Expression Profiling of <i>FLOWERING LOCUS T</i>-<i>Like</i> Gene in Alternate Bearing ‘Hass' Avocado Trees Suggests a Role for <i>PaFT</i> in Avocado Flower Induction

<div><p>In many perennials, heavy fruit load on a shoot decreases the ability of the plant to undergo floral induction in the following spring, resulting in a pattern of crop production known as alternate bearing. Here, we studied the effects of fruit load on floral determination in ‘Hass' avocado (<i>Persea americana</i>). De-fruiting experiments initially confirmed the negative effects of fruit load on return to flowering. Next, we isolated a <i>FLOWERING LOCUS T-like</i> gene, <i>PaFT</i>, hypothesized to act as a phloem-mobile florigen signal and examined its expression profile in shoot tissues of <i>on</i> (fully loaded) and <i>off</i> (fruit-lacking) trees. Expression analyses revealed a strong peak in <i>PaFT</i> transcript levels in leaves of <i>off</i> trees from the end of October through November, followed by a return to starting levels. Moreover and concomitant with inflorescence development, only <i>off</i> buds displayed up-regulation of the floral identity transcripts <i>PaAP1</i> and <i>PaLFY</i>, with significant variation being detected from October and November, respectively. Furthermore, a parallel microscopic study of <i>off</i> apical buds revealed the presence of secondary inflorescence axis structures that only appeared towards the end of November. Finally, ectopic expression of <i>PaFT</i> in <i>Arabidopsis</i> resulted in early flowering transition. Together, our data suggests a link between increased <i>PaFT</i> expression observed during late autumn and avocado flower induction. Furthermore, our results also imply that, as in the case of other crop trees, fruit-load might affect flowering by repressing the expression of <i>PaFT</i> in the leaves. Possible mechanism(s) by which fruit crop might repress <i>PaFT</i> expression, are discussed.</p></div

Evolutionary conservation of the mature oocyte proteome

The proteome profiles of mature ovulated oocytes of the Cnidaria basal eumetazoan, the starlet sea anemone Nematostella vectensis was compared with published data of mammalian mouse mature oocytes. We identified 1837 proteins in N. vectensis oocytes including known oocyte- and germ-cell-specific markers, proteins associated with RNPs and vitellogenin, a major component of egg yolk proteins. Our findings suggest highly conserved enriched functional pathways in N. vectensis and the mouse mature oocytes. This study provides the first catalog of cnidarian oocyte proteins, revealing highly conserved ancient organization of life processes for over 500 million years of evolution. Significance: The current study provides the first proteomic profile of an oocyte of a cnidarian organism the starlet sea anemone N. vectensis and gives new insights on the ancient origin of an oocyte proteome template. The comparative analysis with a chordate oocyte suggests that the oocyte proteome predates the divergence of the cnidarian and bilaterian lineages. In addition, the data generated in the study will serve as a valuable resource for further developmental and evolutional studies

Effect of fruit load on fruit yield in the following year.

<p>Complete fruit removal treatments were performed as described in the legend to <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0110613#pone-0110613-g001" target="_blank">Fig. 1</a>. Total yield per tree was determined in the following commercial harvest season (February, 2013). Values represent means±SE of four trees per treatment and different letters indicate significant differences.</p

Amino acid sequence comparison and structural features of avocado PaFT.

<p>Comparison of the deduced amino acid sequence of PaFT (ac: AIG92770) with four representative FT proteins, including: <i>Arabidopsis</i> AtFT (ac: BAA77838.1), citrus (<i>Citrus unshiu</i>) CiFT3 (ac: BAA77836.1), apple (<i>Malus x domestica</i>) MdFT (ac: ACL98164.1) and populus (<i>Populus tremula</i>) PtFT1(ac: ABD52003.1), together with BFT and TFL1 proteins from <i>Arabidopsis</i>, AtBFT (ac: Q9FIT4.1) and AtTFL1 (ac: AED90661.1). Residues on black, dark gray and light gray backgrounds indicate 100%, 75%, and 50% amino acid similarity, respectively. Dashed lines indicate gaps introduced to achieve maximum alignment. Key FT residues are marked with triangles. PaFT Tyr-84 and Gln-139 correspond to AtFT Tyr-85 and Gln-140. The conserved amino acid sequence LGRQTVYAPGWRQN, which distinguish FT from TFL and/or BFT, is underlined.</p

Effect of fruit load on return to flowering.

<p>Complete fruit removal treatments, starting in July, 2011, were performed as described in the legend to <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0110613#pone-0110613-g001" target="_blank">Fig. 1</a>. Flowering-intensity levels of treated (de-fruited) and untreated (<i>on</i>) trees were ranked at the onset of the following spring (March, 2012). Values represent means±SE of four trees per treatment and different letters indicate significant differences.</p

Effect of fruit load on inflorescence development.

<p>Complete fruit removal treatments, starting in July, 2011, were performed as described in the legend to <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0110613#pone-0110613-g001" target="_blank">Fig. 1</a>. The reproductive fates of individual buds on treated (de-fruited) and untreated (<i>on</i>) trees borne along summer-growing shoots were mapped at the end of the flowering period (May, 2012). Values are expressed as percentage of dormant buds, leaves or inflorescences out of 40 records (of 10 shoots per tree) collected per treatment.</p

Ectopic expression of PaFT in <i>Arabidopsis</i>.

<p>Number of days to flowering and the number of rosette leaves at flowering of wild type plants and four randomly selected lines that constitutively expressed <i>PaFT</i> under LD (<b>A</b>) or SD (<b>B</b>) conditions are shown. Values represent means±SE of n = 8-22 plants per treatment and different letters indicate significant differences. Two randomly selected transforemed lines that constitutively expressed <i>PaFT</i> exhibiting early flowering phenotype relative to wild type plants under LD (<b>C</b>) and SD (<b>D</b>) conditions are shown. Photographs were taken after 22 (<b>C</b>) and 48 days (<b>D</b>).</p