Abstract
BackgroundGeophytes possess specialized storage organs - bulbs, tubers, corms or rhizomes, which allow their survival during unfovarable periods and provide energy support for sprouting and sexual and vegetative reproduction. Bulbing and flowering of the geophyte depend on the combined effects of the internal and external factors, especially temperature and photoperiod. Many geophytes are extensively used in agriculture, but mechanisms of regulation of their flowering and bulbing are still unclear.ResultsComparative morpho-physiological and transcriptome analyses and quantitative validation of gene expression shed light on the molecular regulation of the responses to vernalization in garlic, a typical bulbous plant. Long dark cold exposure of bulbs is a major cue for flowering and bulbing, and its interactions with the genetic makeup of the individual plant dictate the phenotypic expression during growth stage. Photoperiod signal is not involved in the initial nuclear and metabolic processes, but might play role in the later stages of development, flower stem elongation and bulbing. Vernalization for 12 weeks at 4 °C and planting in November resulted in flower initiation under short photoperiod in December–January, and early blooming and bulbing. In contrast, non-vernalized plants did not undergo meristem transition. Comparisons between vernalized and non-vernalized bulbs revealed ~ 14,000 differentially expressed genes.ConclusionsLow temperatures stimulate a large cascades of molecular mechanisms in garlic, and a variety of flowering pathways operate together for the benefit of meristem transition, annual life cycle and viable reproduction results.The circadian clock appears to play a central role in the transition of the meristem from vegetative to reproductive stage in bulbous plant, serving as integrator of the low-temperature signals and the expression of the genes associated with vernalization, photoperiod and meristem transition. The reserved photoperiodic pathway is integrated at an upstream point, possibly by the same receptors. Therefore, in bulb, low temperatures stimulate cascades of developmental mechanisms, and several genetic flowering pathways intermix to achieve successful sexual and vegetative reproduction.
Highlights
Geophytes possess specialized storage organs - bulbs, tubers, corms or rhizomes, which allow their survival during unfovarable periods and provide energy support for sprouting and sexual and vegetative reproduction
The present study aims to identify the molecular mechanisms involved in the regulation of responses to low temperature in a flowering garlic genotype
The apical meristem did not shift to floral initiation, apical dominance did not weaken and lateral meristems did not differentiate into leaves or cloves
Summary
Geophytes possess specialized storage organs - bulbs, tubers, corms or rhizomes, which allow their survival during unfovarable periods and provide energy support for sprouting and sexual and vegetative reproduction. Bulbing and flowering of the geophyte depend on the combined effects of the internal and external factors, especially temperature and photoperiod. Dependence on low temperatures evolved several times in the history of plants, with the consequent involvement of a number of regulatory mechanisms [4, 10, 40, 74]. Cold induction downregulates FLC, enabling the expression of floral integrators and the meristem transition from the vegetative to the reproductive state [44, 60, 79, 80]. High AcFT4 expression inhibits bulb formation by repressing AcFT1 This genotype-specific inhibitory effect gradually weakens when days elongate, and thereafter the inductive photoperiod downregulates AcFT4, with the consequent expression of bulb-promoting AcFT1. TgFT2 is considered to act as florigen, whereas TgFT1 and TgFT3 may have bulb-specific functions [47]
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