Yan, Yan
(Helsingin yliopisto, 2021)
Blue and UV radiation are environmental cues or sources of information that can shape the morphology and development of plants. It was hypothesized that: H1) long-term treatments of solar blue (400–500 nm), long-UV (350–400 nm) and short-UV (290–350 nm) radiation (starting before seedling emergence) are perceived as different and can trigger distinct morphological, physiological and molecular responses; H2) parental long-term exposure to short-UV radiation before flowering affects response patterns to blue and UV radiation in the offspring; H3) long-term exposure to solar blue, long-UV and/or short-UV radiation enhances drought tolerance; H4) the responses in H1, H2 and H3 are accession-dependent and related to the environments where the accessions originate. To test these hypotheses, three experiments assessed morphological, physiological and molecular responses of accessions of two legume species, faba bean (Vicia faba L.) (I, II) and barrel medic (Medicago truncatula Gaertn.) (III). To impose radiation treatments by attenuating different wavebands of sunlight, four types of plastic filters were used in experiments I and III outdoors. Through pairwise filter comparisons, three different solar wavebands were assessed: blue, long-UV and short-UV radiation. In experiment I, two accessions of V. faba (Aurora; ILB938) originating from contrasting UV environments (southern Sweden; Andean region of Colombia and Ecuador) were grown under the four filters in sunlight. To study the transgenerational effect of solar short-UV radiation, experiment II was established using seeds produced by plants from experiment I and a factorial experiment design combining the two V. faba accessions, two parental UV treatments (full sunlight and exclusion of short-UV radiation) and four offspring light treatments, from the factorial combination of UVB and blue radiation manipulations in a controlled environment. In experiment III, the effect of long-term exposure to solar blue, long-UV and short-UV radiation during growth on the tolerance of subsequent progressive drought was studied in three M. truncatula accessions using the same filter treatments as in experiment I combined with progressive drought treatments imposed by withholding watering for 2 and 7 days to half the plants starting 40 days after sowing. The three M. truncatula accessions, Jemalong A17, HM006 and HM020, originate from Australia, France and Tunisia, respectively.
After long-term natural light treatments (I, III), blue light but not long-UV or short-UV radiation, significantly regulated plant morphology and transcript abundance. In contrast, both solar blue and short-UV radiation, but not long-UV radiation, induced the accumulation of total flavonoids in leaves of V. faba (I) and M. truncatula (III). Moreover, simultaneous exposure to blue and UVB radiation had a synergetic effect on the induction of flavonoid accumulation (II). In V. faba, the variations of flavonoid composition and gene expression between the two accessions were consistent throughout the two successive generations (I, II). In V. faba, the transgenerational effect of short-UV radiation altered the morphological responses of the progenies to blue light, and it also affected flavonoid accumulation of the offspring in response to UVB radiation. Moreover, the transgenerational effects differed in the two accessions (II): in Aurora, the parental exposure to solar short-UV radiation led to a near-doubling of total quercetin concentration in response to UVB radiation in the progeny, while this was not observed in ILB938. The difference of responses to blue and UV radiation in these two accessions are consistent with adaptation to contrasting UV environments.
In M. truncatula, long-term exposure to both solar blue and UV radiation pre-acclimated plants to subsequent slowly imposed drought, as observed in the transcriptomic result in accession Jemalong A17 that drought (2 and 7 days without watering) did not regulate differentially expressed genes (DEGs) under the filter transmitting blue and UV radiation. In contrast, drought increased transcript abundance of several previously described stress-inducible genes under all other filters. In the light of transcriptomic and flavonoid responses to filter and drought treatments, two processes potentially contribute to light-driven acclimation to drought: 1) increased flavonoid accumulation under blue and short-UV radiation could enhance the capability to scavenge drought-induced reactive oxygen species (ROS); 2) down-regulation of genes involved in light reactions of photosynthesis by blue light could reduce the generation of ROS when stomata close.
In conclusion, under long-term sunlight treatment, blue light modified plants’ morphology and transcript change while both blue and short-UV radiation induced the accumulation of flavonoids; a transgenerational effect of short-UV radiation influenced offspring responses to blue and UVB radiation differently in the two accessions; both blue and UV radiation contributed to pre-acclimation toward subsequent drought by functioning as environmental cues rather than stressors even if the specific responses differed among accessions. Thus, the results support the four hypotheses.