Stress-Induced Flowering in Pharbitis

Flowering in many plant species is usually regulated by environmental cues, such as night length in photoperiodic flowering and temperature in vernalization. However, some studies have indicated that stress is also a cue to induce flowering. In fact, plants flower as an emergency response when stressed, ensuring their ability to produce the next generation. Through this mechanism, the species is preserved, even under unfavorable environmental conditions. Therefore, stress-induced flowering is universal and as important as photoperiodic flowering and vernalization.

In this article, physiological characteristics, the regulation by salicylic acid (SA) and the expression of flowering-related genes in stress-induced flowering in pharbitis were reviewed. Pharbitis flowered under long-days in response to poor nutrition or low temperature. The pharbitis plants induced to flower by stress reached anthesis, fruited and produced fertile seeds. The progeny of the stressed plants developed normally. Grafting experiments indicated that a transmissible flowering stimulus was involved in poor nutrition stress-induced flowering. Aminooxyacetic acid (AOA), a phenylalanine ammonia-lyase (PAL) inhibitor, inhibited the stress-induced flowering, and this inhibition was overcome by SA. Stress induced PAL activity and SA biosynthesis. PnFT2, a pharbitis ortholog of the flowering gene FLOWERING LOCUS T of Arabidopsis thaliana, was expressed when the plants were induced to flower by stress. The overexpression of PnFT2 induced flowering, and PnFT2RNAi inhibited it. AOA inhibited PnFT2 expression induced by stress, and SA eliminated this inhibitory effect. SA enhanced PnFT2 expression under poor nutrition, but not under non-stressful conditions. Therefore, stress may induce the production of SA and other unknown factor(s) that may work in combination to induce PnFT2 expression and flowering.

In conclusion, pharbitis is typically induced to flower under short-day (SD) conditions and can be induced to flower under unfavorable photoperiodic conditions when exposed to stress. And stress activates PAL and enhances SA levels, promoting PnFT2 expression to induce flowering. However, SA alone cannot induce flowering, and other factors are necessary.

Article by Kaede C. Wada, et al, from Niigata University, Niigata, Japan.

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