47 public questions tagged with this topic.
Lateral meristems, such as vascular cambium and cork cambium, contribute to secondary growth, increasing girth. This follows from NCERT principle where relation explains outcome clearly for students.
Ref: NCERT Biology Textbook for Class XI and XII (Botany section), Chapter: Biology - Botany portion covering relevant concept, Topic: Plant structure, physiology and applications.
Indeterminate growth defines organisms where growth does not terminate at sexual maturity and meristematic or stem cell pools persist lifelong. Plant shoot and root apical meristems containing WUSCHEL and CLAVATA regulated stem cells continuously produce new organs—leaves, stems, flowers, roots—throughout life. Environmental cues modulate rate but growth potential remains unlimited, allowing trees to grow centuries. Determinate growth characterizes animals where growth plates close and size stabilizes under hormonal control. Terms progressive growth and cellular elongation describe components but indeterminate precisely captures perpetual growth capacity unique to plant developmental strategy.
Ref: Taiz & Zeiger, Plant Physiology; Gilbert, 12th ed., Chapter 2: Indeterminate growth.
Seed dormancy is established and maintained during maturation stage of embryogenesis after heart and torpedo phases when embryo growth ceases and expansion growth stops completely. During maturation ABA accumulates inducing late embryogenesis abundant proteins, oleosins, storage proteins, lipids, and desiccation tolerance mechanisms while gibberellin signaling is suppressed and seed coat hardens imposing physical coat dormancy. Maternal and embryonic factors impose dormancy preventing precocious germination vivipary ensuring effective dispersal and survival through unfavorable season. Germination later occurs upon dormancy break via chilling, light, after-ripening, or gibberellin activation mobilizing stored reserves.
Ref: Finkelstein et al., Plant Physiol 2002; Bewley: seed dormancy established during maturation stage via ABA and LEA proteins.
Asymmetric division of highly polarized zygote generates morphologically and molecularly distinct daughters: small apical cell dense with cytoplasm, mitochondria, nucleus destined to become embryo proper undergoing precise patterned divisions to form all plant tissues including shoot and root, and large basal cell highly vacuolated forming suspensor lineage connecting embryo to maternal tissues. Division creates apical and basal cells differing drastically in size, fate, and transcriptional programs including differential WOX2 versus WOX8/9 expression, PIN7 polarity, and auxin response, establishing fundamental apical-basal axis of entire embryo and providing organizer functions.
Ref: Mansfield et al., Arabidopsis Atlas: asymmetric zygote division creates apical embryo cell and basal suspensor cell.
Cotyledons represent first embryonic leaves formed during heart stage when auxin maxima at apical margins trigger localized outgrowth from globular mass converting to bilateral symmetry. During this transition globular embryo converts to triangular heart morphology with two bulging primordia containing emerging provascular strands and accumulating storage products. Cotyledon formation involves coordinated action of PIN1-dependent auxin transport creating convergence maxima, CUC boundary genes establishing separation, and LEC regulators promoting maturation. Their emergence during heart stage establishes photosynthetic and nutritional support organs essential for seedling survival and reflects successful bilateral patterning and lateral organogenesis program.
Ref: Goldberg et al., Science 1989: cotyledons formed during heart stage transition to bilateral symmetry in Arabidopsis embryogenesis.
Heart-shaped embryo stage in Arabidopsis marks crucial transition from radial to bilateral symmetry during intermediate embryogenesis. Globular embryo with uniform cell divisions develops localized auxin maxima at apical flanks mediated by PIN1 convergence points directing periclinal divisions forming two cotyledon primordia protruding outward producing characteristic indented apex enclosing shoot meristem. Morphology indicates successful cotyledon initiation mediated by MP and NPH4 auxin response factors and boundary specification by CUC1/CUC2/STM genes, signifying onset of organogenesis and separation of functional domains in embryonic apex region essential for patterning.
Ref: Jürgens & Mayer, 1994; Capron et al.: heart-shaped embryo indicates initiation of cotyledon primordia via auxin maxima.
Arabidopsis embryogenesis initiates with zygotic stage immediately following double fertilization distinguished by zygote elongation along apical-basal axis, polarization of vacuole and mitochondria, and asymmetric division into apical and basal daughters. This initial stage precedes quadrant stage four-celled embryo, octant, dermatogen, globular, heart, torpedo, mature embryo stages. Zygotic stage encompasses activation of embryonic transcriptome after extensive chromatin reprogramming involving WOX family, auxin signaling, and epigenetic regulators. Therefore it represents earliest morphologically recognizable stage establishing apical-basal polarity foundation for all subsequent patterning and tissue differentiation events during seed formation in planta.
