Transformation of plastids in soil‐shaded lowermost hypocotyl segments of bean (Phaseolus vulgaris) during a 60‐day cultivation period

Abstract

The maintenance but substantial transformation of plastids was found in lowermost hypocotyl segments of soil-grown bean plants (Phaseolus vulgaris cv. Magnum) during a 60-day cultivation period. Although the plants were grown under natural light-dark cycles, this hypocotyl segment was under full coverage of the soil in 5-7 cm depth, thus it was never exposed to light. The 4-day-old plants were fully etiolated: amyloplasts, occasionally prolamellar bodies, protochlorophyllide (Pchlide) and protochlorophyll (Pchl) were found in the hypocotyls of these young seedlings. The 633 and 654 nm bands in the 77 K fluorescence emission spectra indicated the presence of Pchlide and Pchl pigments. During aging, both the Pchlide and Pchl contents increased, however, the Pchl to Pchlide ratio gradually increased. In parallel, the contribution of the 654 nm form decreased and in the spectra of the 60-day-old samples, the main band shifted to 631 nm, and a new form appeared with an emission maximum at 641 nm. The photoactivity had been lost; bleaching took place at continuous illumination. The inner membranes of the plastids disappeared, the amount of starch storing amyloplasts decreased. These data may indicate the general importance of plastids for plant cell metabolism, which can be the reason for their maintenance. Also the general heterogeneity of plastid forms can be concluded: in tissues not exposed to light, Pchl accumulating plastids develop and are maintained even for a long period.

The Role of Membranes and Lipid-Protein Interactions in the Mg-Branch of Tetrapyrrole Biosynthesis

“…The use of etiolated systems to study these processes were often criticized because the major chloroplast differentiation pathway under natural light conditions is the proplastid-to-chloroplast pathway occurring in most seeds germinating on the soil surface or in new leaves produced by the shoot apical meristem (Charuvi et al, 2012 ; Yadav et al, 2019 ). However, several data indicate that etioplasts and accumulation of Pchlide and LPOR ternary complexes occur also under natural conditions in seedlings germinating in the soil (e.g., Vitányi et al, 2013 ; Kakuszi et al, 2017 ), in tissues partially covered by other organs (e.g., Solymosi et al, 2007b ), in inner leaf primordia developing inside closed bud structures (e.g., Solymosi et al, 2004 , 2006a , 2012 ), and in other systems (water plants, etc., reviewed in Solymosi and Aronsson, 2013 ; Armarego-Marriott et al, 2020 ). In such tissues or organs Chl biosynthesis and etioplast-to-chloroplast transformation may be similar to those described in completely etiolated seedlings, but the processes and their regulation still need to be elucidated in such naturally etiolated tissues.…”

Diversity of Plastid Types and Their Interconversions

“…Until the photosynthetic tissues reach a light source, etioplasts develop with stacking prolamellar bodies and numerous small plastoglobuli (Rodríguez-Villalón et al, 2009). According to studies on Arabidopsis and soybeans, etioplast formation is influenced by etiolation time, and the efficient tubular-lamellar arrangement affects subsequent vegetative growth (Kakuszi et al, 2017;Bykowski et al, 2020). The key element to maintaining etioplasts is the completely dark environment.…”

Iron uptake of etioplasts is independent from photosynthesis but applies the reduction-based strategy

“…Therefore, in the absence of light, Chl biosynthesis is inhibited, and proplastids convert into etioplasts accumulating the tubular membrane structure called prolamellar body that lacks the components of the photosynthetic machinery ( Sandoval-Ibáñez et al., 2021 ). This etiolation syndrome commonly occurs in nature, especially in leaf buds ( Solymosi and Böddi, 2006 ; Solymosi et al., 2006 ; Solymosi et al., 2012 ) and soil-covered stem segments ( Vitányi et al., 2013 ; Kakuszi et al., 2016 ; Kakuszi et al., 2017 ).…”