Broad Screening of 43 Cultivars Reveals a Callus-to-Shoot Regeneration Bottleneck in Two Cultivated Luffa Species

Shoot regeneration is an important prerequisite for many plant transformation and genome-editing protocols, but responses can vary among commercial cultivars. We screened 43 Luffa cultivars, recorded as 21 ridged and 22 smooth types, on four MS-based media using seedling-derived cotyledon and cotyledonary-axis explants. At a common six-week endpoint, 10,433 of 10,517 explants formed callus (99.20%), 2074 showed browning (19.72%), and 353 produced a recognizable small shoot bud and/or young leaves (3.36%; descriptive 95% Wilson CI: 3.03–3.72%). Twenty-five cultivars were shoot-positive on at least one medium, whereas 18 were negative on all four. Cuihua2212 had the highest pooled response (60/250 (24.0%)). A cultivar-clustered, denominator-weighted binomial-family generalized estimating equation gave a global medium Wald χ2(3) of 7.48 (p = 0.058). In six exploratory pairwise comparisons, C exceeded D (OR: 2.51, 95% CI: 1.30–4.88; BH-adjusted q = 0.039). The medium-adjusted ridged/smooth OR was 2.54 (95% CI: 1.04–6.17), with substantial uncertainty and sensitivity to cultivar composition. Frequent callus formation, therefore, coincided with infrequent visible shoot recovery under the conditions tested. The screen identifies candidates for independent regeneration optimization; it does not establish a callus-derived shoot pathway, species-wide differences or transformation competence.