Plant Breeding 2

Plant Breeding 2

Adventitious embryony involves embryo formation from:

Adventitious embryony represents sporophytic apomixis subtype where embryos originate directly from somatic diploid cells outside embryo sac, most commonly from nucellus layers or inner integument surrounding megagametophyte. Nucellar initial cell undergoes mitotic division triggered by somatic embryogenesis signaling involving LEC1, WUSCHEL and SOMATIC EMBRYOGENESIS RECEPTOR KINASE expression, protruding into embryo sac cavity and developing into embryo while still within ovule, leading to polyembryony where several nucellar embryos coexist with maybe one sexual zygotic embryo. Because embryo

Ref: Koltunow Plant Cell sporophytic apomixis nucellus; Esau Plant Anatomy adventitious embryony; NCERT Biology polyembryony Citrus example.

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Plant Breeding 2

Non-recurrent apomixis produces embryos that are:

Non-recurrent apomixis rare phenomenon where meiosis proceeds normally producing haploid megaspores, but embryo develops from haploid egg without fertilization through haploid parthenogenesis. Resulting embryo possesses n chromosome number, hemizygous for all genes, exposing recessive lethals and causing severely reduced vigor, frequent albinism and complete male and female sterility because meiosis in haploid sporophyte unable to produce balanced gametes due to absence of homologous pairing leading to random chromosome segregation. Often accompanied by absent or defective endosperm causing se

Ref: Asker & Jerling Apomixis in Plants non-recurrent; Nogler Haploid parthenogenesis review; PubMed haploid apomixis sterility.

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Plant Breeding 2

Recurrent apomixis produces embryos that are:

Recurrent apomixis termed diplosporous and aposporous types maintains chromosome number across cycles because embryo sac develops either from somatic nucellar cell dividing mitotically or from unreduced megaspore mother cell forming restitution nucleus after aborted meiosis. Egg cell therefore diploid containing full maternal chromosome complement 2n. Development proceeds parthenogenetically without fertilization, directly forming diploid embryo genetically identical to mother. Because ploidy same as maternal sporophyte, reproduction recurs stably, hence designation recurrent. Examples widespr

Ref: Nogler 1984 Gametophytic apomixis classification recurrent; Bicknell & Koltunow Review recurrent apomixis; Acquaah definition.

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Plant Breeding 2

Apomixis helps in:

Practical benefit of apomixis lies in fixation of heterosis. Once elite heterozygous hybrid created showing superior yield due to complementation of dominant alleles and epistatic networks, sexual reproduction breaks genotype via segregation. Apomictic seed formation clones maternal genome including heterozygous allelic configuration, thus hybrid vigor maintained generation after generation through seed, eliminating need for annual recreation of hybrid from parental inbreds using costly emasculation and isolation. Economic advantage immense for smallholders lacking access to hybrid seed market

Ref: Koltunow Am J Bot apomixis heterosis fixation; Underwood et al Nature Clonal MiMe; Wang et al Nature Biotech synthetic apomixis rice.

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Plant Breeding 2

Apomixis refers to:

Apomixis signifies alternative reproductive pathway where seeds form without meiosis and syngamy, producing offspring clonal to maternal parent preserving heterozygosity and hybrid genotype. Developmental routes include apospory where embryo sac initiates from somatic nucellar cell bypassing megaspore mother cell differentiation, diplospory where megaspore mother cell undergoes mitosis instead of meiosis forming unreduced embryo sac, and adventitious embryony where embryo develops directly from nucellar tissue. Unreduced egg cell then develops parthenogenetically via activation of egg cell tra

Ref: Bicknell & Koltunow Annu Rev Plant Biol apomixis mechanisms; Carman et al Apomixis genetics; NCERT Biology Apomixis introduction.

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Plant Breeding 2

Barnase–Barstar system is an example of:

Barnase-Barstar system pioneered by Mariani 1990 represents engineered transgenic male sterility mimicking natural tapetal ablation. Barnase from Bacillus amyloliquefaciens encodes extracellular RNase that hydrolyzes single-stranded RNA causing cellular cytotoxicity when driven specifically in tapetum by TA29 promoter active in early anther development, leading to premature tapetal degradation, microspore starvation and complete sterility. Barstar from same bacterium encodes small protein forming stoichiometric complex blocking Barnase active site with high affinity, neutralizing toxicity. Res

Ref: Mariani et al Nature 1990 Tapetal barnase male sterility; Mariani et al 1992 Barstar restoration Nature; Scientific Reports transgenic sterility SeedLink.

