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>