Bract formation

Abstract: The timing of the switch from vegetative to reproductive development is crucial for species survival. The plant-specific transcription factor and meristem identity regulator LEAFY (LFY) controls this switch in Arabidopsis, in part via the direct activation of two other meristem identity genes, APETALA1 (AP1) and CAULIFLOWER (CAL). We recently identified five new direct LFY targets as candidates for the missing meristem identity regulators that act downstream of LFY. Here, we demonstrate that one of these, the class I homeodomain leucine-zipper transcription factor LMI1, is a meristem identity regulator. LMI1 acts together with LFY to activate CAL expression. The interaction between LFY, LMI1 and CAL resembles a feed-forward loop transcriptional network motif. LMI1 has additional LFY-independent roles in the formation of simple serrated leaves and in the suppression of bract formation. The temporal and spatial expression of LMI1 supports a role in meristem identity and leaf/bract morphogenesis.

Abstract: The lobed-leaf character is a clearly recognizable morphologic trait in rapeseed (Brassica napus L.) that has potential advantages in hybrid production. The present study was carried out to analyze the effect of lobed-leaf trait on agronomic traits and to fine map the LOBED-LEAF 1 (BnLL1) gene in B. napus. The lobed-leaf line Yuye 87 and entire-leaf cultivar Zheyou 50 were used to produce four F2 and eight BC1 populations for genetic analysis. Multiple comparison analysis between lobed-leaf character and agronomic traits was carried out in two F2 populations of reciprocal crosses between Yuye 87 and Zheyou 50. Results show that the lobed-leaf phenotype is controlled by a single an incomplete dominant gene and has no significant adverse effects on 10 main agronomic traits. The location of BnLL1 gene was narrowed down to a 36.7-kb homologous region of B. rapa between genes Bra009506 and Bra009511. On the basis of the annotation of orthologous genes in Arabidopsis, Bra009510 is considered to be a promising candidate for BnLL1 gene. The AT5G03790 is the orthologous gene of Bra009510 in Arabidopsis thaliana, with known functions in the formation of simple serrated leaves and in the suppression of bract formation. It is suggested that previously described lobed-leaf gene BnMOSAICLEAF (BnML) and the BnLL1 are the same genes controlling the lobed-leaf phenotype in B. napus, whereas the lobed-leaf gene BcLEAFLOBED 1 (BcLL1) is their homologue in Brassica campestris ssp. chinensis. These findings will facilitate the cloning and functional identification of the BnLL1 gene in future studies.

Abstract: The pistillate flowers of Arceuthobium show a high degree of uniformity and structural simplicity. Because of their simplicity certain structures such as the carpel and the placenta have been difficult to interpret. From this study, the placenta is interpreted as a composite structure consisting of two united ovules fused basally with the tissues of the receptacle. Pollen tube penetration of the placenta at its tip, development of the zygote at the distal pole, and early endosperm formation at the basal pole of the former megagametophyte indicate that the ovule is orthotropous. A theoretical interpretation of gynoecial phylogeny in Arceuthobium is discussed. THE STAMINATE FLOWERS in the genus Arceuthobium, described in a previous paper (Cohen, 1968) are structurally simple and remarkably uniform throughout the genus. The pistillate flowers, the subject of this report, also show a high degree of uniformity and structural simplicity. However, because of their obvious simplicity, certain structures of the pistillate flower such as the carpel and the so-called "placental body" have been difficult to interpret. Oliver in 1870 (cited by Johnson, 1888) was, apparently, the first to notice the placental struc-ture in the ovary of Arceuthobium. From the -material at his disposal he was unable to explain satisfactorily the nature of this structure. Since -then, other workers have presented diverse interpretations (Johnson, 1888; Dowding, 1931). Some of these, however, are based on studies of mature specimens and the evidence adduced is, for a variety of reasons, far from convincing. It was desirable, therefore, to reinvestigate the pistillate flower, with special emphasis on the placental body, utilizing a more complete series of developmental stages than has heretofore been available. From this study, the placenta is interpreted as a composite structure, consisting of two united orthotropous ovules fused basally with tissue of the elongated receptacle. The materials and methods employed in this study are essentially similar to those cited in an earlier paper (Cohen, 1963). OBSERVATIONs-Floral morphology-The young pistillate flower, which barely protrudes beyond the subtending bract, is ovoid or flattened in a transverse plane (Fig. 1) depending on whether it is terminal or lateral in the inflorescence. In the former case the flowers tend to become -adially symmetrical, whereas in the latter, 1 Received for publication 19 December 1969. they are laterally compressed. The two perianth segments are united basally but separate near the tip, forming a pair of lobes through which the stigma protrudes. The pistil consists of an entire or slightly lobed and funnel-shaped stigma, a short style, and a unilocular, inferior ovary containing a massive placenta-like body bearing two embryo sacs (Fig. 8). Whereas the number of perianth segments in the staminate flowers is variable, the dimerous condition predominates in the pistillate flower. Trimerous flowers, nonetheless, do occur but they are exceedingly rare, having previously been cited in only two instances, A. vaginatum and A. pusillum, each consisting of a single specimen (Gill, 1935). Several specimens of A. americanum with trimerous female flowers have appeared in my collection and they will be described later. Sepal initiation and development-The transition from the vegetative phase to flowering in the pistillate inflorescence, as in the formation of staminate flowers, is abrupt. Since the transition from bract formation to floral initiation in the pistillate inflorescence is not accompanied by a change in phyllotaxis, it was not possible to determine. whether the earliest stages in development are those of a vegetative or floral structure. This distinction was possible only in older sepal primordia. The sepals of the pistillate flowers, like those of the staminate flowers, arise as individual primordia; their fused basal regions are formed later in ontogeny. Sepal primordia in both pistillate and staminate flowers are similar in origin as well as in their manner of growth and development. Just before carpel initiation the floral apex is often flat or slightly convex (Fig. 6). In transverse section, the floral apex is elliptical, the sepal primordia forming the long axis of the ellipse. The size of the floral buds varies, the terminal ones characteristically being larger