1. Flowering HORT 301 – Plant Physiology September 29, 2008 Taiz and Zeiger Chapter 25 Web Topics 1.2, 25.1, 25.7 Web Essay 25.2 paul.m.hasegawa.1@purdue.edu Development of floral organs – changes in cell fate in the shoot apical meristem and flower development Mechanisms and processes that transition the shoot apical meristem from vegetative growth to reproductive development (floral evocation) – signals and regulators

2. Floral organ development – shoot anatomical and morphological changes occur during vegetative to reproductive growth Arabidopsis flowering transition (mustard family) is very characteristic Vegetative apical meristem produces short internodes and plant grows as a rosette Transition into a primary inflorescence meristem results in an elongated flower stalk (bolt) Primary and secondary inflorescences, and flowers

3. VegetativeReproductive Reproductive development begins with the transition of the vegetative shoot meristem into a primary inflorescence meristem Primary inflorescence meristem – apical meristem that initiates floral meristems on its flanks Secondary inflorescence meristem – initiates lateral inflorescences

4. Floral organs are initiated from the floral meristem in concentric whorls: sepals (whorl 1), petals (whorl 2), stamens (whorl 3) and carpel/pistil (whorl 4) Stamens – pollen, anther and filament Pistil - ovary with two fused carpels (w/numerous ovules), style and stigma Floral organs initiate sequentially beginning with the sepals and progressing inwards Arabidopsis

5. Three categories of flower development genes, transition development from an inflorescence to a flower: floral meristem identity, floral organ identity & cadastral genes Meristem identity genes – encode transcription factors that are necessary for activation of floral organ identity genes, SUPPRESSOR OF CONSTANS1 (SOC1), APETALA1 (AP1), LEAFY (LFY) Floral organ identity genes – homeodomain transcription factors that regulate genetic programs leading to production of floral organ parts Cadastral genes – delimit the spatial boundaries for expression of floral organ identity genes

6. Floral organ identity genes: Type A – controls organ identity in the first and second whorls; sepals and petals, APETALA1 & 2 (AP1 & AP2) Type B – controls organ determination in the second and third whorls; petals and stamens, APETALA3 (AP3) &PISTILATA1 (PI1) Type C – controls organ determination in the third and fourth whorls: stamens, and pistils/carpels, AGAMOUS (AG) No sepals/petals No petals/stamens No stamens/pistils

7. Floral evocation: Internal and external cues Internal factors - such as meristem size, phase change, hormones, etc. (autonomous) External factors - environmental cues such as photoperiodism, vernalization, etc. Phase transitions are determined in the shoot apical meristem - juvenile, adult vegetative, and adult reproductive phases Juvenile phase – plant is not competent to form reproductive structures Adult vegetative phase - developmentally mature, i.e. competent to form reproductive structures

8. English Ivy (Hedera helix) Transition from juvenile to adult vegetative phase is characterized by morphological and physiological characteristics, e.g. leaf shape, thorniness, rooting capacity, bolting, etc. (Web Topic 25.1) English Ivy (Hedera helix)

9. Adult reproductive phase – flowering may be dependent on environmental cues, which is why plants in vegetative adult phase may not flower Since shoot apex makes the vegetative to reproductive transition, there is a spatial gradient along the shoot axis Reproductive cells/tissues are near the apex, and the most juvenile cells and tissues are at the base of the shoot, e.g. Hedera helix Juvenile phase of annuals can be very short (e.g. few weeks) or may take years (e.g. woody perennials)

10. Adult reproductive phase is stable; reproductive phase scions grafted onto a juvenile root stocks remain reproductive However, continual re-grafting of adult vegetative scions onto juvenile will rejuvenate the scion Juvenile to adult transition involves competence (adult vegetative phase) and determination

11. Size – transition from vegetative to reproductive phase requires that plants attain a certain size Tobacco and Arabidopsis - leaf number is an indicator of size Conditions or factors that impede growth; mineral deficiency, low light, low temperature, defoliation prolong the juvenile phase or cause rejuvenation Translocated substances such as signal regulatory mRNAs or proteins, carbohydrates, hormones, etc. induce the vegetative to floral transition

12. Exogenous cues Light and induction of flowering, light receptors are in leaves Photoperiodism – (Web Topic 25.7) - capacity to track day-length facilitates determination of season Three basic plant categories based on the influence of photoperiod on flowering: short-day, long-day, and day neutral plants Short-day plants – flowering occurs only in or is accelerated by short days (day length is shorter than a certain period) Night break inhibits flowering

