Multiplication, morphological responses and related gene expression to salt stress in protocorm-like bodies of Dendrobium orchids in vitro

Saline water is a significant concern for agricultural productivity. Although the effects of salt stress on food crops have been extensively studied, research on non-food crops, such as orchids, has been limited. This study investigated the impact of salinity on the multiplication of protocorm-like bodies (PLBs) in two Dendrobium cultivars, 'Earsakul' and 'Jindasweet', in which 'Jindasweet' PLBs had already initiated some small shoots, whereas 'Earsakul' PLBs had not. PLBs were cultured under control and salinity conditions (150 mM NaCl). Salinity significantly reduced PLBs multiplication, while increasing electrolyte leakage and photosynthetic pigment content. Salt-treated PLBs also exhibited rougher surfaces, smaller cell sizes and denser tissue, suggesting impaired growth and development. Gene expression after 24 hr of treatment revealed no significant changes in genes related to cell division and development (CDKA1, TUBB3, EXP and CKX1). However, rbcL expression was upregulated in 'Jindasweet'. The expression of LEA was suppressed in 'Earsakul' but not in 'Jindasweet'. 'Jindasweet' PLBs appeared to maintain better multiplication and physiological parameters, supporting the gene expression finding. However, as the two cultivars differed in the stages of PLBs used, the difference in salt tolerance may also be attributed to the developmental stage. These findings highlight the potential of PLBs as an in vitro model for screening salt stress responses. This may provide insight into the mechanisms underlying salt stress in Dendrobium, which could contribute to the development of enhancing salt stress tolerance in orchids, particularly through tissue culture approaches.