On the cover

Colored autoradiographs of Vigna species fed with radioactive Na (²²Na), indicating Na⁺ allocation in plant bodies. While the salt-sensitive species V. angularis (azuki bean, left) shows Na⁺ allocation to the stem and the leaves, the salt-tolerant species, V. nakashimae, V. riukiuensis, V. luteola and V. marina show different patterns of Na⁺ allocation. In this issue, by tracer analysis and mass spectrometry, Noda et al. revealed the diversity of the mechanisms of salt tolerance in the wild genetic resources of the genus Vigna (This issue, p. 326–331).

(K. Naito: Research Center of Genetic Resources, National Agriculture and Food Research Organization)

The induction of 2n pollen is an important technique for breeding polyploid plants. Here, we observed meiosis in the pollen mother cells (PMCs) of six Phalaenopsis cultivars and attempted to induce 2n pollen. The meiotic stage was related to flower bud length. During meiosis, Phalaenopsis cultivars with flower widths of approximately 20–40 mm and 50–60 mm had bud lengths of approximately 3–8 mm and 5–13 mm, respectively. The duration of meiosis ranged from 4.2 to 14 d. This was the first study to characterize meiosis of the PMCs of Phalaenopsis. The natural generation frequency of 2n pollen varied from 0.68% to 1.78%. Meiotic stage and colchicine concentration significantly affected the induction of 2n pollen. The most effective treatment for obtaining 2n pollen was 0.05% colchicine in the leptotene to zygotene stage for 3 d, which achieved a 2n pollen frequency of 10.04%.

Alternaria alternata, the causal pathogen of early blight (EB) disease, is one of the most important diseases in tomato, and other solanaceae family. We analyzed 35 tomato genotypes for quantitative/qualitative traits and biomass growth parameters, as well as the extent and structure of genetic variation associated with EB resistance. Phenotypic comparisons displayed significant differences in leaf blade width (24.95%), stem thickness (30.28%), foliage density (18.88%), and plant size (18.89%), with significant positive correlations with EB resistance (0.18–0.75). Correlation analysis showed that mature fruit size, thickness of fruit pericarp, and leaf type were significantly and negatively correlated with EB resistance (up to –0.41). The susceptible tomato seedlings represented significant reductions in biomass parameters. According to ISSR analysis, the highest resolving power (≥0.79) and heterozygosity (≥0.24) values revealed the presence of high genetic variability among the tomato genotypes. Bayesian model-based STRUCTURE analysis assembled the genotypes into 4 (best ΔK = 4) genetic groups. Combined phenotypic and molecular markers proved to be significantly useful for genetic diversity assessment associated with EB disease resistance.

‘Kitahonami’ is a soft red winter wheat (Triticum aestivum L.) cultivar that has high yield, good agronomic performance and good quality characteristics. It currently accounts for 73% of the wheat cultivation area of Hokkaido the northern island in Japan and 42% of Japan’s overall wheat cultivation. However, this cultivar is susceptible to Wheat yellow mosaic virus (WYMV). WYMV has become widespread recently, with serious virus damage reported in Tokachi and Ohotsuku districts, which are the main wheat production areas in Hokkaido. Here, we report a new wheat breeding line ‘Kitami-94’, which was developed over four years by repeated backcrossing with ‘Kitahonami’ using DNA markers for WYMV resistance linked to the Qym1 and Qym2 from ‘Madsen’. Basic maps of Qym1 and Qym2 were created and used to confirm that ‘Kitami-94’ reliably carried the two resistance genes. ‘Kitami-94’ demonstrated WYMV resistance, and had agronomic traits and quality equivalent to ‘Kitahonami’ except for higher polyphenol oxidase activity and lower thousand grain weight. ‘Kitami-94’ may be useful for elucidating the mechanism of WYMV resistance in the background of ‘Kitahonami’, and for developing new cultivars.

The pathogen Colletotrichum orbiculare is causal fungus of cucurbit anthracnose. Multiple races have been identified in the United States, suggesting that it is necessary to cultivate suitable resistant cultivars and breed new cultivars with the most suitable resistance gene. This study examined the pathogenicity and virulence of 20 strains in Japan to clarify the existence of races and virulence differences. Based on the symptoms on inoculated cotyledons and true leaves of watermelon, we could evaluate the compatibility of each strain to each host cultivar. Our analysis based on the reaction to the host cultivar harboring the resistance gene Ar-1 (Cla001017) revealed the existence of three races in Japan. An alarming result was that a race that overcame Ar-1, which is a target gene in current watermelon breeding in Japan, is present in Japan. The cucumber and melon host cultivars showed diverse symptoms, whereas a squash cultivar was resistant to all strains. Three strains caused severe damage even to the most resistant cucumber cultivar ‘Ban Kyuri’ and resistant cultivars harboring Cssgr, a well-known gene conferring loss-of-susceptibility resistance. Screening genetic resources for novel resistance genes using strains with high virulence is of vital importance for watermelon, cucumber, and melon production.

sd1-d has been utilized to develop short-culmed indica varieties adaptable to higher fertilizer-applications. Its tall alleles SD1-in and SD1-ja are harbored in indica and japonica subspecies, respectively. SD1-in possesses a higher effect on elongating culm than SD1-ja. The sd1-d of indica IR36 was substituted with SD1-in or SD1-ja through recurrent backcrossing with IR36, and two tall isogenic lines (“5867-36” and “Koshi-36”) were developed. IR36, 5867-36 and Koshi-36 were grown in a paddy field, and the effects of sd1-d, SD1-in and SD1-ja on morphological characteristics concerning dry-matter production and photosynthesis were compared mutually. sd1-d diminished dry weight of total brown rice/m² and total dry matter weights, but enhanced harvest indexes, compared with SD1-in. In IR36, shorter lengths of the first (flag) to third leaves, and more panicle-bearing stems, caused by sd1-d, compared with SD1-in-carrying 5867-36, and erect first leaves, not caused by sd1-d, could construct the canopy structure appropriate for obtaining a high rate of photosynthesis at an optimum LAI. Koshi-36 could be used for a mid-mother line to develop indica varieties adaptable to middle and low fertilizer-applications, due to higher effect of SD1-ja on yielding ability, compared with that of sd1-d, no breaking-type lodging, and resistances to diseases and pests.

Wild species in the genus Vigna are a great resource of tolerance to various stresses including salinity. We have previously screened the genetic resources of the genus Vigna and identified several accessions that have independently evolved salt tolerance. However, many aspects of such tolerance have remained unknown. Thus, we used autoradiography with radioactive sodium (²²Na⁺) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) to visualize and compare Na⁺ allocation in Vigna angularis (Willd.) Ohwi & H.Ohashi (azuki bean), Vigna nakashimae (Ohwi) Ohwi & H.Ohashi, Vigna riukiuensis (Ohwi) Ohwi & H.Ohashi, Vigna luteola (Jacq.) Benth. and Vigna marina (Burm.) Merr.. The results indicated: 1) Tolerant accessions suppress Na⁺ accumulation compared to azuki bean. 2) V. nakashimae and V. marina does so by accumulating higher amount of K⁺, whereas V. riukiuensis and V. luteola does so by other mechanisms. 3) V. luteola avoids salt-shedding by allocating excess Na⁺ to newly expanded leaves. As the mechanisms of the tolerant species were different, they could be piled up in a single crop via classical breeding or by genetic engineering or genome editing.