(a) Explain what is secondary active transport. Discuss its importance in ion acquisition in plants. 10 (b) Discuss what is…

(a) Secondary active transport and ion acquisition

Active transport moves solutes against their electrochemical gradient. In primary active transport a pump uses ATP directly. The plasma-membrane P-type H⁺-ATPase (AHA family) pumps H⁺ out of root cells. This makes the outside acidic (pH about 5.5) and the inside strongly negative (about −120 to −200 mV), a store of energy called the proton-motive force.

In secondary active transport a carrier uses this H⁺ gradient, not ATP, to move another solute uphill.

Symport (co-transport): the solute moves in the same direction as H⁺.

• Nitrate: the NRT1/NPF and NRT2 transporters take in NO₃⁻ with 2 H⁺. NRT1.1 (CHL1) is dual-affinity and also senses nitrate. NRT2.1, with its partner NAR2, is the high-affinity system.
• Phosphate: PHT1 transporters take in H₂PO₄⁻ with H⁺.
• Potassium: HAK5 takes in K⁺ with H⁺ when soil K⁺ is very low. At higher K⁺, uptake runs passively through channels such as AKT1.
• Other solutes: sulphate (SULTR), amino acids, and sucrose (SUC2) in phloem loading.

Antiport (counter-transport): the solute moves opposite to H⁺.

• SOS1: a plasma-membrane Na⁺/H⁺ antiporter that pumps Na⁺ out of the cell.
• Tonoplast antiporters: NHX (Na⁺, K⁺/H⁺) and CAX (Ca²⁺/H⁺) load the vacuole, driven by the V-ATPase and H⁺-pyrophosphatase.

Importance in ion acquisition

• Soil solution is very dilute (nitrate and phosphate often micromolar), yet cells need millimolar concentrations. Only energy coupling can build gradients of 100–1,000-fold.
• Epstein's dual isotherms reflect this: a high-affinity system (mechanism I, carrier-mediated) works at low concentration, and a low-affinity system (mechanism II) at high concentration.
• Salt tolerance depends on antiporters. Over-expressing AtNHX1 or SOS1 improves salinity tolerance.
• Root H⁺ extrusion also acidifies the rhizosphere and releases bound phosphate and iron.
• Blocking the ATPase (for example with vanadate) or collapsing the gradient (with uncouplers) stops uptake. This shows that the energy source is the H⁺ gradient.

(b) GOGAT and its catalytic function

GOGAT (glutamine:2-oxoglutarate aminotransferase), or glutamate synthase, works with glutamine synthetase (GS) in the GS–GOGAT cycle. This is the main route by which plants assimilate ammonium from nitrate reduction, N₂ fixation and photorespiration. It was found in bacteria by Tempest and colleagues (1970) and in plants by Lea and Miflin (1974).

The reactions

• GS: glutamate + NH₄⁺ + ATP → glutamine + ADP + Pi. GS has a very high affinity for NH₄⁺, so toxic ammonium never builds up.
• GOGAT: glutamine + 2-oxoglutarate + 2 e⁻ (from 2 reduced ferredoxin, or NADH + H⁺) → 2 glutamate.
• The cycle: one glutamate returns to GS. The other is the net product, a donor of amino groups to all other amino acids through transaminases, and to chlorophyll and nucleotides.

Catalytic mechanism

GOGAT is an iron–sulphur flavoprotein with two linked active sites.

1. An N-terminal glutamine amidotransferase domain hydrolyses glutamine, releasing NH₃ and the first glutamate.
2. The NH₃ passes through an internal tunnel of about 30 Å to the synthase site. It never leaves the enzyme, which prevents loss and toxicity.
3. At the synthase site NH₃ reacts with 2-oxoglutarate to form 2-iminoglutarate.
4. This is reduced by FMN to the second glutamate. The electrons come from ferredoxin through a [3Fe–4S] cluster (Fd-GOGAT), or from NADH through FAD and Fe–S centres (NADH-GOGAT).

