(a) What are the consequences of domestication of plants? Explain. 10 (b) Describe how successive cambia cause thickening of the…

(a) Consequences of domestication of plants

Domestication is the genetic change of wild plants under human selection. It produces a set of traits called the domestication syndrome.

Morphological and physiological changes:

• Loss of seed shattering, so seeds stay on the plant until harvest (sh4 and qSH1 in rice, Br genes in wheat and barley).
• Loss of seed dormancy and more uniform germination.
• Larger seeds and fruits, and more compact growth with fewer tillers or side branches (the tb1 gene in maize).
• Synchronous ripening, reduced sensitivity to day length (allowing spread to new latitudes), and loss of bitter or toxic compounds, as in sweet almond and cucurbits.
• Genetic bottleneck: only a small part of the wild gene pool passed into crops. Cultivated rice (Oryza sativa) carries much less diversity than wild O. rufipogon. Monocultures of a few varieties made crops vulnerable to epidemics, such as the Bengal famine brown-spot epidemic of 1943, Southern corn leaf blight in 1970, and the Irish potato famine.
• Dependence on humans: many crops can no longer survive or spread without people. Maize cobs cannot shed their seed, and bananas are seedless and propagated by suckers.

Human and ecological consequences:

• Settled agriculture, surplus, population growth and cities followed.
• In South Asia, barley and wheat were grown at Mehrgarh (about 7000 BCE), with cotton appearing later. Rice, pulses (mung, urd, pigeon pea), sesame, sugarcane and mango were domesticated in the subcontinent.
• Forests were cleared, soils eroded, weeds and crop pests co-evolved (for example weedy rice), and zoonotic diseases spread from domestic animals.
• Modern extension: the Green Revolution's semi-dwarf wheat (with Rht genes, bred by Borlaug) and rice (IR8) intensified selection for yield. It raised production but narrowed diversity. This makes gene banks, crop wild relatives and landraces vital for breeding resistance and climate tolerance.

(b) Successive cambia and stem thickening

In normal dicot stems, one vascular cambium works throughout life, adding secondary xylem inwards and phloem outwards. In some plants the first cambium is short-lived and is replaced by a series of new cambia. This is a form of anomalous secondary growth.

• The first cambium: it forms normally (fascicular and interfascicular) and produces a limited amount of secondary xylem and phloem, then stops dividing.
• New cambia: a new meristematic layer arises outside the phloem, from the pericycle, phloem parenchyma or inner cortex.
• Each new cambium: it produces a fresh ring of small collateral vascular bundles, with xylem inside and phloem outside, embedded in conjunctive tissue. This is lignified or thin-walled parenchyma, also cut off by the cambium, which later becomes lignified. That cambium then stops too, and another arises outside it.
• Result: repetition gives several concentric rings of vascular bundles, with the oldest innermost. The stem, root or tuber thickens even though no single cambium persists.

The phenomenon is seen in:

• Amaranthaceae (now including the former Chenopodiaceae): Amaranthus, Achyranthes, Chenopodium, Beta vulgaris (the rings in the beet root).
• Nyctaginaceae: Boerhavia, Bougainvillea, Mirabilis.
• Other groups: Menispermaceae (Cocculus, Tinospora) and, among gymnosperms, Cycas and Gnetum.

(c) Types of agamospermy; practical importance of apomixis

Agamospermy (apomixis in the strict sense) is the formation of seeds without meiosis and fertilisation. The embryo is therefore genetically identical to the mother plant.

• Adventive (sporophytic) embryony: embryos arise directly from diploid cells of the nucellus or integument, next to or beside a sexual embryo, so seeds are often polyembryonic. Examples are Citrus, mango (Mangifera), Opuntia and Eugenia.

Gametophytic apomixis: an unreduced (2n) embryo sac forms, and its egg develops into an embryo without fertilisation (parthenogenesis).

