Question

1)Describe how Rhizobium infects members of the Fabaceae and how the subsequent relationship between the bacteria and plant benefits both organisms. 100 marks

2) Ferns are vascular plants but they are predominantly found in humid habitats, explain why this is the case.100 marks

Answer 1

1) When legume plants require nitrogen, their roots release flavonoids. Rhizobium is sensitive to these molecules and activate product of nodD gene, a transcription factor that controls the expression of genes involved in the synthesis of the Nod Factor (NF). NFs are secreted LCO whose modifications depends upon species  from which they are  produced which  determines host plant.

this process requires anaerobic conditions i.e, absence of traces of oxygen as nitrogenase enzyme responsible for this is irreversibly inactivated by aerobic conditiions. in aerobic conditions nitogenase enzyme need to be protected. To combat this root nodules are formed ro limit oxygen exposure Iin certaiin plants like fabaceae family.

This symbiotic nitrogen fixation is restricted to a limited number of bacterial groups, including rhizobium,frankia,etc

Infection proceed by Root hairs formation they elongate through the process of tip growth. Infection threads develop from growing root hairs, are thought to be tip-growing structures, and therefore most likely elongate by using at least some of the machinery that was supporting root hair growth before infection took place.

Bacteria belonging to the genera Rhizobium and Azorhizobium ,etc grow in the soil as free-living organisms but can also live as nitrogen-fixing symbionts inside root nodule cells of legume plants. The interactions between several rhizobial species and their host plants have become models for this type of nitrogen-fixing symbiosis. Temperate legumes such as alfalfa, pea, and vetch form indeterminate nodules that arise from root inner and middle cortical cells and grow out from the root via a persistent meristem. During the formation of functional indeterminate nodules, symbiotic bacteria must gain access to the interior of the host root. To get from the outside to the inside, rhizobia grow and divide in tubules called infection threads, which are composite structures derived from the two symbiotic partners. symbiotic infection and invasion during the formation of indeterminate nodules. It summarizes root hair growth, how root hair growth is influenced by rhizobial signaling molecules, infection of root hairs, infection thread extension down root hairs, infection thread growth into root tissue, and the plant and bacterial contributions necessary for infection thread formation and growth.

Bacteria is deriving nutrition from the leguminous plants and while legumes are getting nitrogrn fixed through root nodules.Rhizobia sp. bacteria can be found in the root nodules of legumes. These are swellings (clusters of cells) that can be found along the roots. The Rhizobia carry out the process known as nitrogen fixation. Plants need nitrogen to produce protein but they can’t take it directly from the air where it is abundant in the form of diatomic nitrogen gas (N2). In order for plants to get nitrogen it must first be incorporated into organic compounds such as amino acids in a process known as nitrogen fixation. The Rhizobia absorb nitrogen and use it to make proteins and as a by-product it produces nitrogen-containing ammonium which, unlike the N2 in the air, the plant can take up and use for growth (protein production).

2) FERNS (pteridophytes) are the first terrestrial plants to possess vascular tissues – xylem and phloem. These are found in cool, damp, shady places though some can also survive in sandy conditions. In ferns the main plant body is a sporophyte which is differentiated into true root, stem and leaves.These organs have well-differentiated vascular tissues. The leaves in ferns are large i.e, macrophylls. The sporophytes bear sporangia that are subtended by leaf-like appendages called sporophylls.The sporangia produce spores by meiosis in spore mother cells. The spores germinate to give rise to inconspicuous, small but multicellular,free-living, mostly photosynthetic thalloid gametophytes called prothallus.

These gametophytes require cool, damp, shady places to grow. Because of this specific restricted requirement and the need for water for fertilisation, the spread of living pteridophytes is limited and restricted to narrow geographical regions. that is why although being vascular they are found in damp place,humid,shady places.The gametophytes bear male and female sex organs called antheridia and archegonia.

Water is required for transfer of antherozoids – the male gametes released from the antheridia, to the mouth of archegonium. Fusion of male gamete with the egg present in the archegonium result in the formation of zygote. Zygote thereafter produces a multicellular well-differentiated sporophyte which is the dominant phase of the pteridophytes. The megaspores and microspores germinate and give rise to female and male gametophytes, respectively. The female gametophytes in these plants are retained on the parent sporophytes for variable periods. The development of the zygotes into young embryos take place within the female gametophytes.