Isoforms (both in plastids)

• Fd-GOGAT: in leaf chloroplasts, powered by ferredoxin reduced in photosystem I. It re-assimilates the ammonia released by photorespiration, which is about ten times the primary assimilation rate. Arabidopsis mutants lacking it die in normal air but survive at high CO₂ (Somerville and Ogren, 1980).
• NADH-GOGAT: in roots, legume nodules and developing seeds. It is important for ammonium from N₂ fixation.

The alternative route, glutamate dehydrogenase, has a low affinity for NH₄⁺ and plays only a minor role. GS–GOGAT activity therefore limits nitrogen-use efficiency, a target of crop improvement.

(c) Hormonal regulation of seed dormancy and germination

Dormancy is the failure of a viable seed to germinate even under favourable conditions. Quiescence is simply a lack of water.

Types of dormancy

• Physical: a hard, impermeable coat, as in legumes. It is broken by scarification.
• Physiological: controlled by the embryo and by hormones.
• Morphological: an immature embryo.

The ABA : GA balance decides the outcome, set by environmental signals.

Abscisic acid (ABA) induces and maintains dormancy

• It is made in the embryo during seed maturation; NCED is the key enzyme.
• Through the PYR/PYL receptors and SnRK2 kinases, it activates the transcription factors ABI3, ABI4 and ABI5. These keep storage-protein and LEA genes on and embryo growth off.
• DOG1 (Delay of Germination 1) raises the depth of dormancy by increasing ABA sensitivity.
• ABA-deficient or insensitive mutants show vivipary, for example the maize viviparous (vp) mutants that germinate on the cob.

Gibberellins (GA) promote germination

• GA binds the GID1 receptor. This causes DELLA growth-repressor proteins to be destroyed by the proteasome.
• The embryo's growth potential rises, and the endosperm cap and testa weaken through expansins and hydrolases.
• In cereals, GA from the embryo induces α-amylase and proteases in the aleurone (through GAMYB) to mobilise the reserves.
• GA-deficient mutants (such as ga1 in Arabidopsis) do not germinate without added GA.

Environmental signals act through this balance

• Light: red light acts through phytochrome B and removes PIF1/PIL5, raising GA synthesis and lowering ABA. This explains why lettuce 'Grand Rapids' seed germinates in red light but not after far-red (Borthwick, 1952).
• Cold stratification and dry after-ripening: these lower ABA and raise GA sensitivity.
• Temperature: high temperature induces thermodormancy.

Other hormones and signals

• Ethylene and brassinosteroids promote germination and counter ABA; ethylene also breaks dormancy in some seeds.
• Cytokinins oppose ABA.
• Nitrate and karrikins from smoke (perceived by KAI2) stimulate germination after fires.
• Root-parasite seeds (Striga, Orobanche) germinate only in response to strigolactones from host roots.

Significance: dormancy times germination to the favourable season and preserves seed viability in the soil seed bank. Too little dormancy causes pre-harvest sprouting in wheat and rice, so breeders select for moderate dormancy.

(d) Metallophytes and their practical importance

Metallophytes are plants that grow naturally on soils rich in heavy metals, such as serpentine (ultramafic) soils rich in Ni, Co and Cr, calamine soils rich in Zn, Pb and Cd, and copper outcrops. Most are tolerant populations or endemics that have evolved rapidly. Bradshaw and Antonovics showed that grasses such as Agrostis on mine spoils became tolerant within decades.

Baker's (1981) strategies

• Excluders: keep metal out of the shoot, for example metal-tolerant ecotypes of Festuca rubra and Agrostis capillaris.
• Indicators: shoot metal reflects soil metal.
• Hyperaccumulators: store extraordinary amounts in shoots, usually in the vacuoles of leaf epidermis, bound to citrate, malate, histidine or nicotianamine. Common thresholds (mg per kg dry leaf) are above 100 for Cd, above 300 for Cu and Co, above 1,000 for Ni, As and Pb, and above 3,000 (older 10,000) for Zn.

Examples

• Zn and Cd: Noccaea (Thlaspi) caerulescens and Arabidopsis halleri.