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Plant Breeding 2

R-line in CGMS contains:

R-line known as restorer line harbors dominant restorer of fertility genes Rf encoding pentatricopeptide repeat proteins targeted to mitochondria where they bind and cleave aberrant ORF transcripts such as orf79 associated with WA cytoplasm in rice, restoring normal oxidative phosphorylation and ATP supply for anther development. Genotype RfRf, cytoplasm may be either S or N but crucial nuclear contribution dominates. Cross between A-line rfrf and R-line RfRf produces F1 heterozygote Rfrf where restorer protein suppresses sterility causing ORF, enabling normal pollen development and full male

Ref: Huang et al Nature 2015 Rf cloning rice; Bentolila et al PPR restorer mechanism; PubMed Restorer genetics review.

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Plant Breeding 2

B-line in CGMS is used to:

B-line or maintainer line defined as fertile counterpart isogenic to A-line for nuclear genes but carrying normal fertile cytoplasm N and homozygous non-restorer alleles rfrf. Function maintains male sterile A-line because when A-line cytoplasm S mother crossed with pollen from B-line, offspring inherits S cytoplasm from maternal parent causing male sterility while nuclear genome remains rfrf identical to both parents. B-line itself self-fertile due to N cytoplasm compensating, producing sufficient seeds for propagation. Without B-line, A-line cannot be multiplied sexually because selfing yiel

Ref: Singh BD Maintainer line role; Acquaah Principles B-line multiplication; ICAR Hybrid seed Handbook.

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Plant Breeding 2

In CGMS, A-line is:

In CGMS three-line scheme A-line designated as cytoplasmic male sterile female parent carrying sterile cytoplasm S derived from wild abortive source and non-restorer genotype rfrf in nuclear background, so phenotype fully male sterile with whitish anthers containing aborted pollen but female fertility normal. Because cytoplasm maternally inherited, progeny from any cross where A-line used as female inherits S cytoplasm and remains sterile unless restorer allele introduced from male parent. To multiply A-line, cross made with maintainer B-line sharing identical nuclear genome but normal fertile

Ref: Singh BD CGMS A-line definition; Rice Knowledge Portal Three line system; Acquaah Hybrid breeding A-line.

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Plant Breeding 2

CGMS involves interaction between:

Cytoplasmic-genic male sterility arises from antagonistic interaction between mitochondrial genome carrying sterility causing chimeric open reading frames and nuclear genome harboring restorer of fertility Rf genes. Mitochondrial ORFs such as orf79 in rice WA cytoplasm, orf138 in Brassica Ogu cytoplasm resulting from rearrangement generate cytotoxic peptides disrupting mitochondrial electron transport complex assembly reducing ATP supply critical for high energy demand of tapetal cells during microsporogenesis. Cytoplasm S with ORF confers sterility when nuclear background homozygous for non-r

Ref: Hanson MR Bentolila Annu Rev Genetics CMS molecular basis; Fujii PPR restorer mechanism Plant Cell; Singh BD CGMS interaction.

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Plant Breeding 2

PGMS expression depends on:

Photoperiod-sensitive genetic male sterility fertility restoration strictly dependent on day length perception. Phytochrome and circadian oscillator genes modulate expression of long noncoding RNA LDMAR also called PMS3 essential for normal tapetal development under short photoperiod. Under long day conditions exceeding threshold around 13.5 to 14 hours, LDMAR promoter hypermethylated reducing transcript accumulation leading to precocious programmed cell death of tapetum and microspore abortion causing complete male sterility. Under short day conditions below threshold methylation relieved, LD

Ref: Ding et al Nature Genetics LDMAR PGMS; Yuan LC Photoperiod sterility review; PubMed NK58S PGMS mechanism.

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Plant Breeding 2

TGMS expression depends on:

Thermo-sensitive genetic male sterility expression modulated primarily by ambient temperature experienced by developing anthers during critical window from secondary branch differentiation to meiosis. Molecular switch involves temperature-dependent processing of UbL40 mRNA or activity of RNase Z encoded by tms5 gene. At high temperature exceeding critical sterility inducing point typically 26 to 28 Celsius for rice, abnormal accumulation of mRNA triggers ribonuclease dysfunction, persistence of tapetum and premature microspore degeneration resulting in complete pollen sterility with shriveled

Ref: Zhang et al PNAS 2012 tms5 RNase; Lee DS Rice TGMS molecular mechanism; Frontiers thermo-sensitive GMS review.

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