13. Long-day plants – flowering occurs only in or is accelerated by long days (day length is longer than a certain period) Night break of sufficient duration induces flowering

14. Long-day plants – day length greater than a certain period Short-day plants – night length greater than a certain period Day neutral plants are not responsive to photoperiod

15. Plants are adapted to day-length at different latitudes Distinguishing fall and spring – plants couple a juvenile phase (annual plants), temperature requirement (vernalization/overwintering), short-day and then long-day (vice versa) etc. Leaf perceives the photoperiodic signal (via photoreceptors), which is translocated to the meristem and induces flowering Phytochromes and cryptochromes are the photoreceptors that regulate flowering based on molecular genetic evidence

16. Circadian biological clock - periodic rhythm of plant processes that is regulated by an internal timepiece that keeps track of diurnal day and night, e.g. leaf movement, stomatal movement, gene expression Light is the major modulator for “setting” the clock for a particular daily light and dark cycle (entrainment) Molecular genetic evidence indicates that clock function is necessary for flowering

17. After entrainment, the plant will continue a cyclic rhythm even in continuous darkness (few cycles) Evidence of an endogenous mechanism that “keeps time” Light entrains the clock with phytochrome and cryptochrome being the photoreceptors

18. CONSTANS (CO), flowering regulator in Arabidopsis (long day plants) - CO (zinc finger transcription factor) transduces light signal perception (leaves) CO activates FLOWERING LOCUS T (FT), which results in flowering CO activity is controlled by transcriptional and post-translational regulation

19. CO expression (mRNA) is clock controlled – occurs in short days only in darkness and begins in long days just before dusk In long days, CO protein accumulates at the light to dark transition (dusk), but rapidly degrades (proteasome degradation) in darkness In long days, there is sufficient CO expression in the light to produce CO protein before dusk, while in short days there is no CO expression in light and CO protein does not accumulate CO protein is necessary for FT expression and flowering, long days only

20. When CO expression is driven by a constitutive promoter under short days, then CO protein accumulates CO accumulation is rhythmic implicating circadian clock control involvement PHYA and cryptochromes stabilize CO in the light period prior to dusk

21. Rice (short-day plant) - Heading-date1 (Hd1) = CO and Heading-date3 (Hd3) = FT However, Hd1 inhibits Hd3 activation of flowering Short days – similar to CO, Hd1 does not accumulate because Hd1 expression occurs only in darkness, Hd3 is activated and flowering occurs Long days – similar to CO, Hd1 accumulates, inhibits Hd3 activation and no flowering phyA is the photoreceptor for flowering in rice/short-day plants, light signaling that leads to Hd1 protein production

22. Vernalization (Web Topic 25.7) - cold treatment that is required before plants are responsive to photoperiodic-induced floral transition; an adaptation that reduces the likelihood of flowering during winter Vernalization induces acquisition of competence to flower, shoot apex is the target Vernalization induces flowering in winter-annual types of Arabidopsis thaliana, 4 °C for 40 days, note difference in number of rosette leaves (indicator of age)

23. Different species respond at different developmental stages to vernalization, e.g. during germination to vegetative maturity Prolonged exposure to low temperature saturates the vernalization requirement Vernalization regulates gene expression through epigenetic processes that involve chromatin remodeling, which inhibits expression of the floral repressor FLOWERING LOCUS C (FLC)

24. Florigen, the light regulated, flowering hormone – Web Essay 25.2 Floral stimulus moves through phloem, and is conserved in plants 25.30 Transfer of the floral stimulus between different genera, induced henbane (Hyoscyamus niger) (left) is the stock grafted with a non- induced petunia scion (right)

25. Zeevart (2007) Taiz and Zeiger Web Essay 25.2 Light entrains the circadian clock (photoperiodic effect), which activates CO to activate FT FT or Hd3 is translocated from the leaves to the inflorescence meristem and activates floral meristem identity genes (SOC1, AP1 and LFY1), floral organ identity (homeotic) genes and flowering Other internal and external cues regulate for flowering, such as low temperature, size, GA, carbohydrates, etc. Light → CO → FT → SOC1 → LFY → AP1/AP3/PI & AG