Nickel:

• Odontarrhena (Alyssum) murale in the Balkans.
• Pycnandra acuminata of New Caledonia, whose latex holds about 25% Ni.
• Rinorea bengalensis, reported from ultramafic soils of the Andaman Islands.
• Arsenic: the fern Pteris vittata.
• Copper: the "copper flower" Haumaniastrum katangense of the Katanga copper belt.
• Zinc indicator: the calamine violet Viola calaminaria.

Practical importance

Phytoremediation:

• Phytoextraction: removing Cd, Zn or As from soil by harvesting hyperaccumulators; Pteris vittata is used against arsenic.
• Phytostabilisation: holding toxic mine tailings in place with tolerant excluder grasses.
• Chelate-assisted extraction of Pb with Brassica juncea (Raskin, 1994).
• Phytomining (agromining): growing Ni hyperaccumulators on low-grade ultramafic land, burning the biomass and recovering nickel salts. Field trials with Odontarrhena in Albania have been reported.
• Geobotanical prospecting: indicator species reveal ore bodies.
• Research: these plants are models for metal homeostasis genes, such as HMA4 and ZIP transporters, used to breed crops that exclude Cd from grain or are biofortified with Zn and Fe.
• Conservation: metal-soil floras are rare and endemic, and mining threatens them.

(e) Invasive alien species and their impact on biodiversity

Under the Convention on Biological Diversity, an invasive alien species (IAS) is a species introduced outside its natural range, deliberately or by accident, whose establishment and spread threaten ecosystems, habitats or species.

The invasion process: introduction, then establishment, then a lag phase, then spread and impact. Invasiveness is favoured by:

• fast growth and prolific seed or vegetative reproduction;
• allelopathy;
• broad tolerance;
• escape from natural enemies (the enemy-release hypothesis);
• disturbed habitats.

Global scale: the IPBES assessment (2023) found more than 37,000 established alien species, over 3,500 of them harmful. It reported costs of over US$423 billion a year (2019), and found invasive species a major factor in about 60% of recorded global extinctions.

Indian examples

• ***Lantana camara*:** introduced as an ornamental. It forms dense thickets in the understorey, suppresses native regeneration, and increases fire risk. A 2020 study found it threatens about 40% of India's tiger habitat.
• Parthenium hysterophorus (congress grass): spread across India after the 1950s. Its allelopathic chemical parthenin reduces crop yields, and it causes dermatitis and allergy.
• ***Prosopis juliflora*:** introduced for fuel and to check desertification. It has taken over the Banni grasslands of Kachchh, lowered the water table, and displaced native grasses and pastoral livelihoods.
• Eichhornia crassipes (water hyacinth): chokes wetlands and lakes, reduces oxygen and light, harms fisheries, and breeds mosquitoes.
• ***Mikania micrantha*, Chromolaena odorata and Senna spectabilis:** smother vegetation in the North-East and the Western Ghats; Senna has spread in Mudumalai and Wayanad.
• Animals: the African catfish (Clarias gariepinus), tilapia and the giant African snail.

Impacts on biodiversity

• Competition, allelopathy and smothering of native plants.
• Homogenisation of communities, and hybridisation with natives.
• Spread of disease.
• Changes to ecosystem processes: fire regimes, hydrology and nutrient cycling.
• Loss of fodder for wild herbivores, which affects carnivores.
• Economic losses in agriculture, fisheries and health.

Management

• Prevention: quarantine under the Plant Quarantine Order 2003 and the Destructive Insects and Pests Act 1914, backed by early detection.
• Mechanical removal: for example the cut-root-stock method for Lantana.

Biological control:

• the beetle Zygogramma bicolorata against Parthenium;
• weevils Neochetina eichhorniae and N. bruchi against water hyacinth.
• Use of the biomass: Lantana furniture and Prosopis charcoal.
• Restoration: with native species.
• Global target: Target 6 of the Kunming–Montreal Global Biodiversity Framework aims to halve the introduction and establishment of IAS by 2030.