Ref: Jürgens et al., Annu Rev Plant Biol 1994: first stage of Arabidopsis embryogenesis is zygotic stage with asymmetric division.
Asymmetric division of zygote produces small apical cell forming embryo proper and large basal cell dividing transversely generating file of suspensor cells connecting embryo to maternal tissue and endosperm. Thus embryonic suspensor derives entirely from basal lineage specified by high WOX8/WOX9 expression, auxin efflux toward basal pole, and activated downstream transcriptomes. Suspensor facilitates nutrient transport, hormone supply, and mechanical positioning of embryo within seed cavity. Its uppermost cell hypophysis contributes quiescent center and columella stem cells. Basal origin demonstrates early lineage separation between embryonic and extraembryonic supportive tissues.
Ref: Yeung & Meinke, Bot Gaz; Jürgens: embryonic suspensor derived from basal cell after zygotic asymmetric division.
MP/ARF5 and NPH4/ARF7 auxin response factors become transcriptionally active when auxin triggers TIR1/AFB-mediated ubiquitination and proteasomal degradation of BODENLOS and other AUX/IAA repressors containing EAR motifs. Activated ARFs bind canonical auxin response elements TGTCTC in promoters of PLETHORA genes particularly PLT1, PLT2, PLT4/BABYBOOM, PLT5/AIL5 rapidly inducing their expression in basal and root pole during embryogenesis. MP-PLT module directly converts auxin accumulation into root stem cell specification program, stabilizing PIN expression creating self-reinforcing auxin-PLT-PIN loop crucial for embryonic root establishment and post-embryonic maintenance.
Ref: Berleth & Jürgens, Development 1993: MP and NPH4/ARF7 promote PLT expression establishing root fate downstream of auxin.
SHOOT MERISTEMLESS encodes KNOX1 class homeobox protein expressed throughout shoot apical meristem dome but sharply downregulated in incipient leaf primordia by ASYMMETRIC LEAVES1/2 repression pathway. STM prevents premature differentiation by promoting cytokinin biosynthesis through IPT7 activation, repressing gibberellin 20-oxidase genes maintaining low active GA, repressing CUC boundary regulators at proper domains, and protecting stem cell identity. Strong stm mutants fail to establish embryonic SAM, exhibit fused cotyledons, lack vegetative growth, arrest as seedlings, demonstrating absolute requirement for maintaining shoot meristem identity and indeterminate self-renewal capacity.
Ref: Long & Barton, Development 1998; Laux et al.: STM maintains shoot meristem identity and indeterminate growth, KNOX.
REVOLUTA belongs to class III HD-ZIP family specifying adaxial leaf fate, shoot meristem identity, and interfascicular fiber vascular patterning regulated by microRNA165/166 degradation. Ectopic constitutive 35S-driven expression of REV in root pole suppresses PLT genes, reprograms root meristem into shoot-like organization initiating leaf-like primordia expressing shoot markers STM, WUS, CLV3, and capable of producing shoots upon cytokinin treatment. Root-to-shoot conversion illustrates antagonistic relationship between shoot-promoting REV pathway and root-promoting PLT pathway, where misexpression overrides intrinsic root networks and reprograms meristem identity toward aerial fate.
Ref: Smith et al., Plant Cell 2004; Prigge et al., Plant Physiol: REV misexpression induces root-to-shoot conversion via shoot programs.
TOPLESS encodes Groucho/Tup1-type transcriptional corepressor that directly binds EAR motif of AUX/IAA repressors and other EAR-containing factors recruiting HISTONE DEACETYLASE 19 to silence auxin-responsive genes via chromatin compaction. tpl topless mutants exhibit dramatic embryonic patterning defects where shoot apical domain loses shoot identity and transforms into second root pole producing seedlings with double roots, ectopic expression of PLT and WOX5 throughout upper axis, loss of shoot markers STM and WUS. Shoot-to-root conversion reveals that TPL-mediated active repression of root programs in upper embryo is essential for stabilizing apical fate.
Ref: Long et al., Nature 2006; Szemenyei et al., Science: topless mutant shows shoot-to-root conversion in Arabidopsis embryo.
