Problem 9 - Campbell Biology

Question

The specific relationship between a legume and its mutualistic Rhizobium strain probably depends on (A) each legume having a chemical dialogue with a fungus. (B) each Rhizobium strain having a form of nitrogenase that works only in the appropriate legume host. (C) each legume being found where the soil has only the Rhizobium specific to that legume. (D) specific recognition between chemical signals and signal receptors of the Rhizobium strain and legume species.

Step-by-Step Solution

Answer

(D) specific recognition between chemical signals and signal receptors of the Rhizobium strain and legume species.

Step 1 Title - Identify the key aspect of the relationship

The exercise is asking to understand the basis of the relationship between a legume and its mutualistic Rhizobium strain. Pay attention to the mutualistic nature of the relationship, which implies a cooperative interaction where both organisms benefit.

Step 2 Title - Examine each option

Go through each option to determine how it relates to the key aspect of mutualistic relationships: (A) Chemical dialogue with a fungus - This mentions fungi, not Rhizobium. Fungus interactions are not relevant here. (B) Nitrogenase specificity - Focuses on an enzyme's functionality rather than the interaction between legume and Rhizobium. (C) Soil-specific Rhizobium - Implies environmental, not biological, requirements for interaction. (D) Chemical signals and receptors - This talks about molecular interactions between legume and Rhizobium, pertinent to mutualistic relationships.

Step 3 Title - Choose the most appropriate option

Given the options, the only one that directly relates to the mutualistic interaction is (D), which describes a specific recognition between chemical signals and receptors of both Rhizobium and the legume species. This specificity is aligned with mutualistic relationships.

Key Concepts

mutualistic relationships

Mutualistic relationships are partnerships between two different organisms where both benefit. In the context of legumes and Rhizobium, the mutualism revolves around nutrient exchange. Legumes provide carbohydrates to Rhizobium bacteria. In exchange, Rhizobium helps legumes by fixing atmospheric nitrogen into a form the plant can use, like ammonia. This collaboration allows legumes to thrive even in nitrogen-poor soils.

chemical signals

Chemical signals are crucial for establishing mutualistic relationships between legumes and Rhizobium. These signals include compounds secreted by both organisms that help initiate and maintain the interaction. The legume roots release flavonoids, which are detected by Rhizobium. In response, Rhizobium secretes nod factors, signaling molecules that inform the legume it's ready to form a symbiotic relationship. This exchange of chemical messages ensures the proper communication and compatibility between the partners.

signal receptors

Signal receptors in legumes and Rhizobium are specialized proteins that detect chemical signals and trigger appropriate responses. In legumes, receptors on root cells recognize nod factors from Rhizobium. Once these receptors detect the signals, they activate pathways that lead to nodule formation. These nodules are specialized structures where nitrogen fixation occurs. Similarly, Rhizobium has receptors to detect flavonoids from legumes, signaling them to produce nod factors. This mutual recognition is vital for the symbiotic partnership.

specific recognition

Specific recognition refers to the precise identification of chemical signals by receptors in both the legume and Rhizobium. This specificity ensures only compatible species interact. Each legume species releases unique flavonoids that only specific Rhizobium strains can detect. Likewise, the nod factors produced by Rhizobium are recognized only by the appropriate legume's receptors. This specificity prevents incompatible interactions and ensures successful mutualism. Therefore, the specific recognition between legumes and Rhizobium is fundamental for forming a successful partnership.

nitrogen-fixation

Nitrogen-fixation is the process where Rhizobium converts atmospheric nitrogen into ammonia, a form of nitrogen plants can use. This is done via the enzyme nitrogenase, which operates within the root nodules formed during the mutualistic relationship. Nitrogen is a critical nutrient for plant growth, and its availability often limits productivity in many soils. Through nitrogen fixation, Rhizobium enhances soil fertility, benefitting both the legume and surrounding plant life. This process is a cornerstone of sustainable agriculture because it reduces the need for synthetic nitrogen fertilizers.