• Diplospory: the megaspore mother cell itself forms the embryo sac, with meiosis skipped or restitutional. Examples are Taraxacum (dandelion), Antennaria and Eragrostis.
• Apospory: a somatic nucellar cell forms the embryo sac. Examples are Hieracium, Cenchrus/Pennisetum ciliare (buffel grass), Panicum maximum and Poa.
• Pseudogamy: in many aposporous grasses, pollination is still needed, either to form the endosperm or to stimulate development.
• Non-recurrent apomixis: the embryo develops from a haploid egg (haploid parthenogenesis) or from another cell of a reduced embryo sac (apogamy). The resulting haploids are usually sterile.

Why it matters in practice

• Fixing hybrid vigour: hybrid seed can be saved and resown without segregation, so farmers would not need to buy fresh hybrid seed every season.
• True-to-type propagation: crops can be multiplied true to type through seed, which is easier to store and carry than cuttings and is often virus-free. Nucellar seedlings of Citrus and mango are used as uniform rootstocks.
• Plant breeding: apomixis rescues and fixes useful genotypes, including sterile or odd-ploidy ones.
• Synthetic apomixis: researchers have engineered apomixis in rice. The MiMe mutations turn meiosis into mitosis, and egg-cell expression of the BABY BOOM1 gene triggers the embryo. This gave clonal hybrid seeds (Khanday and colleagues, Nature, 2019), with later reports of much higher frequencies. It makes apomixis a realistic tool for fixing hybrids.

(d) Five fibre-yielding plants

• Cotton: Gossypium hirsutum (American upland cotton), G. arboreum and G. herbaceum (desi cottons). Family Malvaceae. The fibre is the seed-coat epidermal hairs (lint).
• Jute: Corchorus capsularis (white jute) and C. olitorius (tossa jute). Family Malvaceae (formerly Tiliaceae). The fibre is bast (phloem fibre) of the stem.
• Flax (linen): Linum usitatissimum. Family Linaceae. The fibre is stem bast.
• Sunn hemp: Crotalaria juncea. Family Fabaceae. The fibre is stem bast.
• Sisal: Agave sisalana. Family Asparagaceae (formerly Agavaceae). The fibre is the vascular fibre of the leaf.

Other examples:

• Coir (Cocos nucifera, Arecaceae): fibre from the fruit's mesocarp.
• Mesta or kenaf (Hibiscus cannabinus, Malvaceae): stem bast.
• Ramie (Boehmeria nivea, Urticaceae): stem bast.
• Manila hemp or abaca (Musa textilis, Musaceae): leaf-sheath fibre.
• Silk cotton or kapok (Ceiba pentandra, Malvaceae): hairs from the fruit wall.

(e) Zygotic versus somatic embryos

A zygotic embryo develops from the zygote formed by fertilisation of the egg inside the embryo sac. A somatic embryo develops from a somatic (non-gametic) cell. This happens in vitro from explants or callus under an auxin such as 2,4-D, first shown in carrot by Steward and by Reinert in 1958–59, or in vivo as nucellar embryos in Citrus.

Differences

• Origin: a zygote (sexual) versus any somatic cell (asexual), directly from the explant or indirectly via callus.
• Genetic make-up: the zygotic embryo is a new recombinant combining both parents. The somatic embryo is a clone of the source plant, though somaclonal variation can occur.

Environment and nutrition:

• The zygotic embryo develops within the ovule and seed, nourished by endosperm (triploid in most angiosperms) and protected by the seed coat.
• The somatic embryo develops in culture, has no endosperm or seed coat, and depends on the medium. It can be encapsulated in alginate to make "synthetic seed".

Suspensor and dormancy:

• The zygotic embryo usually has a well-developed suspensor, and seeds undergo maturation, desiccation and often dormancy.
• The somatic embryo has a reduced or absent suspensor, often lacks a desiccation or dormancy phase, and may germinate early.
• Polarity and connection: both are bipolar, with shoot and root poles. A somatic embryo has no vascular connection with the parent tissue, which distinguishes it from an organogenic shoot bud.
• Number and synchrony: usually one zygotic embryo per seed, versus hundreds of somatic embryos, often developing unevenly.
• Uses: the zygotic embryo is the basis of sexual reproduction and variation. Somatic embryos are used for mass clonal propagation (oil palm, coffee, date palm, Eucalyptus), genetic transformation, cryopreservation and studies of embryogenesis.