PMC

Ovule number per flower is determined by the early and late ovule initiation processes, while the early ovule initiation process has been demonstrated previously that partially controlled by BR-PIN1 pathway. Here, we discovered the late ovule initiation process mainly through BR-PIN3 pathway. BR positively regulates PIN3 expression which in turn is required for auxin polar transport to form an auxin maxima in the placenta for ovule primordia initiation. Moreover, PIN3 indirectly regulates STK expression, which is required for pistil growth and ovule identity, to promote pistil elongation providing enough space for the late ovule initiation.

Ovule morphogenesis in Aconitum taipeicum, Actaea asiatica and Beesia calthaefolia. (A–G) Aconitum taipeicum: (A) ovule primordium; (B) inner integument is initiated annularly; (C) ovule with cup-shaped inner integument; (D) ovule with hood-shaped integument; (E) lateral view of ovule, showing no appendage presents; (F) a mature ovule with an arrow indicating the micropyle (endostomic); (G) micropylar end showing lobed inner integument. (H–J) Actaea asiatica: (H) developing ovule with cup-shaped inner integument and hood-shaped outer one; (I) a mature ovule with an arrow indicating the micropyle (endostomic); (J) magnified part of the micropylar end shown in (I), showing lobed inner integument. (K–M) Beesia calthaefolia: (K) developing ovule with cup-shaped inner integument and hood-shaped outer one; (L) a mature ovule with an arrow indicating the micropyle (endostomic); (M) magnified part of the micropylar end shown in (L), showing lobed inner integument. F, Funicle; II, inner integument; N, nucellus; O, ovule primordium; OI, outer integument. Scale bars: A, B, J, M = 50 µm; C = 120 µm; D, G, I, K = 200 µm; E = 0·30 mm; F = 231 µm; H, L = 150 µm.

Examples of barley genotypes showing differences in ovule morphology at maturity. (A) A representative large ovule in Lina. (B) A representative large ovule in Foxtrot. (C) A representative large ovule in Wren. (D) A representative large ovule in Salka. (E) A representative small ovule in Gant. (F) A representative small ovule in Cecilia. (G) A representative small ovule in Akita. (H) A representative small ovule in Forum. Composite images were created by overlaying sequential optical sections from the z-stack. Scale bar = 100 µm.

BEL1 Expression Is Regulated by Cytokinin.
(A) Wild-type ovule, stage 2-III.
(B) bel1-1 ovule, stage 2-III.
(C) Wild-type ovule, stage 2-III, 2 d (2D) after treatment.
(D) In situ hybridization on wild-type ovule with BEL1 probe.
(E) In situ hybridization on wild-type ovule treated with using the BEL1 probe 2 d after treatment.
(F) In situ hybridization on cre1-12 ahk2-2 ahk3-3 triple mutant ovule with BEL1 probe.
(G) Quantitative BEL1 expression analysis by real-time RT-PCR. Wild-type mock-treated or -treated 2 d after treatment, wild-type and cre1-12 ahk2-2 ahk3-3 triple mutant flowers, cre1-12 single mutant, and ahk2-2 ahk3-3 double mutant 2 d after mock treatment or treatment.
(H) to (J) WUSpro:WUS-GUS activity in wild-type ovule (H), bel1-1 ovule (I), and in a wild-type ovule 2 d after treatment (J). The ovules are at stage 2-III/3-I. The white arrowhead indicates ectopic WUSpro:WUS-GUS expression in the aberrant structures of the ovules ([I] to [J]).
(K) PIN1pro:PIN1-GFP in bel1-1 ovule.
(L) DR5rev-pro:GFP in bel1-1 ovule. The ovule is at stage 2-III.
(M) bel1-1 ovule treated with . The arrowhead indicates the region where the bel1-1 structure is formed.
chal, chalaza; f, funiculus; ii, inner integument; nu, nucellus; oi, outer integument.
Bars = 20 μm.