogens. These mechanisms, central to topics like chemical defense in plants for CSIR NET Life Sciences, involve both constitutive (pre-formed) and inducible responses. Understanding specifics, such as pisatin’s role or avenacin’s antifungal action, is key for exams.

Phytoalexins in Plant Defense

Phytoalexins like pisatin accumulate post-infection in pea (Pisum sativum) as inducible antimicrobials, not pre-formed in Ricinus communis, which produces casbene. This distinction highlights phytoalexins’ role in active defense signaling.

Lectins Against Insect Pests

Phaseolus vulgaris lectin Phaseolus agglutinin I (PHA I) toxifies cowpea weevil (Callosobruchus maculatus), inhibiting development, though α-amylase inhibitors may enhance effects. Lectins bind gut epithelia, disrupting digestion.

Cyanogenic Glycosides Mechanism

Cyanogenic glycosides release toxic HCN via dual enzymes: β-glucosidase cleaves sugar to form cyanohydrin, then hydroxynitrile lyase liberates HCN—not a single non-enzymatic hydrolysis. Tissue damage activates this compartmentalized process.

Saponins like Avenacin

Oat root triterpenoid saponin avenacin prevents Gaeumannomyces graminis infection, a take-all pathogen; mutants lacking it become susceptible. Pathogens evolve detoxifying enzymes like avenacinase.